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|Year : 2009 | Volume
| Issue : 42 | Page : 69--90
A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it
John G Casali1, William A Ahroon2, Jeff A Lancaster1,
1 Auditory Systems Laboratory, Virginia Tech, Blacksburg, VA, USA
2 U.S Army Aeromedical Research Laboratory (USAARL), Fort Rucker, AL, USA
John G Casali
Grado Dept. of Industrial and Systems Engineering, 519G, Whittemore Hall, ISE Department, Virginia Tech University, Blacksburg, VA 24061
Operational hearing protection and maintenance of audibility of signals and speech are considered force multipliers in military operations, increasing Soldier survivability and lethality. The in-field research described in this paper was conducted to examine operational performance effects of three different hearing enhancement protection systems (HEPS) that are intended to provide both protection and audibility. The experiment utilized operationally-defined measures in full-scale, simulated combat scenarios with Army ROTC Cadet Soldiers as subjects. The Soldiers' operational performance was evaluated in two missions: reconnaissance and raid (attack on enemy camp). Both missions had substantial hearing requirements, including communications, signal detection/recognition, and distance judgments. Operational performance was measured by objective metrics of Squad performance, including the distances required to detect an enemy insurgent camp under each HEPS, and by subjective metrics, such as the Army's dimensions of combat-related mission success as evaluated by Army Officers who served as training leaders/observers. Other subjective ratings were obtained after each training exercise from both the Officers and the Soldiers, including detailed impressions about each HEPS after extended use. Two of the three HEPS were electronic sound transmission devices (comprising an ambient sound pass-through filtering and amplification circuit): a Peltor Comtac II circumaural headset (NRR=21; 16 dB maximum gain); and a Communications Enhancement Protection System (CEPS) (NRR=29; 36 dB maximum gain). One passive, level-dependent HEPS was used, the yellow end of the Combat Arms Earplug, which provides amplitude-sensitive attenuation that sharply increases when the ambient sound is above about 110 dB (e.g., due to a gunshot), but which provides an NRR of 0 and very little attenuation below 1000 Hz in lower ambient noise levels. In the military mission entailing location of and attack on an enemy camp, the CEPS device resulted in the longest (earliest) average auditory detection distance of the camp (400 feet), followed by the Peltor (233 feet) and then the Combat Arms Earplug (150 feet), in comparison to detection by the unprotected, normal ear at about 220 feet. Commanding officers' ratings of mission performance and overall success slightly favored the electronic HEPS, but these ratings were dependent upon the particular mission undertaken. Ergonomics and usability issues abounded with the electronic HEPS, and the Soldiers' subjective ratings showed variability across all three devices, with device preference depending upon the particular dimension being rated (e.g., comfort vs. hearing ability). Clearly, the results of this in-field experiment demonstrate that more development is needed to achieve the levels of hearing performance and user acceptance from the HEPS that is desirable and needed for combat conditions. In this vein, it is important to note that HEPS designs are continually evolving, and certain upgrades to the three devices evaluated in the late 2006 timeframe of this study have occurred and further evaluations are thus warranted.
|How to cite this article:|
Casali JG, Ahroon WA, Lancaster JA. A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it.Noise Health 2009;11:69-90
|How to cite this URL:|
Casali JG, Ahroon WA, Lancaster JA. A field investigation of hearing protection and hearing enhancement in one device: For soldiers whose ears and lives depend upon it. Noise Health [serial online] 2009 [cited 2022 Jul 4 ];11:69-90
Available from: https://www.noiseandhealth.org/text.asp?2009/11/42/69/48564
Background and Prior Research
Many different varieties of so-called conventional hearing protection devices (hereafter, "HPDs"), in the form of ear canal inserts (earplugs), earmuffs which encircle the outer ear, and semi-insert devices which cap the ear canal at its rim, are available to protect individuals who are exposed to the hazards of loud noises that can result in noise-induced hearing loss. These conventional hearing protection devices reduce (attenuate) noise at the ear solely by passive means, and the attenuation provided by them is the same regardless of incident sound level; that is, the devices are level-independent or amplitude-insensitive.  A major impetus for the development of augmented or enhanced HPDs has been the sometimes negative influence that conventional HPDs have on the hearing ability of users. One major category of augmented HPDs is the level-dependent or amplitude-sensitive class of device. These devices have the ability to either increase its attenuation as incident noise levels rise and/or to automatically adjust the amplification of its pass-through sound transmission circuit. Thus, certain sounds can be amplified and heard in quiet conditions but also show reduced amplification in high incident sound levels to prevent overexposure. These comprised the types of devices investigated in the field experiment discussed herein. For the interested reader, an exhaustive review of all technologies comprising conventional and augmented HPDs, as well as the research studies which have evaluated their effects on signal detection and speech communication may be found in reference. 
Conventional HPDs have often been implicated in compromised auditory perception, degraded signal detection, reduced speech communication abilities, and diminished situational awareness. , Depending upon the specific hearing abilities of users, as well as the acoustical circumstances, these effects can create hazards for the wearer, or at the very least, resistance to the use of hearing protection.  Examples in industry include the hazard created when a construction worker in a ground crew may not hear a backup alarm on an approaching heavy vehicle, due to a hearing protector which provides too much attenuation in the alarm's frequency bandwidth.  Furthermore, and more to the subject of this paper and the detailed Department of Defense contractor's report  from which it emanates, is the very real dilemma faced by military personnel who definitely need hearing protection from gunfire and other loud sounds, many of which can occur suddenly and unexpectedly, but who also need to hear important and sometimes critical signals and speech that are associated with mission tactics, inter-soldier communications, and enemy threats.
Due to the life-threatening implications of not hearing important signals for Soldiers, many of them do not use hearing protection, especially when performing certain missions, such as scouting and reconnaissance.  Unfortunately, recent information from the Iraq conflict suggests that approximately one-third of U.S. Soldiers who return from service in that war have permanent noise-induced hearing loss that is believed to be associated with military operations, and furthermore, the problem of noise-induced hearing loss is the most common military disability, as evidenced by the approximately $1.2 billion spent on personnel hearing loss and tinnitus in 2005 alone.  It is, in part, for these reasons that it has become ever more critical that military personnel be provided with hearing protectors which effectively defend their hearing against noise, whether that noise occurs during in-camp activities, troop transport, or combat engagement. However, due to the huge risks posed by not hearing certain signals in military operations, it is also absolutely essential that Soldiers' hearing protection devices not overly compromise their ability to auditorially detect, recognize, and localize sounds that are important to their survival and to mission success. Such specialized HPDs are commonly known in military vernacular as hearing enhancement protection systems (hereafter, "HEPS") and three examples of these devices that were of interest to the U.S. Army as to their combat-implementation effectiveness were investigated in the research described herein. The emphasis in this field research investigation was to compare the influence of the HEPS systems on metrics of "operational performance effectiveness" of the Soldiers in carrying out their missions.
There is limited precedent for examining the effects of hearing protection and communications systems on operational performance effectiveness, and those that have been conducted have examined only specific military and aviation systems. An early experiment by Whitaker, Peters, and Garinther  manipulated communications intelligibility for crews in a tank simulator, and ascertained the effects on operational effectiveness as measured by the percentage of time an incorrect target was shot, and by the tank gunner's response times. Gunner performance did decrease significantly but only at the point wherein communications intelligibility dropped to 50%, whereas ratings of tank crew workload increased as soon as intelligibility dropped from 100% to 75%. In a subsequent investigation in a tank simulator, Peters and Garinther , found that operational performance metrics consisting of tank crew mission time, enemy target identification, mission communication errors, and gunner error (percent of time the target was hit on the first round) all degraded as a function of the tank crew's communications intelligibility becoming poorer. These Army studies were the first of their kind to provide empirical data which suggested that much-needed advancements in communications headsets, such as the enhanced low-frequency attenuation of active noise cancellation devices, should offer improved overall noise attenuation and more intelligible communications through the earphone, and thus as a result, should have benefit in combat performance effectiveness. More recently, a flight simulator-based experiment by Casali, Lancaster, Valimont, and Gauger  demonstrated that certain noise-attenuating aviation headsets which were found to result in higher speech intelligibility and lower protected exposure levels during the flights, also had concomitant flight mission success benefits as measured by in-flight metrics of actual aircraft navigation and control performance. The salient point is that this flight experiment yielded a significant relationship between the better communications/noise-attenuation capabilities of certain headsets and the corresponding performance of the pilots in safer and more accurate flight control, which is clearly an operational performance benefit of improved hearing for the pilot.
Research Objective and Operational Performance
The experiment described herein followed directly from the aforementioned acute need for improved hearing protection enhancement systems (HEPS) for U.S. Army personnel, coupled with the promise conveyed from the aforementioned limited prior research which had evidenced that operational performance improvements can be realized with improved hearing, at least in tank and aircraft operations. Thus, the experimental objective was to compare three Army HEPS systems in a realistic field infantry-based experiment wherein the final arbiter for HEPS recommendations depended upon measures of the Soldiers' operational performance in two assigned training missions consisting of a raid on camp exercise and a reconnaissance exercise. Each of three Soldier Squads used a single HEPS device while performing both of the two assigned training missions.
Operational performance was measured by objective metrics of the Soldier squad's performance, such as the distances required to detect an enemy camp under each HEPS, and by subjective metrics, such as indicators of combat-related mission success as rated by Army officers who served as training leaders and observers. Other subjective ratings were obtained after the two training mission exercises from both the commanding officers and the participants, including detailed impressions about each HEPS under investigation. Finally, after observing both of the exercises, interviewing the participants, and experiencing the HEPS themselves, the research team provided its own evaluation regarding positive and negative features of each HEPS, specifically from an ergonomics and usability standpoint. Due to length considerations, this latter evaluation is not covered herein, but can be found in the final Virginia Tech technical report for the Army. 
