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|Year : 2005 | Volume
| Issue : 26 | Page : 11--20
Protection efficiency of hearing protectors against military noise from handheld weapons and vehicles
R Paakkonen1, K Lehtomaki2,
1 Finnish Institute of Occupational Health, Tampere, Finland
2 Finnish Defence Forces, Helsinki, Finland
Tampere Regional Institute of Occupational Health, P.O.Box 486, FIN-33101 TAMPERE
Noise attenuation against military noises has been measured in several cases under practical field conditions. Commercial and military versions of earmuff noise attenuation were measured against rifle noise. All the tested earmuffs attenuated the C-weighted peak level to less than 135 dB, which is less than the proposed recommendation value. Combat and shooting exercises create a risk of hearing damage, reaching a peak level of 180 dB. Measurements were done during attack exercises with blank and normal cartridges and during a defence exercise with normal cartridges. The noise exposure levels were relatively moderate (outside the ear 9597 dB, in ear canal 82-85 dB) for military exercises. Peak levels of 110-120 dB for military trainers were measured in the ear canal during the conscript use of small-bore weapons. Combat vehicles and tanks are noisy, and for noise control during their use headgear with communication properties is worn. Noise inside such headgear was found to reach up to 120 dB, and the noise doses varied between 90 and 105 dB. Noise was also measured for aviation pilots in Finnish jet fighters. The cockpit values averaged 96 dB - 100 dB over the flight, whereas noise in the ear canal averaged 88 dB - 95 dB. The analyses indicated that radio noise is 4-10 dB higher inside the helmet than the background noise is, when measured as equivalent noise. The technicians on the ground were exposed to noise levels varying from 93 to 97 dB over the day.
In practice, hearing protectors attenuate noise by 10-30 dB, depending on the frequency content of the noise sources. However, the difference when measured outside and inside hearing protectors varies by 5-10 dB because communication increases the noise level at the entrance of ear the canal. Currently the best protection for soldiers seems to be active noise cancellation ear muffs that are equipped for communication purposes and worn during the entire military exercise.
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Paakkonen R, Lehtomaki K. Protection efficiency of hearing protectors against military noise from handheld weapons and vehicles.Noise Health 2005;7:11-20
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Paakkonen R, Lehtomaki K. Protection efficiency of hearing protectors against military noise from handheld weapons and vehicles. Noise Health [serial online] 2005 [cited 2021 Mar 8 ];7:11-20
Available from: https://www.noiseandhealth.org/text.asp?2005/7/26/11/31644
The noise of military weapons can reach peak levels of up to 180 dB (NATO 1987). Impulse noise from assault rifles and other small firearms used in military training have peak sound pressure levels (SPL) of 155-168 dB when measured as the C-weighted peak level (Paakkonen et al. 2000a). In the Finnish Defence Forces regulations on use of hearing protectors have been established for each type of weapon. Combat and shooting exercises create the most significant risk of hearing damage.
Troops are transported through hostile environments to their destination by combat vehicles. Steel crawlers of these vehicles generate vibration that proceeds to interior spaces and creates noise when the vehicle is driven. Noise exposure exceeds the limit values, but few reliable exposure analyses have been reported for headgear attenuation and the influence of communication (Carter 1992; Fitzpatric 1988; Ivey et al. 1987; Ribak et al. 1985; Toivonen et al 2003).
According to international studies, noise that exceeds a personal daily exposure level of 85 dB or a sound peak level of 140 dB is considered harmful to hearing. These values are included in a European Council directive (2003/10/EC), in recommendations of the American Conference of Governmental Industrial Hygienists (ACGIH 2003), in military standards in the United States (e.g. MIL-STD-1474D), and in many national documents, for example, the Finnish VNp 1404/93 (EEC 1986). These norms evaluate noise exposure as measured at the site of the head of the exposed subject. For industrial impulse noise, the often-applied recommendation is for peak levels below 140 dB and exposure to fewer than 100 impulses a day (ACGIH 2003; EEC 1986; NATO 1987). Dancer and Franke (1995) have estimated that, except for peak level, impulse noise exposure should be analysed according to the 8-hour exposure level principles.
Despite strict orders to always use ear protectors while shooting, 1.5-2.5 % of Finnish conscripts develop acoustic trauma (AT) during their military service (Savolainen and Lehtomaki 1996). Most of the accidents occur (over 80%) after or before actual combat training, while protectors are not in use. Most ATs occur in surprise situations, when earplugs have been applied but lost, or when protectors were not used because conscripts were handling their weapons after the exercise and normal hearing was necessary (Savolainen and Lehtomaki 1996). It has also been suspected that earplugs are not suitable for all conscripts and that some conscripts can not insert earplugs properly (Berger 1988; Berger 1986; Casali et al. 1996; Pekkarinen 1989; Paakkonen et al. 2003; Toivonen et al. 2002).