Field Experiment Methodology
The research investigation was embedded into the Virginia Tech (VT) Army ROTC battalion field training exercises and was conducted at the Radford Arsenal Ammunition Plant (RAAP) located in Radford, Virginia which comprises a U.S. Army base. The Cadet Soldier trainees (hereafter, "Soldiers") participated as volunteer subjects and all were members of the VT-ROTC. They participated as an increment to the course of their normal in-field training at the RAAP, but did so by choice after attending a briefing on the experiment conducted by the experimenters (without the commanding officers present to remove any possibility of coercion), and after reading and signing an informed consent form. To fulfill the stated objective of evaluating the HEPSs' operational performance effectiveness under as realistic combat or combat support conditions as possible, the Army ROTC Cadet Soldiers underwent the two mission simulation exercises on two different field training settings (i.e., training "lanes"), after undergoing training by the research team and practice in the proper use of their assigned HEPS. A total of 43 Soldiers participated, with 40 males and 3 females. Rank of the Soldiers ranged from E1 (Private) to E5 (Sergeant), and time in the Army ranged from 0.75 to 4 years. The Soldier participants were each trained on, and assigned to, only one of the three HEPS, as described below.
Affixed to the back of each Soldier's helmet was a waterproof identification number which enabled the experimenter-observers to record data by participant, albeit anonymously. All participants wore a standard camouflage combat ensemble with helmets and carried full ruck sacks.
Participant familiarization and training on HEPS
Briefing and informed consent
In a classroom setting on the VT campus two weeks prior to the field experiment, VT-ROTC Commanding Officers briefed the Cadet Soldiers such that they understood the training exercise objectives to be conducted on each lane. Once familiarized with the experimental exercises in the classroom setting, Soldiers were asked to consider participating in the HEPS experiment portions of the ROTC training exercise, and informed consent was obtained at this time from those who indicated an interest. In the on-campus briefing, Cadets were provided with mission objectives, tactics, and tasks, and were also familiarized in detail with the specific HEPS to which their Squad was assigned.
Ear canal inspection and hearing health inquiry
A simple screening procedure was employed to determine if Cadets qualified to participate in the HEPS experiment. This procedure consisted of:
An ear canal inspection with a lighted otoscope to check for excessive earwax or other foreign body occlusion of the tympanum. (This was performed by an Army audiologist.) A question as to whether Cadets had and used a hearing aid. A question as to whether Cadets had any known and significant hearing impairments.
If any of the above three were affirmative, the Cadet did not qualify for the experiment and he or she was assigned to one of the other four non-experimental training lanes. The identities of those Cadets who did not qualify or who declined to participate for any reason (in the latter case, none declined), were held in confidence.
Next, in a classroom-setting, a graphics-augmented presentation was given by the lead author to the qualified and consenting Soldier participants concerning noise, hearing protection, and a high-level description of the experimental rationale. The participants were made fully aware of the importance of the HEPS devices to the Army, the importance of the experiment to Army needs, and the realism of the upcoming field training exercises with the embedded experiments. The Soldiers listened, again in a classroom setting, to a broad sample of threat-related sounds (and the sources, for example, AK-47 magazine preparation, of those sounds were identified for them), but the participants were not told which sample sounds they would actually encounter on the training lanes. The Soldiers were then divided into HEPS-assigned Squads in separate classrooms wherein they were trained in detail by an experimenter in the proper fitting, use, gain control (if applicable), and special features of the specific HEPS device assigned to them, and were given HEPS use instructions to read and take home for further study. Soldiers were told that they could ask questions about the fit or adjustment of their HEPS at any time before or during the exercises, but that they did have to fit and adjust the HEPS on their own when on the training lanes. It was made clear to the Soldiers that they were to wear their HEPS continuously throughout the entire training exercise on the lane, and that they were not to remove it until the exercise was completed. They were briefed on the hazards posed to hearing in removing the HEPS in the presence of noise, or potential noise, such as explosions. They were also briefed in detail on the importance of the auditory sense in carrying out tactics to meet mission objectives.
Training mission exercises and conditions
The two separate missions consisted of reconnaissance of an enemy camp (without enemy engagement) and raid on an enemy camp (with full engagement), with both implemented as full-scale, in-field combat exercises. The ROTC Cadets actually participated in a total of four separate missions for their training; however, careful analysis of these mission scenarios by the research team revealed that the reconnaissance and raid missions had the best potential for embedding tasks that imposed the highest communications and signal detection/identification demands (i.e., auditory loads). Therefore, these two missions were selected for the experiment. Simulated combat conditions and scenarios were designed to impose inter-Soldier and Soldier-Command communications requirements and other listening burdens (e.g., threat detection) on the Soldier participants in the presence of battleground noise, while they carried out specific assigned tasks using prescribed tactics in which they had been prior trained as ROTC Cadets. The missions were designed by the research team with assistance from the Commanding and Training Officers of the VT-ROTC Corps, all of whom were veterans of Middle Eastern conflicts including Operation Iraqi Freedom. Each Soldier participant was assigned to one of three Squads of 13 to 15 Soldiers in each. Each Squad wore and relied upon one of the three HEPS, and proceeded through both the reconnaissance and raid missions in that order. Thus, each of the three Squads completed the two mission scenarios sequentially and independently on both experimental lanes, with noise and all other conditions held as constant as possible (within the constraints of the in-field setting) and with each Squad utilizing a different HEPS. Thus, HEPS was a between-Squad variable, while mission scenario was a within-Squad variable. Both missions included a significant component of inter-Soldier communications and signal (threat) detection requirements.
Reconnaissance mission exercise
The single RAAP training lane deemed as having the mission scenario with the lowest noise levels, in part due to the need for Soldiers' stealth while locating and scouting an enemy camp, yet still maintaining a high communications burden, was the reconnaissance (RECON) experimental lane. The RECON lane required the Squad to transverse a longitudinal distance of about 450 meters (492 yards) in a lane ranging up to about 300 meters (328 yards) in width, and the Training Officers and research team observers accompanied the Squad to perform data collection and measures of mission performance. This lane was equipped with three separate battery-powered sound generation/loudspeaker systems (2 Sony CFD-F10 CD/radio/cassette recorders and an AIWA EX-20 CD/radio/cassette recorder) for presentation of looped "threat sounds" from compact discs, and digital video cameras for recording of Soldiers' movements. The configuration of the RECON lane, as well as placement and loudness of the threat-related sounds, were selected and refined in conjunction with the aforementioned Army Training Officers. These Officers assisted the research team in spacing of the three threat stations so that each station provided a realistic noise level for the particular threat (as adjudged by the Army Training Officers), being just audible to the open, normal ear at about 45 to 50 feet, but was not audible from the location of any of the other threats. The three threat detection stations were situated so as to "intercept" the Squad's approach toward the enemy camp, and an experimenter was unobtrusively located in a position to observe the Soldiers' reactions to the threat stations. These three threat stations presented: a) enemy conversation in Arabic, b) sniper station weapons preparation noise (magazine/ammunition handling), and c) quiet enemy movement through the woods (i.e., footfall noise in leaves). Soldiers' reactions to the threats included auditory detection at a "safe" distance, and then taking appropriate tactical action but not engaging the threat. The enemy camp itself on the RECON mission including only personnel who were conversing, but no gunfire, vehicular, or other significant noises.
Obviously, the RECON mission involved the need for extremely careful listening by the Soldiers while they were wearing the HEPS, and these activities included intra-Squad whisper-level speech communications and threat detection, all occurring during the approach toward and scouting of the enemy camp, and all done without the Squad being detected by the enemy. A Squad of 13 to 15 Soldiers operated in the RECON lane on a given exercise, with each Squad using only one HEPS. The Squad, in accomplishing their reconnaissance mission, had to break into separate small groups of 2 to 4 members for scouting and detection of the enemy. Basically, the RECON lane required all aspects of reconnaissance, including the need for quiet movement and communication, locating and identifying an insurgent camp (without engaging), auditorially detecting the aforementioned threat stations before moving too close to them, and reporting information about the scouted insurgent camp back to Command, all without being detected by the enemy. The total time required for the RECON mission was approximately 1 to 1.5 hours, depending upon the Squad's performance.
RAID mission exercise
The single RAAP training lane with the highest noise levels and communications burden comprised the RAID mission, and it involved navigation over a long distance, detection of a noisy enemy camp located at the end of that distance, followed by strategic planning for a full attack on that camp. The RAID lane encompassed about 400 meters (437 yards) of linear distance over which the Soldiers navigated and traversed, and its width ranged from 100 to 200 meters (109 to 218 yards). At the location of the enemy camp near the end of the RAID lane, there was a sound system that presented pre-recorded sounds characteristic of: a) insurgent camp noise (e.g., group conversation in Arabic, weapons preparation, vehicle movement, improvised explosive device [IED] munitions practice, small arms target practice), and b) battleground noise (e.g., from weapons fire, ground vehicles, and artillery, that was presented at 100 dBA once the camp was engaged during the Squad's raid on the camp). The 100 dBA level was limited to be a safe exposure level as approved by the Virginia Tech Institutional Research Board.
The RAID lane sound system consisted of four Bose 802 Series II loudspeakers, each mounted at a height of seven feet on tripod stands, and amplified using two Bose Series VI Professional Amplifiers. Power was provided by a Honda EU-2000i generator, which was stationed behind an enclosed equipment trailer that housed the sound generation equipment. The sounds of insurgent camp and battle noises were taken from Network Sound Effects (Network Music, Inc.) compact discs, and included mixes of the following sound files. Insurgent camp sounds included firing of a shotgun and AK-47 rifles, cannon firing, single small explosions, 81 mm mortar firing, large dynamite explosions, foreign open air bazaar vendors, pickup trucks passing by, diesel trucks pulling in and idling, jeep driving away, shifting gears, and other noise characteristic of a busy camp. Battle noise (presented after engagement) included "military battle 20 th century noises," firing a shotgun rapidly, ricochets and gunfire from an M-14 rifle, firing an M-16 assault rifle, firing an AK-47 rifle repeatedly, firing a mounted machine gun, firing an M-60 portable machine gun, firing semi-automatic handguns, 81 mm mortar firing, and shouting in Arabic. All files were mixed using the Adobe Audition professional digital audio software package and recorded onto CDs. A Larson-Davis 824 Series sound level meter was used to measure and calibrate all of the sounds that were applied on the lanes and at the camp. Documentation of the equipment setup may be found in the final report to the Army, reference. 