If military shooters are to be protected against noise levels above 140 dB in the use of handheld weapons, as in industrial noise, then peak level attenuation of more than 16 dB is needed (Paakkonen et al. 2000a). The noise attenuation of hearing protectors under field conditions is suspected to be significantly worse than in laboratory tests (Berger 1988). In addition, ear plugs may not always fit properly in the ear canal of military trainers (Berger 1988). Military training officers should maintain their hearing capability during exercises and at the same time protect their hearing against high-level shooting impulses. We have developed a method for measuring exposure to impulse noise in field conditions (Paakkonen et al. 2000a; Paakkonen et al. 2003; Paakkonen 1998; Paakkonen 1991; Toivonen et al. 2002). The SPL from exposure to rifle impulses is usually attenuated only by hearing protectors, especially among conscripts and military trainers. We have also studied other possibilities, but hearing protection by earmuffs or earplugs is the most important for practical training conditions in shooting range action. However, classical passive earmuffs or earplugs deteriorate communication between military trainers and conscripts and can therefore increase the risk of accidents.
When the noise exposure in vehicles is considered, thus only a few studies on the noise attenuation of noise helmets or aviation helmets have been published (Ivey et al. 1987). In practice, protectors have been found to leak or have defects that deteriorate sound reduction properties. For aircraft pilot noise exposure also involves radio communication, which increases the noise exposure. The speech intelligibility of the radio communication is a significant safety parameter, and, in order to improve it cockpit noise should be minimized, and the noise attenuation of the helmets and headsets maximized. These actions could allow some reduction of the volume of radio communication.
The purpose of this study was to gather information on the efficiency and noise attenuation properties of different types of hearing protectors under military field conditions.
Materials and methods
The measurements were made during different experiments on noise from Finnish assault rifles, other rifles and military weapons, and vehicle noise was analysed from combat vehicles and aircraft. The measurements were carried out both outside (exposing noise) and inside hearing protectors, helmets or the like at the entrance of the ear canal or directly from the ear canal.
Attenuation properties of moulded ear plugs against rifle noise
A method was developed for measuring noise levels directly in the ear canal between the ear plug and the ear drum (Paakkonen et al. 2000a). For this purpose, miniature microphones (Sennheiser KE4-211-9) were connected through thin insulated wires (diameter less than 0.1 mm) to an amplifier and a tape-recorder (Sony TCD D7/8), and the tape-recorded samples were analysed by sound-level analysis systems (B&K 2260, B&K 2131, Advantest R9211B). At the same time, the exposing noise was measured simultaneously at a distance of 100 mm from the ear plug by a noise analyser B&K 2260 using 1/4-inch condenser microphones (B&K 4136).
Microphones have different properties in measurement of high intensity noise peaks. Therefore a comparison measurement was done by two signal pistols (6.3 mm and 9 mm) equipped with and without suppressors at the distance of 2.6 m in a laboratory room. [Figure 1] shows the results for microphones B&K 4136 (sound level meter B&K 2209), B&K 4165 (-20 dB attenuator and sound level analyser B&K 2260), Sennheiser KE 4-211-9 (self constructed amplifier) and sound dose meter (CEL 460). The results show that CEL 460 starts to clip peak levels at about 142 dB, but the other sensors measure noise levels up to 150 dB. Microphone B&K 4136 can measure sound pressure peaks up to 170 dB without distortion. Also the frequency responses on measurement systems were stated to be as informed by the manufacturers.
Ear plugs were moulded for 17 professional soldiers. The moulded ear plugs had a filter that had certain properties allowing sound to pass to the ear canal. These moulded communication earplugs (Elacin Compact®) were tested in a shooting exercise. Nine officers were equipped with this measurement system, and each of them fired 30 shots during 2 minutes at the same time at a free-field shooting range.
Attenuation properties of electronic hearing protectors against rifle noise
The following hearing protectors (ear muffs) were tested for their attenuation properties: Peltor H61, Peltor H7, Peltor H6, Bilsom Marksman and Ear Ultra 9000.
The tests were carried out on a free-field (more than 50 m radius) shooting range on a hot, calm sunny day. The measurement sites were in the opening of the left ear canal of the shooter and at a distance of 50 mm from the tested left ear cup of the shooter. The assault rifle was the same at all times, and the cartridges were from the same series of production. At the measurement site at least five similar shots were fired for each protector/participant combination. At least five different military volunteers were used. Therefore, the noise results are average (A) values of 25 shots and their standard deviations (SD) both outside and inside the protector. The study was carried out by occupational military volunteers, and oral consent was obtained prior to the tests. When the measurements were started, the ear microphone (Sennheiser KE4211-9) was inserted into the ear canal opening and held there by tape. The thin microphone wire (diameter about 1 mm) ran between the ear cushion and the skin. This caused a small (typically less than 2 dB) measurement error that was, nevertheless, the same for all the measurements (Paakkonen 1991).
Attenuation properties of earplugs in combat exercises
A group of male military trainers (age 20-45 years) participated in the earplug test during combat exercises. The ear canals and ear drums of each man were carefully inspected, and the diameter of the ear canals was measured with an E.A.R/Aearo EARGAGE™ earplug sizing device. For each man, Elacin Compact Soft moulded ear plugs were manufactured, and the men also selected an earplug, either Bilsom 303S or Bilsom 303L (Bilsom Sweden), that he felt best fit his ear. The Ethics Committee of the Finnish Defence Forces approved the research plan, the study was voluntary, and the subjects signed an informed consent form.
Measurements were carried out during two attack exercises with blank cartridges, two exercises with normal cartridges, and one defence exercise with normal cartridges. The combat noise was caused by assault rifles, bazookas, machine rifles or demonstration explosives. The training sessions took about 0.51 hours. On the shooting range the noise exposure was measured simultaneously at a distance of 10 mm from the ear plug by a noise analyser (B&K 2260) using ¼-inch condenser microphones (B&K 4136). Measurement equipment was available for three trainers at a time. It consisted of a logging noise dosimeter (CEL-460) and a two-channel MIRE (microphone in real ear) measurement and recording system. In the MIRE system one microphone ( Noise exposure of combat vehicle personnel and attenuation by hearing protector devices
Noise measurements were carried out in combat vehicles (BMP-2, CV9030) and tanks (T-72 and Leopard 2A). The headgear was usually Russian HTSh 3 or 4 equipment or Norwegian active noise reduction protectors. Noise was measured in the areas of the driver, the commander, the gunner and the squad (in the rear) during the driving of a combat vehicle. Noise was also measured outside and inside the headgear of the vehicle personnel. Inside the headgear it was measured at the entrance of the ear canal and was comprised of the noise entering the headgear and radio communication. Outside the headgear, noise was measured both by noise dosimeters (CEL 460) and by recording (Sony TCD-D8).
Aircraft, helmets and protectors
Noise measurements were carried out in Finnish fighters (Hornet F-18 (HN), Saab 35 Draken (DK), Mig 21 Bis (MG)) and jet trainers (BA Hawk MK-51 (HW)). The measurements took place during normal flight rounds at the pilots´ own air bases. The pilots used Gentex helmets in the F-18, FFV Flyghjalm helmets in the Draken, ZSH-5A helmets in the Mig and Alpha helmets in the Hawk. Some of the pilots also used earplugs (Bilsom Propp 1000) in addition to the helmet. All the helmets were personally adjusted for the pilots before their flights. The volume of the radio was set freely by the pilots. The technicians used Silenta Super earmuffs.
The noise exposures were evaluated by noise dosemeters (MIP 6074) outside and inside the flight helmet. The outside noise dose was measured above the seat belt on the shoulder, and the inside noise dose was measured near the ear canal in the right ear of the pilot by a miniature microphone (Knowles BL1785). Therefore, it caused some error in the measurement results, but the error was the same for all the tested ear muffs. The noise dosemeters were started after the pilot had received his flight task in the flight squadron, and the meter was read after the pilot returned to the same location after the flight. Therefore, the noise doses contain exposure before and after the jet engine was started or stopped. For similar plane types noise results were summarized as average values and their standard deviations (SD).
For the continuous time and frequency analyses, a two-channel tape-recording system was constructed in which two miniature microphones (Sennheiser KE4-211-1) were connected through a self-constructed amplifier to a digital tape-recorder (Sony TCD-D7-8). These two microphones were set similarly as in the noise dose measurements but were located on the left ear and left shoulder of the pilot. The microphone wires were led into an angle pocket inside the flight suit for safety reasons. The tape-recorded samples were analysed and averaged over the flight (from the starting to the stopping of the engine) in the frequency domain by a realtime analyser (B&K 2131) and instantaneously in the time domain as the A-weighted noise level (F-time constant) by an amplifier, A-weighting filter and recorder (B&K 2307). For the time analyses, the background noise levels (cockpit noise into flight helmet) and the noise levels caused by the radio were evaluated. The results were again averaged (SD) for similar plane types. With the use of the measured values, it was also possible to differentiate between the exposing noise in the ear canal, the cockpit noise and the significance of the radio noise.