As with the RECON mission, the layout of the RAID lane, as well as the insurgent camp noise and battle sounds, were configured by the research team with the assistance of the ROTC Commanders and Training Officers. In particular, Army Officers listened to all insurgent camp sounds and reviewed the RAID lane configuration to ensure that they were representative of actual combat conditions and would enhance the training experience for the Cadet Soldiers in training. These Officers accompanied the research team to the training lanes on three occasions prior to the actual experiments, to assist with the layout, configurations, and loudspeaker volume settings for the various sound sources.
In carrying out the small-unit tactics on the RAID lane, substantial intra-Squad communications were required. For example, the Squad Leader was briefed orally by a Commanding Officer as to the mission objectives and available timeline. Furthermore, the Squad Leader had to communicate with at least three other Squad members to give orders and to receive information throughout the Squad's progression up the lane toward the enemy camp. The Leader then returned to the Squad, and verbally briefed the Squad. In carrying out the RAID mission, the Squad members interacted with each other, and also with the Commanding Officer. Most communications were done directly either via whisper, quiet voice, or hand signals. All of these activities were observed and scored by the Training Officers and the experimenters who followed the Squad on the lane, and they performed data collection on prescribed forms.
The most important hearing requirement on the RAID lane entailed early auditory detection and identification of the insurgent camp as the Squad approached the camp at the lane's end. This point of detection, as achieved by each Squad, was observed and scored by the experimenters using footage markers that had been unobtrusively located on the lane. The RAID lane was laid out on a specific terrain such that the Squad had to proceed up a long hill of about 300 meters (325 yards) length and progressively increasing slope toward the insurgent camp, and the camp was located just over the crest of this hill such that the Squad could not achieve visual contact with the camp before actually cresting the hill, at a distance of about 45 meters (150 feet) from the camp. (At this point, the Squad would have been visually detected by the enemy, so the mission would have been deemed a failure. The insurgent camp sounds were set at a level at which the normal, trained ear (i.e., the first author's ear) could just detect the sounds at a distance of about 220 feet, this distance being about 70 feet from visual access.) Therefore, auditory detection (in advance of visual contact) of the camp was critical to mission success, so that the Squad could evaluate the situation and plan its attack, from a position well down the hill, prior to being spotted by insurgents at the camp, and this required early auditory detection and identification of the enemy camp. Auditory detection and identification of the camp's presence was based on the Squad members' ability, while wearing their assigned HEPS, to hear and identify the specific pre-recorded sounds emanating from the camp, as produced through the aforementioned system. Upon engagement by the Squad, the insurgent camp noise transitioned into the battleground noise, as described previously. The RAID mission entailed approximately 1 to 1.5 hours, depending upon the Squad's performance.
Mission exercise lanes: Terrain and weather conditions
The terrain of both the RAID and RECON lanes was rough, and consisted of rather steep hills and valleys. Being late Fall, the ground was covered with leaves, but some leaves were still on the trees, partially obscuring vision. Foliage was relatively thick throughout, consisting mostly of large and small deciduous trees. There was a minimal amount of dense underbrush, but where this did exist, it was used for hiding of the sound systems for the threat-related noises. Walking was difficult in most places, largely due to the uneven terrain and slippery rocks, and the full rucksacks, M-16 and M-249 weapons carried by the Soldiers exacerbated this. Weather conditions were very similar on both days of data collection, as follows: temperature from 44○ F to 62○ F, precipitation (rain) of 0.13-inch to 0.76-inch, and wind speeds up to 12 mph with an average of 5 to 6 mph. There were believed to be no significant effects of weather differences across the two days of field experimentation, in which both RECON and RAID lanes were used on each day. All exercises were conducted from early morning to dusk, but in daylight hours in all cases.
Hearing enhancement protection systems
The three hearing enhancement protection systems (HEPS) included in the field experiment were prior selected by the USAARL for head-to-head comparison and were all specifically designed for Soldiers' use. Each Squad was trained on only one HEPS and used that device on both mission exercises. There was no mixing of HEPS devices within a Squad.
The three HEPS were: a) the Combat Arms Earplug (CAE), b) the Communication and Enhancement Protection System (CEPS), and c) the Peltor Comtac II headset (Peltor). Cadet Soldier assignment to Squads was randomized, subject only to scheduling constraints. For the CAE, there were 13 males and 2 females; for the CEPS, there were 14 males, and 1 female; and for the Peltor, there were 13 males and 0 females.
Combat Arms Earplug
The Combat Arms Earplug (CAE) is a two-ended, earplug-type hearing protector that was designed to mitigate the noise hazard produced from continuous noise exposures (with its green end), as well as from impulsive noises (such as gunfire and/or explosions) while providing minimal reduction of desired signals and speech (with its yellow end). The CAE is depicted in [Figure 1]. It is a preformed, triple-flanged earplug, and its conventional passive end, colored olive drab green, is intended to be worn during hazardous continuous exposures, such as in a troop transport or near heavy machinery. The other end, colored bright yellow, has a tube through the plug and is intended to be worn when hearing protection is needed against impulsive noise, but when audibility of external sounds is also needed during quiet periods (the green end then being exposed for camouflage). This sound pathway tube is internally occluded by a thin wafer with an orifice at its center. Sounds that are up to moderately high levels, such as speech communications and signals, can pass through this orifice in relatively unimpeded fashion since their waves exhibit laminar flow in the tube. Thus, in relative quiet, signals and speech can be heard through the yellow end of the earplug, albeit with a small amount of attenuation, while a sharp increase in attenuation occurs at gunshot levels. [Table 1] provides the reported attenuation and Noise Reduction Rating (NRR) values, per ANSI S3.19-1974  as reported by AEARO Company  for the yellow and green ends of the CAE. However, at the point where a sound reaches 110 to 120 dB in level, such as during a gunshot or explosion, the flow in the yellow end's duct becomes turbulent and the orifice acoustically behaves as if it "closes," resulting in a sharp increase in attenuation, and thus increased protection during gunfire. , This behavior is the "nonlinear" aspect of the CAE; that is, its attenuation changes with incident sound level.
This nonlinear property is intended to provide reasonable hearing ability when needed, such as on a scouting mission, but also instantaneously increased attenuation in the presence of gunfire and explosions, such as those produced by improvised explosive devices (IEDs). In this research experiment, the Soldiers used the nonlinear yellow end of the CAE during the field experiments, as it is intended to be used on actual combat and reconnaissance missions where hearing is needed. More information about the CAE may be found in references , and at URL: http://www.e-a-r.com/e%2Da%2Dr.com/premod_detail.cfm?prod_family=Combat%20Arms andind_prod_num=370-1000001.
Communication Enhancement and Protection System
The Communication Enhancement and Protection System (CEPS), manufactured by Communications and Ear Protection, Inc. is designed to control the sound level that arrives at the ear and provide the user with radio communications ability and hearing of external (ambient) sounds via a sound-transmission system, all in a small insert-type earphone that is connected to two separate gain control and power supply/radio control modules [Figure 2]. Ambient sounds are attenuated by a compressible Comply™ foam eartip, which attaches to the tube of a "hearing enhancement module (HEM)" housing that comprises the insert earphone. [Table 2] provides the passive spectral attenuation values for the CEPS (in dB, and as determined when unamplified by the sound transmission system, which greatly affects the at-ear noise levels).
A microphone mounted on the exterior end of each HEM of the CEPS captures ambient sound for processing (through embedded circuitry), and true binaural hearing is provided since the microphone pickup and ambient signal processing are separate for each ear. The Comply™ eartip is attached to the interior end of the ear insert module via threads on a tube that exits the HEM. The processed signal is input to the ear through a miniature loudspeaker inside the HEM from which the signal travels. The user is able to simultaneously control the intensity (gain) of the signal that reaches both ears via momentary contact switches located on the separate gain control module that is itself attached to the user's gear, as well as to turn the system off. A boom microphone is also provided to allow the Soldier to communicate via radio, and this microphone clamps to the helmet. After positioning the three basic elements of the CEPS, the Soldier is able to "cinch" the various connector cables such that they fit snugly against the clothing and helmet (e.g., to reduce the possibility of cables getting caught on branches, bushes), but are free enough to allow movement without stressing them (and possibly pulling the HEM out of the ear canal).
The CEPS has a wide dynamic range of gain that is set via use of two miniature volume pushbuttons on the gain control module (one for increasing and one for decreasing). The gain of the CEPS was tested in the Auditory Systems Laboratory using an acoustical test fixture (ATF) in a reverberant, diffuse sound field, with pink noise spectra presented at 70 dB(linear) and 85 dB(linear). The dynamic range of gain provided by the CEPS was found to be approximately 36 dB of total gain from the lowest to the highest gain settings. With about 63 discrete gain settings, each comprised of a single press of either one of the two gain pushbuttons, the gain step size is indeed very small, ranging from a measured step size of about 0.3 dB to 3.0 dB depending upon the starting point in the dynamic range. It is questionable whether such a small step size (at least those step sizes below about 2 dB) is necessary or prudent, because the Soldier must press the buttons numerous times to get a significant change in output level at the ear - and this is very time-consuming when the need to hear better is often a very critical, quick one. Also, it is an issue when the Soldier needs to decrease the at-ear levels quickly in the presence of continuous loud noises. The CEPS also provides an automatic decrease in its amplification of ambient sounds when their level is high, such as during gunfire. However, the amount of reduction of this signal amplification, its trigger level for initiation, and its attenuation profile over time and across frequency, were not specified in the product literature; however, more information on this device may be found at the following URL: http://chppm-www.apgea.army.mil/documents/FACT/51-005-0504.pdf.
Peltor Comtac II
The Peltor Comtac II is a circumaural (earmuff-style) headset design that incorporates amplitude-sensitive sound-transmission technology, and which has low-profile earcups designed to be worn under an Army helmet [Figure 3]. All power supply and other circuitry reside within the headset itself; that is, no attached modules are required. The passive qualities of the headset provide attenuation against continuous noises as well as impulsive noises, and the passive spectral attenuation values and NRR in dB (when the amplification system is off), are listed in [Table 3]. Attenuation values are as per the Peltor website: ">http://www.aearo.com/pdf/hearing/atten_peltor.pdf.
The Peltor device is amplitude-sensitive in that the sound-transmission amplification technology boosts the levels of external low intensity sounds (such as surrounding voices or threats), within a midrange frequency passband, in an attempt to make them audible to the user. The Peltor device provides two microphones (one on each earcup) that face the front of the user, providing true binaural hearing via two separate microphone/earphone circuits, and having a common gain control that is set via an up/down pushbutton switch located on the bottom on one of the earcups. In similar fashion as discussed above for the CEPS device, the dynamic range of gain provided by the Peltor was measured by the research team and found to be narrow, constituting approximately 15 dB of total gain from the lowest to the highest gain settings. With only four discrete gain settings, each comprised of a single press of either side of the gain control rocker switch, the gain step size is adequate and is relatively linear across the range, averaging about 4.0 dB per switch depression. So, the Soldier can achieve the desired gain setting quickly, if not precisely. The electronics, located inside the earcups, are designed to instantaneously shut off the sound-amplification during high-level noise, thus providing the amplitude-sensitive quality. The Peltor can be interfaced with various communications options, including boom microphones that interface to the earcups, waterproof radio push-to-talk (PTT) adaptors, and connectors for vehicle and aircraft communications. More information on sound-transmission earmuffs may be found in references, , while more specific information on the Peltor Comtac II may be found at http://www.peltor.com/peltor.com/comm_detail.cfm?prod_family=Comtac%20IIandind_prod_num=MT15H69FB-09001 .
Dependent measures and experimenter observations
In addition to four Army Training Officers, five research team members observed and recorded data on the Soldiers while they traversed the RECON and RAID lanes, performing their missions. The Soldiers were told in advance that experimenters and Officers, both serving as "observers," would accompany them as they progressed down the lanes, and that they may also see other stationary experimenters (who were surreptitiously taking data), but to ignore them. The research team members communicated with each other on the lanes using walkie-talkies. Observers carried special data collection forms that were specific to their observation objectives; these forms appear in Casali and Lancaster.  Observers identified individual Soldiers and catalogued their responses using the helmet-mounted numbers, noting, for example, when Squad members communicated with each other, either through hand signals, by mouth (i.e., voice and/or lip movements), or some combination of both, if and how Squad members detected and/or responded to a threat, and whenever any Squad member removed, re-fit, adjusted, or lifted/pulled their HEPS.
On the RECON lane, the three individual "threat" stations were observed by a single experimenter standing near each, and these were separated by sufficient distance such that one threat could not be heard with the open ear in the vicinity of another. The primary dependent measure associated with these threats was whether the threat was auditorially detected by an approaching Soldier before he/she was within about 30 feet of the threat, and if so, was the Soldier's response a correct or incorrect one (based on established VT-ROTC training protocols). As described earlier, in rough terms, the threats were just barely audible to the open, unassisted ear at a distance of about 45 to 50 feet. The video cameras captured images of the immediate vicinity surrounding the threat station, and as stated, were positioned facing the direction from which the Squads came toward the reconnoitered camp. Two experimenters and the Training Officers accompanied the Soldiers throughout the RECON lane (as on the RAID lane as well), recording communications, listening, and other behaviors which related to mission performance.
The primary objective measure for the RAID lane was the linear distance (in feet) at which Squad members, particularly the Soldier at the "point position," could first detect and identify the "normal" (i.e., non-battle-related) sounds of the insurgent camp. (Recall that the camp was purposely located over the crest of a hill, so that if visual detection, which could only be established at the 150-feet mark, occurred prior to auditory detection/identification, which was set at about 220 feet for the normal, unoccluded ear, it meant that the Squad would be visible to the insurgents at the camp.)
To prepare the RAID lane, the research team measured along a path that was perpendicular from the face of the loudspeaker closest to the RAID lane and adjusted the battle-related noise to be 100 dBA at 30 feet from the closest loudspeaker. The 30-foot distance represented the front edge of the insurgent camp. From that point, 450 feet of distance was measured in a straight line back into the region of the lane over which the Squad was to progress toward the camp, and small plastic pipe markers were placed in 50-foot increments along that distance. These distance markers were used to determine the initial or "first" detection distance to the enemy camp for a particular HEPS-equipped Squad. Whereas the battle noise was set as described above, the insurgent camp activity noise (which was critical for the detection task), was adjusted such that it would be just audible (using the open ear, without any hearing enhancement devices) at a distance of approximately 220 feet. "First" detection was evidenced by a Squad member, usually the "point man" putting his/her fist up and freezing (signaling to the other Squad members that something was detected), getting into a lower position, readying a weapon, giving hand signals, and/or continuing the approach with caution. After it was visually apparent to the experimenters that auditory detection of the insurgent camp had indeed occurred, the experimenter verified this by asking the Soldier why he/she had reacted. Once insurgent camp detection was accomplished and the Squad had reverted into attack planning, the accompanying experimenters notified (by radio) the member of the research team who controlled the presentation of sounds at the insurgent camp to be ready to change from insurgent camp sounds to battle noise. Thereafter, when the Squad attacked the insurgent camp, the battle noise was started and continued until the Squad attained control of the camp, whose insurgent actors offered stiff resistance.
At the completion of each lane's training exercise, Squad members were allowed to rest, and while doing so were given two questionnaires, each of which included rating scales that tapped impressions of situation awareness, comfort, communications, mission objective success, etc., as well as open-ended questions and comments. Additionally, ROTC Training Officers were given a rating scale and open-ended comment form with which to "score" a particular Squad's mission success/ failure based on ROTC indices (e.g., moving tactically satisfactorily, communicating tactically unsatisfactorily, etc.). The scale used in the Officers' rating form was "Unsatisfactory, Satisfactory, Excellent," which relied on the rating categories currently used by the Army ROTC. Two questions were asked about "Overall Mission Success," while six others tapped into specific dimensions or tasks that were necessary during the mission. All questionnaires appear in Casali and Lancaster. 
Results and Discussion
Overall data analysis
Where and as appropriate, statistical tests including analysis of variance (ANOVA) were computed on sets of dependent measures using the JMP™ statistical analysis program, from http://www.jmp.com/software/. Nonparametric Fisher's Exact Tests were conducted on binary data (e.g., "yes/no" answers). The probability ( p ) of a Type I (i.e., alpha) error of 0.05 was chosen as the cutoff level for statistical significance on all tests. On any significant F -ratios revealed by the ANOVA analyses, post-hoc, unplanned comparisons were made using Tukey's test to isolate significant main effects and interactions. Tukey's test was chosen because it is conservative with respect to Type I error, and it tests all pairwise comparisons. Complete statistical tables for all tests performed, along with extensive statistical descriptors for the data, appear in Casali and Lancaster. 
The following discussion of the field experiment's results are separated into those from the RECON and RAID lanes. Under each training mission heading, the discussion begins with coverage of the ROTC Training Officers' expert assessment of mission success, and various dimensions of mission performance, since the focus of this research was on operational performance effectiveness. Following that is a discussion of the data collected by the research team. In each case (Officers or experimenters), the discussion is broken down into the three HEPS under study.
It should be noted that the Training Officers were encouraged by the experimenters to use the particular HEPS that was assigned on a lane. The Officers did end up using the two electronic HEPS (CEPS and Peltor Comtac II), but not the CAE (by their choice), and thus their assessments below include their comments about their personal experiences with these two HEPS.
RECON lane exercise results
At the completion RECON lane exercise, the three Training Officers were given an Army rating form upon which to assess a particular HEPS-equipped Squad's performance. The performance was rated based on several indices of performance or activities that are of import to the U.S. Army, and the rating given by each of the three Officers is reported in [Table 4]. All ratings were based on U.S. Army assessments of mission performance, and included space for comments. The Officers' ratings for each HEPS-equipped Squad are discussed in turn.
Officer ratings on RECON using CAE [Table 4]
All three Officers rated the CAE (yellow nonlinear end inserted) as "satisfactory" with respect to overall mission success on the RECON training lane. When asked about any issues related to the Squad's overall mission success, raters indicated that once the CAE was in the ear, the entire Squad began to talk louder than normal, and that although the mission was completed satisfactorily, the Squad operated overall "louder than normal," which is disadvantageous to maintaining stealth. Two of the three Officers rated tactical movement performance as "satisfactory," with the third not observing this activity. In comments specific to this activity, the Officers indicated that the Squad had to use more hand and arm signals that, while affording desirable tactical advantage, resulted in more and perhaps louder bodily movements. They also noted difficulties in hearing from a distance. Two of the three Officers scored communicating tactically as "unsatisfactory" (with the third as "satisfactory"). Comments indicated that Squad members had to speak "louder than what was tactically sound," and had to do so when talking in close proximity. With respect to the conduct of reconnaissance , two of the three Officers scored the Squad as "satisfactory," while the third scored performance as "unsatisfactory." Comments specific to this activity indicated that the Squad members were not able to hear the enemy and, as a result, they had to position themselves closer than normal -- too close, in fact. Also, there was difficulty amongst Squad members in hearing each other moving and speaking. Perhaps most sobering was an Officer's comment that the Squad "would have been heard had the enemy been real, and not simulated." Finally, after reconnaissance of the objective, the Squad consolidated and reorganized . All three Officers rated this activity as "satisfactory." Comments indicated that the Squad continued to talk louder than normal, that "too many branches were crunching," and that the Squad had difficulty in hearing from a distance. To the former comment, and based on the Squad members' own comments, the CAE seemed to reduce the Squad members' ability to hear their own footfalls and, as a result, they may have walked "more loudly."
Experimenter ratings on RECON using CAE
There were not many Soldier-to-Solider communications on the lane. Two different Squad members were noted to communicate, each by hand, and one by hand and by mouth. With respect to all three of the threat stations, not one Squad member detected any of the threats while wearing the CAE, and this result was noted to be the case across all HEPS. After discussion with the ROTC Officers at the completion of the study, it was determined that the threat stations were perhaps not ideally located on the RECON lane, as they were at positions that either flanked the insurgent camp or were too close to it, or were at positions that would probably not be (and were not) traversed by any Squad member (even though they were in positions deemed as "defensive" for the camp). As earlier mentioned, the placement and sound levels of these threat stations was based on the input of two senior Army Training Officers. But in hindsight, the location of the threats probably could have been improved from an experimental standpoint, and this constituted a flaw in the field experiment.
Near the "Arabic speaking" threat, two Soldiers were noted to remove the CAE (one twice, and another one time); also, these same two Soldiers were observed to adjust the CAE (one once, and another three times). When queried after the lane's completion, these two Squad members indicated that the adjustments were made because "it was being rubbed by my Kevlar helmet," and "I pushed it in too far and had to adjust it." The experimenters collected other post-RECON lane comments from the Squad members who wore the CAE. These included communication difficulties: "it was hard to whisper, and I relied more on hand signals." Others stated that they could not hear twigs and branches crack, thereby affecting their ability to gauge their own noise when moving, and that they "could not hear as well generally." Many Squad members commented that they felt "compromised," and that they could hear their heartbeat and breathing more than normal, which suggests an occlusion effect. Other Squad members indicated that they experienced localization issues: "I could not tell direction" and "it was hard to localize."
Officer ratings on RECON using CEPS [Table 4]
As for the CAE, all three Officers rated the CEPS-equipped Squad's overall mission success on the reconnaissance lane as "satisfactory." When asked about any issues related to the Squad's overall mission success , Officers' comments indicated that the "Squad Leader had trouble hearing things when he briefed and received," and that "the Squad Leader lost connection with his Team Leader." All Officers rated mission planning as "satisfactory." Comments revealed that mission planning was "easily done, but people spoke louder because of the ear buds," and that "the Squad Leader had a tough time hearing;" however, the Squad was "able to maintain communication that was tactical." In comments specific to tactical movement , again all three Officers rated performance as "satisfactory." In comments specific to this activity, raters stated that the CEPS provided "good amplification of [the] sound [signal] without being drowned out by ambient sound." Another comment was that the Squad "was able to move tactically using the [CEPS]," but that the "Squad Leader was pretty loud during movement." One Training Officer who used both of the two electronic HEPS commented that the CEPS was "much better than [the] Peltor device," but that "a different way to attach the big box to [the] uniform would make [the] device better." With respect to communicating tactically , two of the three Officers rated performance as "satisfactory," with the third member not observing. With respect to the conduct of reconnaissance, one each of the Officers rated the Squad's performance as "satisfactory," "excellent," and "not observed." In a comment specific to this activity, one Officer stated that the CEPS was "the only device [with which] I heard simulated sounds." With respect to Squad consolidation and reorganization after reconnoitering the insurgent camp, only one Officer observed the activity, and rated performance as "satisfactory." In a specific comment, this Officer stated, "the Squad consolidated after actions on the objective, and did use effective verbal passwords."
Experimenter ratings on RECON using CEPS
The experimenters observed four Soldiers actually communicating with the CEPS: three by mouth (i.e., voice and/or lip movements), and one by mouth and hand. One of these three Squad members also was observed to communicate only by hand as well. As for the CAE and somewhat surprisingly, no Soldier wearing the CEPS was noted to detect any of the threats presented at the three threat stations on the RECON lane. At the post-lane meeting point, Squad members made several comments regarding their impressions of the CEPS. Comments included: "more, longer wire [is] needed for personal/optimum/protected placement." Many lamented that the "switches were too small, and/or were not labeled." Those Soldiers of shorter stature had "problems with the system," probably due to longer cable slack across their torsos. Other comments were that "the volume switch high at [the] chest was good," but that "Velcro attachments of the boxes would be better than clips," because "infantry runs into things all of the time." One Soldier stated that he "[inadvertently] bumped up the volume as [he] entered the insurgent camp area," but another "inadvertently lowered the volume." Two Soldiers indicated that they had to refit the CEPS, but in so doing they would "simply try to shove [the CEPS] back in with gloves." The Squad Point man stated that he "had difficulty hearing," and that "anything touching the helmet was amplified." Several Squad members thought that the CEPS "would be a good idea," but that the "wires were long, and would catch on things." One Squad member pointed out that his "voice was distorted," and another indicated, "My [own] voice was extremely amplified."
In addition to the Squad members' comments, one Training Officer who used the CEPS stated, "I can't think right now because I can hear everything," which may or may not be a benefit. Another stated that the CEPS was "awesome, easy to whisper," but that his "own voice sounded loud." Officers reiterated what the Squad members had commented on regarding noise amplification of items touching the helmet, and of "branches against the helmet produced loud sounds," and "rubbing wires against [the] helmet" did as well. "The wires were cumbersome [and the] box was unwieldy," said one, and "the switches need labeling, such as the volume switch," said another. The Officers also had issues with the system with respect to its design shortcomings. "I hear too many details, and the left-right orientation was okay, but the front-back [sounds] were lost." Finally, another Officer indicated "the fact that we hear too many details makes us more careful of our own steps."
Officer ratings on RECON using Peltor [Table 4]
Two of the three Training Officers scored the Peltor Comtac II-equipped Squad's overall mission success performance on the RECON lane as "satisfactory," while the third scored it as "unsatisfactory." Comments specific to overall mission success indicated that the Peltor Comtac II "amplified all noises with the exception of the human voice," and that "when [I] decreased the volume, my heartbeat was amplified." Other comments were that "the Squad was much louder than usual for RECON," that the system "amplified all forest noises." With respect to mission planning , and similar to the overall mission success rating, two of the three Officers rated Squad performance as "satisfactory," with the third rating it as "unsatisfactory." Comments again indicated that the system "amplified forest noises over human voice most of the time," and that, as a result, "it was hard for the [Squad members] to hear each other." Another Officer stated "planning was adequate, but people spoke louder due to earmuffs." The planning process "took a little longer because outside noises disrupted the Squad Leader." With respect to tactical movement , again two of three Officers rated performance as "satisfactory," with the third rating it as "unsatisfactory." Comments indicated that the Squad "[was] able to move tactically," but that the "Squad was rather loud when conducting movement." Again, a general comment was that the system "magnifies ambient sound too much while moving." With respect to communicating tactically, two of three Officers rated performance as "unsatisfactory," while the third rated it as "satisfactory." Specific comments indicated, "It was hard to hear each other," and that "Squad members were yelling to hear each other." With respect to conduct of reconnaissance, two Officers rated performance as "satisfactory," with the third indicating that performance was not observed. Comments specific to this activity were that "the reconnaissance was loud," and that (again), "it was difficult to hear because of ambient sound." With respect to actions on the objective, two Officers rated performance as "satisfactory," with the third indicating that performance was not observed. The sole comment specific to this activity was that "the actions on the objective were loud." With respect to the consolidate and reorganize activity following actions on the objective, two Officers rated performance as "satisfactory," with the third indicating that performance was not observed. One Officer stated that the activity "went rather well because voices were projected."
Experimenter ratings on RECON using Peltor
Four Squad members were noted to communicate, two by voice, and two by hand (with one of these also communicating by whistle), while wearing the Peltor HEPS. During the mission planning activity, the Peltor push-to-talk (PTT) system was heavily used to communicate with the base camp's Tactical Operations Center (TOC), and Squad members reported that communications did work while at rest but, during movement on the lane, some connectivity was lost. During the mission planning stage, the Squad Leader was observed by the experimenters to be talking very quietly and he was heard by all Squad members through the Peltor's ambient sound pass-through system, with no occlusion effect reported. Thus, tactical communication and mission plans discussions were accomplished without any Squad member needing to remove the Peltor Comtac II to do so.
With respect to threat detection, and similar to the other HEPS, no Soldiers were scored as detecting any of them with the Peltor device; however, the experimenter located at the "weapons preparation" threat indicated that four Soldier silhouettes were observed nearby, but that none of them seemed to react to the threat. Post-RECON lane comments from the Squad members indicated that they "heard clinking and words," suggesting that at least some Soldiers detected both the "weapons preparation" and "Arabic speaking" threats; however, at the time the Squad was near the threats, it was not possible for the experimenters to confirm this. Other Squad comments indicated that they "could hear their own footsteps, but could not tell how much noise [they] were making [themselves]." Several soldiers noted that the HEPS was "very directional" in that it was "good for frontal sounds," but as a result they were "worried about [enemy] approach from behind." They "heard well in the front hemisphere only," and that there was "good hearing of colleagues' voices in mid-whisper." However, "twigs splitting caused the headset to cut off," causing some Soldiers to "turn down the volume when moving, but (turn it) up when still." "Distance was tough to judge," for some of the Soldiers, and the system was "not good for patrol, but [was] good for sitting at a listening post." With respect to comfort issues, there were many. Squad members experienced "discomfort with clasping with helmet," the "earmuff interferes with [the] stock," and the HEPS was "not comfortable with glasses."
RAID lane exercise results
Detection distance from camp
As discussed previously, the primary objective measure for the RAID lane was the distance at which Squad members could detect the sounds emanating from the insurgent camp located near the end of the RAID lane, and this distance was observed by the experimenters who accompanied the Squads. Analysis of distance detection data revealed a difference in detection distances across the HEPS; ANOVA: F , = 175.0, p visual detection was available at 150 feet, and not before that point, and that open-ear detection was available at approximately 220 feet for the normal ear.) Therefore, those Squad members in the CAE condition achieved visual detection prior to or concurrent with CAE-occluded auditory detection, and the Squad members' comments corroborated this finding. Post-hoc analysis of distance detection data using Tukey's test revealed significant differences between all three HEPS, with the CAE resulting in the closest (worst) detection distance, followed by the Peltor Comtac II headset, and then the CEPS resulting in the furthest (best) detection distance.
Both Squad and Training Officers reported during the post-lane debriefings that when using the CAE's yellow earplug, the camp was seen before it was heard, which has clear implications for Army operations. Every Squad member voiced this contention with the CAE. Conversely, for both the Peltor Comtac II and the CEPS, all Squad members so equipped indicated that they heard the camp before seeing it. However, the question remains as to whether they would have heard it before being detected themselves.
Officer Ratings on RAID using CAE [Table 5]
Two Officers rated the Squad's overall mission success performance with the CAE as "satisfactory," with the third rating it as "unsatisfactory." Comments specific to overall mission success indicated that "communication between the Squad Leader and Team Leader in the joint operational environment was awful," but it was unclear as to whether or not this applied to an effect of the CAE or to the Squad members' abilities in particular. With respect to mission planning, the Officers were mixed in their ratings, with each providing a different rating (i.e., "unsatisfactory," "satisfactory," and "excellent"). With respect to tactical movement, all three Officers rated performance as "satisfactory." Comments specific to this activity indicated that the Squad "relied too much on verbal communications, and spoke too loud continuously." Another stated that "Squad members failed to hear certain noises; e.g., gunfire, explosions in the distance," all of which were part of the insurgent camp sounds from the research team's sound system, and which were (obviously), heard by the unoccluded Officers before the CAE-occluded Soldiers. With respect to communicating tactically, two Officers rated performance as "satisfactory," while the third rated performance as "unsatisfactory." Comments specific to this activity were that the "Squad members spoke too loud continuously," and that "at one point, [a Squad member] spoke loud when it [would have been] appropriate to whisper." With respect to the conduct of reconnaissance, two Officers indicated that they did not observe the activity, while the third rated performance as "unsatisfactory." This Officer said that the Squad "did not respond to [the] audio signature until ~ 50 meters away. [The insurgent camp] is first identified usually at 100 meters." With respect to actions on the objective, two Officers rated performance as "satisfactory," with the third rating it as "unsatisfactory." Comments indicated that the Squad's performance was "slow and disjointed, but communication breakdown came from poor guidance and vague instructions from the Squad Leader," suggesting that at least part of the performance rating was due to a particular Squad member's deficiencies and was not necessarily due to the CAE. This is corroborated by another comment, "[the] Squad failed to react to enemy fire [that] may have been due to failure of leadership." The third Officer stated, "communication [was] not so good, [as the Squad had] trouble understanding each other." With respect to consolidate and reorganize, again the three Officers provided different performance ratings (i.e., "unsatisfactory," "satisfactory," and "excellent"). Comments specific to this activity indicated that there was "slow, poor guidance and instructions from the Squad Leader," and that the "Squad Leader failed to control the Squad." Another comment that may or may not be due to either the Squad Leader or the CAE was that "Leaders were using "inside" voice; unsure if results were because of hearing or failure on Leader's part to communicate effectively-most likely the latter."
Experimenter ratings on RAID using CAE
The observers noted that 13 Squad members communicated with the CAE, with 16 instances of communications by mouth (voice and/or lip movements), 11 instances of communications by hand, and 5 instances of a combination by hand and mouth. Only one individual was noted to have lifted or pulled the CAE out of the ear during the RAID lane march. During the march, the accompanying Training Officer advised on four occasions that the Squad members were "speaking too loud," and to "use a tactical whisper." Once the insurgent camp was seen (recall that no Squad member using the CAE actually detected the camp auditorially before seeing the camp), the Squad member at Point responded by freezing, getting down, crawling, and using hand signals. Once the Squad Leader gave the order to engage the insurgent actors at the camp, several Squad members began to communicate by voice exclusively, and commands were largely shouted to overcome the noise and the CAE. Several difficulties in communication were noted between Squad members, including one instance in which two Squad members yelled to each other without understanding after having tried whispering twice, and using normal conversational levels once. No Squad member was observed to have removed his/her CAE during the battle with the insurgents at the camp, in which all weapons fired blanks, producing high sound levels. One Team Leader was observed to have yelled a command to a Squad member that was approximately 15 feet away, but the receiver did not understand the command. After the Squad had taken control of the insurgent camp, one Squad member attempted to yell to another a warning about an IED, but the receiver did not respond. Comments from Squad members post-RAID were many. One Squad member indicated that his "[CAE] earplug suction came loose while running." Others expressed frustration that they could not communicate while wearing the CAE. "I had a difficult time hearing whispers," and "if I whispered, they didn't hear me, but when I whistled, they did." However, during the traverse portion of the RAID mission prior to camp detection, at least some Squad members were noted not to have any apparent difficulty whispering and understanding each other. Other comments were that the "CAE did not distinguish itself from a foam earplug," and similarly, that there was "no difference between [the CAE] and any other foam earplug."
Officer ratings on RAID using CEPS [Table 5]
In the CEPS condition, two Officers rated overall mission success performance as "satisfactory," while the third rated it as "excellent." Comments included "successful reconnaissance of the objective assisted the Squad in successful mission completion." Another Officer noted that the "CEPS caused [Squad] members to speak very loudly and they made significant noise while moving." With respect to mission planning, all three Officers rated performance as "satisfactory." Comments specific to this activity indicated that "communication between [the] chain of command went well," and that there were "no issues with CEPS," and that the "Squad could have spoken well below the typical level, but were not able to regulate [the] volume of [their] voices." With respect to tactical movement, two Officers rated performance as "satisfactory," while the third rated it as "unsatisfactory." Comments were that "movement was loud, people didn't consider the noise [that] they were making." In regard to communicating tactically, two Officers rated performance as "unsatisfactory," while the third rated it as "satisfactory." Comments were that the Squad members "seemed to be yelling at each other," and that "verbal communications and Squad members spoke very loudly, and rarely used hand/arm signals or radios." Another positive comment was that "whispers could be heard from [a] long distance." As to conduct of reconnaissance, two Officers rated performance as "excellent," with the third indicating that this activity had not been observed. Related comments were that "Squad members could tell [the] relative distance to [the] objective, and could hear some of what was going on," and that the Squad "gained very important, pertinent information, and effectively conducted reconnaissance on the enemy. [The Squad] heard the enemy beyond 200 meters, but needed to be focused on listening for [the] enemy."
As for actions on the objective, two Officers rated performance as "satisfactory," with the third rating it as "excellent." Comments included "communication was good," but also "some Squad members could not hear commands, possibly due to volume regulation," and that "the Squad responded rapidly and effectively, but was somewhat disjointed. Squad members were initially focused but [this was] due to poor instructions from the Squad Leader. There were no issues with hearing. After specific instructions were given, [the Squad] responded very rapidly." On consolidate and reorganize, two Officers rated performance as "satisfactory," with the third not observing it. Comments indicated that it was "fast and effective," and "everybody heard the command to get off the objective," although "some Cadets could not hear some of the commands."
Training Officers who also wore the CEPS provided their impressions of the device on the RAID lane, as follows: "Amazing sound gain, and excellent gunfire noise reduction. Too much in the area of "footfall" noises-twigs and sticks cracking and peripheral noises were serious distracters. I became hyper-aware of the noise I made during movement [to the point that] I was distracted from the objective's noises initially." Another Officer's comment was that "the plug and cord design does not support active movement in any environment," and that "it is easy to dislodge the plug or snag the cords. I would be more apt to remove the device after I dislodged the plugs than I would be to keep replacing them."
Experimenter Ratings on RAID using CEPS
Fifteen Squad members were observed by the research team to have communicated during the RAID lane march with the CEPS device, some multiple times and/or using different communication modes. There were 27 instances of communications by mouth (voice and/or lip movements), 11 instances of communications by hand, and 10 instances of communications using both; all of this evidenced that the CEPS seemed to promote and facilitate inter-Soldier communication. One Squad member was noted to have removed the CEPS, while another was observed to have lifted/pulled the CEPS slightly from its position. There were six recorded instances of CEPS adjustment, with one of these being to both sides of the device, and another occurring after the earpieces had fallen out during a low crawl, and just before the terrain rise immediately before the insurgent camp. (However, in post-RAID comments, 10 of 15 Squad members indicated that they had refit their earpieces while on the lane). Training Officers who wore the CEPS were also noted to often adjust it or lift/pull on it. When communicating by mouth, most CEPS-wearing Soldiers appeared to be successful, although at least one was noted to have talked very loudly, and another to have whispered loudly. It was also notable that shouts to a particular Squad member from behind seemed difficult for that Squad member to perceive. Once the insurgent camp was auditorially detected, quiet conversation ensued, but no Squad members were noted to immediately move to a lower position, or to ready their weapon. As mentioned previously, all CEPS-occluded Squad members heard the camp well before they saw it. Comments from the Squad members post-RAID were many. Some indicated that it felt as if the Comply™ foam earplug tips were falling out of their ears, that the tips would not hold shape, and/or that there was considerable difficulty in reinserting them.
Interestingly, some Squad members indicated that the CEPS, while a benefit on the RECON lane, was a hindrance on the RAID lane. In low crawling movements, the CEPS was very noisy from the cords rubbing on the ground. Metal-on-metal contact was quite loud, and the Squad members had difficulty with the switches while wearing their cotton gloves. Squad members indicated that they would like to hear an audible "click" for volume changes, but not for power actuation. One individual had inadvertently lowered the volume when in a low crawl, and at least two Squad members stated that their own voice was either too loud, or was distorted electronically. One Squad member suggested that the system would be easier to fit and use if it were part of an earpiece that fit around the pinnae. CEPS fit difficulty was exacerbated by the fact that many Soldiers could not roll-down the foam eartips and refit them while the helmet was in place, and that they just "shoved it in the ear" as a result. Other suggestions included an "immediate muting" function, and there was a need to either shorten the wires or to provide some capability with which to ensure that excess wire did not "stick out too far from gear."
Officer ratings on RAID using Peltor [Table 5]
All three Officers rated overall mission success performance as "satisfactory." Comments included "the Attack was successful," but that "some basic tasks were not completed." Another commented that the "Squad failed to find [the] objective in a timely manner." With respect to mission planning, two Officers rated performance as "satisfactory," with the third rating it as "excellent." Officers stated that the Squad "spoke in hushed tones," and that "hearing was not a problem, so effective communications occurred." As for tactical movement, all three Officers rated performance as "satisfactory." Comments included that the "Squad was very loud in their footsteps," and that there was "poor situational awareness on [the] part of the Squad Leader." Another was that the Squad "failed to hear some noises, like car sounds," and that the car sounds "should have drawn attention, [but the Squad] did not notice [the] objective sounds-even when close." It is assumed that this Officer was referring to the insurgent camp and vehicle noises produced from the loudspeakers of wheeled vehicles (e.g., jeeps, trucks). As for communicating tactically, two Officers rated performance with the Peltor as "satisfactory," with the third rating it as "excellent." The Officers related that the Squad was "able to speak to each other tactically-in a low voice," that they "talked more softly to each other," and that the Squad "mostly used hand signals, but no one had to speak in a loud voice to get a message across." As for conduct of reconnaissance, two Officers indicated that this activity had not been observed, with the third rating performance as "satisfactory." In relation to reconnaissance, an Officer stated that the "Squad Leader heard [the] enemy on [the] objective at 125 meters, [and] saw [the] enemy on [the] objective at 75 meters." With respect to actions on the objective, two Officers rated performance as "satisfactory," with the third rating it as "excellent." The Officers stated that there were "no issues hearing at all," and that the "sound of fire was less harsh, making it easier to think." However, they also observed that the Squad "failed to look for IEDs even though [there were] sounds of explosions." With respect to consolidate and reorganize, two Officers rated performance as "satisfactory," with the third rating it as "excellent." Officers again stated that there were "no issues hearing at all," and that "I did not hear yelling, but actions took a long time to complete." Another stated that the Squad "did consolidate effectively, but leadership was [the] only setback-not hearing."
Training Officers who wore the Peltor HEPS also provided their comments on the device's performance on the RAID lane. One stated that the Peltor provided "effective amplification of certain sounds," but that "noise made by wind through trees, [from] my clothing and equipment rubbing, my footsteps, etc., was amplified to the point that it distracted me and blocked out other noises [that] I needed to hear; that is, the enemy on the objective, vehicles on the objective, gunfire on the objective." Also that "I removed the device in order to hear the above-mentioned at a range of about 150 meters. Good reduction of gunfire (personal weapons) noise." Further, the device was "very effective at amplifying internal, verbal communications," and that "I would use this for range training." However, the same Officer stated, "I would not wear [the Peltor HEPS] on a combat patrol, especially in a city." A second Officer provided impressions of the Peltor: "ambient noises drowned out important noises, such as far-off speech, car sounds, etc. The ambient noise was overwhelming."
Experimenter tatings on RAID using Peltor
Eleven Squad members were noted to communicate while wearing the Peltor device during the RAID lane march, some multiple times, and using different communication techniques. There were 20 observed instances of communications by mouth (voice and/or lip movements), 17 instances of communication by hand alone, and 3 instances of communications by both hand and mouth. During the mission planning stage before the approach toward the camp commenced, the Squad members were noted to be able to speak at a much lower volume, and the Officers were not observed to warn Squad members to "quiet down." During the approach, an Officer noted to a member of the research team that the Squad was walking more quietly because the Peltor picked up and amplified the noise from foot movement. Squad members were noted to communicate by mouth at distances of greater than 25 feet and be understood with no apparent difficulty. However, Squad members were noted to talk too loudly when close to the insurgent camp, possibly due to the excitement of the imminent engagement. Upon detection of the camp, all Squad members were noted to freeze, get into lower positions, ready weapons, and give hand signals. Once the camp was in sight, weapons went up, and all Squad members were in the prone position. In post-RAID comments, Squad members indicated that they were able to hear the Squad Leader during the mission planning stage. Research team observers, when listening to the mission plans at the same distance from the Squad Leader as the other Soldiers, but without the Peltor device in place (i.e., open ear), could not hear the conversation at all.
When traversing the RAID lane, however, wind noise was an issue for many Squad members with the Peltor HEPS, and several stated that it was difficult to hear spoken commands and other signals due to the sounds of twigs and leaves being amplified. Other Soldiers suggested that the Peltor produced annoying static when turned on. Eight Squad members indicated that they heard the insurgent camp before they saw it, but five indicated the opposite. During the march, Squad members were noted to generally communicate very quietly with each other. However, during the firefight, several Soldiers stated that they could not communicate, and that this was a disability as commands had to be yelled because the amplification system had been automatically "deactivated" due to weapons noise. The Squad reported that distance estimations were difficult, and that some members could not hear each other due to the front/back directional microphones on the Peltor device.
HEPS adjustments, refits, and lifting
As mentioned earlier, two members of the research team accompanied each HEPS-equipped Squad on its march through the RAID lane. These observers noted whenever a particular Squad member physically lifted, adjusted (not including gain adjustments), or removed his/her HEPS, but it should be noted that this was not an exhaustive observation process, as Soldiers were spread out on the lane and there were at least 14 of them for any given HEPS. Nonetheless, the sampling by the observers showed that the CEPS-equipped Squad was noted to adjust, refit, or lift its HEPS more times  than either of the other two HEPS-equipped Squads on the RAID lane march. These data were supported by numerous comments by Squad members and Training Officers (as detailed above and below) regarding CEPS eartips falling out of the ear, and/or being pulled out by snagging of the cabling on tree foliage. For the CAE, only one Squad member was observed to adjust, refit, or lift the HEPS, although comments did indicate that more occurrences actually did take place. This result suggests that the CAE did not present particular difficulties for the Squad so equipped with respect to hearing protection or fit. (It should also be noted that the CAE was redesigned in early 2008 to provide additional sizing, as well as a lower profile in the ear.) The Peltor Comtac II-equipped Squad was not observed to adjust, refit, or lift the HEPS on the RAID lane. This result was influenced by the fact that the Squad so equipped could not reposition the Peltor muffs under the Kevlar™ helmet without removing it first.
Post-mission rating scales
As implied by its name, in an operational performance experiment such as the one described herein, the measures of mission performance and success are the most important ones, and these were provided from the Training Officers' observations and from the objective measures, both discussed above. However, in addition to those operational performance measures, two comprehensive rating scales were completed by each HEPS-equipped Squad after they had accomplished both the RECON and RAID missions. The questionnaires included a 22-question form of the research team's development, denoted as the "ASL (Auditory Systems Lab) questionnaire" and a 34-question form from the USAARL, denoted as the "Hill questionnaire." Due to space limitations, only a subset of questions from the two questionnaires will be discussed herein; whereas, the complete data sets from both questionnaires appear in Casali and Lancaster. 
[Table 6] presents responses on each HEPS as provided by Soldiers on the four yes/no questions on the ASL questionnaire. There were no significant differences across HEPS for any of these binary responses according to Fisher's Exact Test. However, Question 4 for the CEPS-equipped Squad (i.e., would you recommend that the Army invest in the HEPS?) approached significance, ( p = 0.09, Fisher's Exact Test), wherein 12 Soldiers responded "yes" and 3 responded "no" for the CEPS, as compared to seven yes and seven no responses for the CAE and the Peltor HEPS. Question 1 asked whether or not Squad members received adequate training in the use of a particular HEPS; all Squad members indicated that they had received adequate training for their particular assigned HEPS. When asked whether or not Squad members had used prior Army-issued hearing protection at training sites or firing ranges ( Question 2 ), a majority also indicated that they had used it. Responses on Question 3 regarding the use of Army-issued hearing protection in combat training indicated that a majority of the Soldiers did not use it. When asked to explain why they did not use hearing protection in combat training, most Cadets stated variations of the same response: "blanks are not loud," or that hearing protection was "not issued." However, the fact is that while blanks may not be as loud as live ammunition, they indeed are hazardous to hearing and warrant hearing protection (Ohlin, D., 2008, personal communication). Others suggested that hearing protection made it "hard to hear," that it was "unnecessary," or that it was "not emphasized."
Likert-type Rating Responses
In the ASL rating scales, squad members were asked the degree to which they agreed with a particular statement about their HEPS. An ANOVA was conducted on the rating scale responses, as the response categories represented interval data, followed by post-hoc Tukey's Tests analyses for comparisons among HEPS. A subset of the rating scale results are reported next.
On the rating scale of HEPS improved situation awareness , significant differences between all three HEPS were found [Figure 5]. The CAE-equipped Squad members disagreed with the statement that their HEPS improved situation awareness. The Peltor Comtac II-equipped Squad was neutral as to situation awareness, while the CEPS-equipped Squad agreed that it improved situation awareness.
On the rating scale of HEPS enhanced my ability to hear friendly sounds and speech, CAE-equipped Squad members disagreed with the statement that their HEPS enhanced their ability to hear friendly signals and speech, while the other HEPS-equipped Squads agreed that their HEPS did enhance this hearing ability, and equally so [Figure 6].
When asked whether they agreed with the statement that a particular HEPS enhanced the ability to hear enemy signals and speech , CAE-equipped Squad members disagreed with the statement, while the Squad members who were equipped with the Peltor Comtac II felt neutral, and the CEPS-equipped Soldiers reported that their hearing on this task had been enhanced [Figure 7].
When questioned about their agreement with the statement that a particular HEPS was comfortable, the Soldiers reported significantly different impressions for all three HEPS [Figure 8]. Peltor Comtac II-equipped Squad members disagreed with the statement that their HEPS was comfortable, while the Squad members who were equipped with the CAE were neutral in their feelings about comfort. The CEPS-equipped Squad, however, agreed with the statement that their HEPS was comfortable.
On the rating scale regarding whether a particular HEPS was more effective than the current Army-issued hearing protection, there was no significant difference between the three HEPS [Figure 9]. Squad members agreed with the contention that all of the HEPS were more effective than the current Army-issued hearing protectors, which in the case of these Soldiers, were conventional foam earplugs.
On the important question about whether a particular HEPS will improve battlefield survivability, there were significantly different answers among the three HEPS [Figure 10]. The CEPS-equipped squad members were positive about their battlefield survivability, while the Squads issued with the Peltor and CAE slightly disagreed that their survivability had improved.
When asked the degree to which they agreed with the statement that a particular HEPS will improve command, control, and battlefield communications, the CAE-equipped Squad disagreed that their HEPS improved command, control, and battlefield communications, while the Peltor- and CEPS-equipped Squads agreed slightly that they had improved [Figure 11].
When Squad members were asked if the device is easy to use, there were significant differences between the CEPS and the other two HEPS, which did not differ from each other. Squad members reported that both the CAE and the Peltor Comtac II were almost totally adequate with respect to ease of use, while the CEPS was rated lower, but still adequate. When Squad comments are considered, this result may be due to the small switches (which Squad members indicated they had trouble manipulating with gloves on) and/or the lack of labels on the switches. Also, the CEPS's ease of use is somewhat hampered by its design, which uses separate controls and components that are not part of one unit, but positioned at various locations on the body and connected by various cables and cords [Figure 12].
When Squad members were asked if they could crawl as needed, a significant effect of HEPS was found [Figure 13]. CAE-equipped Squad members thought that their HEPS was very adequate in this regard as did the Peltor-equipped Squad. However, the CEPS-equipped Squad members thought that their ability to crawl was somewhat poorer with the CEPS, perhaps attesting to the presence of cabling and modules that were clipped to the Soldier's chest area.
When asked if their HEPS stayed attached, Soldiers reported differences between the CEPS and the other two HEPS, which did not differ [Figure 14]. CEPS-equipped Squad members thought that their device was only somewhat adequate with respect to staying attached, while Squads equipped with the other two HEPS thought that their devices were very adequate in staying attached.
When Squad members were asked if their HEPS interfered with stealth, there were significant differences among the HEPS [Figure 15]. The analyses revealed significant differences between the CEPS and the CAE, with the CAE interfering more with stealth than the CEPS, while the Peltor Comtac II did not differ from either of the other two HEPS.
When Squad members were asked about their ability to localize sounds while wearing their HEPS, a significant main effect of HEPS was not found, and the mean values ranged from 2.6 to 3.6 [Figure 16]. Squad members reported that their HEPS were somewhat inadequate to slightly adequate when asked about their ability to localize sounds, and this impression is disconcerting, given the importance of Soldiers' needs to locate the direction of sounds in their environments.
Human factors design issues with the HEPS
During the course of the above-described research, the senior author gained considerable experience with each of the HEPS in writing the instructions for the Cadet Soldiers, in fitting and adjusting the HEPS, and in using the HEPS. Based on these experiences, and through additional in-laboratory tests conducted to ascertain the electronic gain characteristics of the Peltor Comtac II and CEPS devices, a human factors evaluation of the three HEPS was performed. The results of this evaluation are beyond the scope of this paper, but they may be found in Casali and Lancaster.  The bottom-line conclusion of this human factors evaluation was that numerous issues, some of serious proportions, warrant attention in future developments of these devices, or even before continued deployment of the devices in their current state . As noted in the abstract, certain features of the devices have indeed been redesigned since the late 2006 timeframe of this experiment.
While no research study is ever perfect, and in-field research is particularly challenging due to the presence of extraneous factors and the difficulty of replication of conditions, the research described herein was indeed successful toward its objective; that is, to compare an Army-selected group of three HEPS as to their influence on the operational performance effectiveness of Soldiers functioning in infantry operations that heavily involve auditory tasks. The field exercises used were realistic training exercises that involved raid (RAID) and reconnaissance (RECON) missions, and a comprehensive battery of measures was obtained, including objective metrics of squad performance such as detection distances, subjective ratings from Army Training Officers, subjective ratings from Soldiers, and direct observations by experimenters who accompanied the troops on the training lanes.
It is probably now obvious that the optimal bottom line in this research would be to state with confidence which HEPS was "best" at facilitating successful "operational performance effectiveness." To this end, it must be said that none of the three devices were optimal for either mission, and each HEPS had its own, and in some cases unique, strengths and weaknesses. But if an assumption is made [and it is indeed an arguable assumption] that the single dependent measure that is most reflective of "operational performance effectiveness" is the rating of mission success by the ROTC Training Officers, then the CEPS device was associated with the highest ratings across the seven rating dimensions for the RECON mission, producing mostly "satisfactory" ratings and no "unsatisfactory" ratings, while the CAE and Peltor Comtac II devices each had several "unsatisfactory" ratings [Table 4]. But such an operational performance advantage for the CEPS did not carry over into the RAID mission, wherein the CEPS and Peltor Comtac II were very close in ratings given by the Officers [Table 5], with a slight advantage to the Peltor.
On the other hand, if the simple (but intuitively important) insurgent camp detection distance metric for the RAID exercise is used for ranking the three systems, then the CEPS device, with its superior gain characteristics, offers nearly a 1.7-to-1 advantage in detection distance over the Peltor Comtac II, and a 2.6-to-1 advantage over the CAE, the only unamplified device in the experiment. Detection distance constitutes a measure that directly correlates with survival and mission success, but it is certainly not the only measure. Other issues associated with comfort, compatibility with other gear, suppression of gunfire impulses, switch functioning, and effects on auditory perception of the Soldiers' self-produced sounds (such as footfalls), were not the CEPS device's strong suits, and the CEPS was also the device which was most frequently refitted or adjusted by the Soldiers.
It is also of concern that no matter which HEPS was used, the Squad members failed to detect any of the three threats (footfalls, Arabic speaking, and sniper weapons preparation) that were staged on the RECON lane. These threats were clearly audible at about 45 to 50 feet to the normal ear; however, they were not detected under any of the three HEPS. Realizing that these threats were audible to the experimenters under the two electronic HEPS (i.e., CEPS and Peltor Comtac), there are two plausible explanations as to why the Squad members did not detect them. First is that the relatively novice Soldiers were not cognizant of these particular sounds, even though they were familiarized with them in the classroom briefing sessions prior to the field exercises. Second, the Squads did not typically select a path to the reconnoitered camp that led by the threats, although the Training Officers did establish the location of the threats.
Of course, in view that it is a passive device with no gain circuitry to enhance hearing, the CAE is at clear disadvantage in scenarios wherein auditory detection and communications are required. Even though it does have a pass-through sound duct in its yellow (i.e., nonlinear) end, the CAE still provides 10 to 15 dB of attenuation of those sounds that fall in the critical speech/signal bandwidth of 1000 to 4000 Hz. Also of particular concern is the fact that the Training Officers noted that the CAE resulted in squad members speaking "louder than what was tactically sound," which could result in their being more readily detected by the enemy.
After much observation, experience with, and testing of the three HEPS in this study, the authors conclude that none of the devices were optimally suited for deployment for the two types of missions involved in this research, and that each device warranted more attention to design refinements and performance testing as detailed in Casali and Lancaster.  Although each of the HEPS may have been suitable in their tested state for certain mission scenarios or environments, none of them was fully developed as a well-human-engineered example of personal protective equipment. Indeed, it is a high calling to function both as a hearing protector and as a hearing enhancement system, but as the two electronic-amplification products (CEPS and Peltor Comtac II) clearly evidence, such duality of function is achievable, if not fully optimized in these two embodiments.
There is a clear dilemma in drawing conclusions about "overall superior performance" of a single HEPS, or to rank the HEPS on a composite of the measures obtained as to superior performance. In part, this dilemma is due to the fact that the three HEPS differed widely in their performance across the different dependent measures and observations (i.e., the results on different metrics would favor different devices), so it is difficult to pick one device that is "best" for both reconnaissance and raid objectives. Furthermore, while one device (i.e., the CEPS) may appear to be superior because of its performance on the important detection distance metric, it has other, perhaps overarching ergonomics issues that argue against its use in combat situations without further development and refinement. Based on all of the data obtained, it is quite clear that each of the three HEPS was in need of more design refinement and subsequent test/evaluation, particularly in the realm of human factors considerations, and indeed since the late 2006 juncture of this research, certain changes have been made in all three HEPS that were tested. Thus, a recommendation for further research is to conduct an evaluation of the "design-improved" versions of the three HEPS evaluated herein, along with other recently developed devices such as the NACRE Quiet Pro, the Silynx C40OPS, and the Silynx QuietOPS.sup>
This research was conducted for the United States Army Aeromedical Research Laboratory (USAARL) at Fort Rucker, Alabama, under Contract Number DAAD 19-02-D-001/Delivery Order 0946, Subcontract Agreement TCN 06214, Battelle, Inc, through the Scientific Services Program.
A special thanks is given to the Virginia Tech United States Army Reserve Officer Training Corps (VT-ROTC), commanded by COL Christopher St. Jean, for their participation and assistance.
Institutional Review Board (IRB) approval for Human Subjects experimentation was obtained from the Virginia Tech (VT) IRB, the U.S. Army Medical Research and Materiel Command-Office of Research Protection, and Battelle, Inc., who authorized the VT IRB to serve as IRB authority for Battelle.
The views, opinions, and/or findings are those of the authors and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other documentation.
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