Noise attenuation of moulded ear plugs against rifle shots
The noise attenuation of ear plugs during rifle shots averaged 18-20 dB, depending on the measured parameter (C-weighted peak level or 1 second A-weighted level). The average noise exposure for the shooting range exercise was 116 dB (LAeq, 5min). The peak levels ranged from 129 to 145 dB in the ear canal and from 152 to 154 dB outside the ear canal. However, for two persons out of nine, the ear plug did not attenuate the noise at all, and improper insertion of the ear plug or improper adjustment between the ear canal and the moulded ear plug was indicated. The comments of the subjects about communication during the use of the ear plugs were positive.
A summary of the ear muff measurements is shown in [Table 1]. According to the results, the peak levels measured inside the protectors were less than 140 dB (variation 130-134 dB). These values are lower than the limit recommended for hearing damage risk. The peak levels were attenuated by 22-35 dB. Whether the electronic communication circuit was open fully or not did not influence the attenuation values against shooting impulses. Therefore, the communication circuit cut the signal as it should have.
A summary of the results recorded from the trainers during the attack and defense training of the conscripts is presented in [Table 2]. The noise exposure levels were relatively moderate (outside 95-97 dB, in ear canal 82-85 dB). However, there were exceptions in which officers had problems with the noise attenuation of the ear plugs (for two persons with noise exposure of 101 and 107 dB). In addition, the use of cellular phones caused disturbances in the measurement equipment, but these could, however, be cut out of the samples before the analyses. The ear canal diameter of the trainers (n=16) averaged 9.3 (SD 1.1) mm.
The noise exposure over the whole measurement period averaged 105 dB for the driver, 93-96 dB for the commander, and 101-102 dB for the squad area. At the entrance of the ear canal, the noise level averaged 98 dB with the Russian HTSh-4 helmet and 86 dB for the driver with the Norwegian ANR helmet [Figure 2].
A summary of the noise exposure in the combat vehicles is shown in [Table 3].
A summary of the noise analyses in the aircraft is given in [Table 4]. At low frequencies (less than 100 Hz) the noise level was higher inside the helmet due to the occlusion effect of the ear. At higher frequencies (200-10000 Hz) the noise attenuation was 5-25 dB, depending on the fighter, helmet and the amount of communication over the flight round.
[Table 4] shows the averaged results of the noise dose analyses.
[Table 5] shows an overview of the results. In practice hearing protectors attenuate 10-30 dB, depending on the frequency content of the noise sources. However, the difference when measured outside and inside hearing protectors differs typically by 5-10 dB because communication increases the noise level at the entrance of the ear canal.
Moulded ear plugs
The noise attenuation of the communication ear plugs was slightly less than that of contemporary ear plugs, for which the noise attenuation was found to be 28-31 dB in earlier studies (Paakkonen et al. 2000a; Paakkonen et al. 2003; Paakkonen et al. 1998; Paakkonen et al. 2000b; Toivonen et al. 2002; Toivonen et al. 2003). However, the communication properties of moulded ear plugs were considered to be positive. The improvement in communication was due to the lower attenuation at higher frequencies (3000-6000 Hz) by 10-15 dB. The success of inserting ear plugs should always be carefully examined to avoid the risk of hearing loss. We have earlier noted that training in the insertion of regular ear plugs improved noise attenuation by 10 dB (Toivonen et al. 2002). Therefore, a quick method should be developed that can be used to ensure that noise is attenuated by ear plugs after being inserted.
The peak of a shot impulse is too short for the human ear to hear whether the protector attenuates sufficiently or not. However, high peak levels can enter the hearing organ and can cause significant damage to hair cells (Bruel 1975). The peak level attenuation seemed to be sufficient even when the standard deviation of the results are examined. The standard deviation was greater for the values inside the protector than for the values outside it.
On military shooting ranges communication is vitally important for prevention purposes and for situations involving accidents. Communication also enables effective training. Currently, the tested ear muffs are in use only in shooting range work. They are needed even more in combat exercises, in which nowadays only ear plugs are used. In practice, ear muffs are not easy to use in these situations, and ear plugs have been effective if properly applied. However, in Finland most cases of hearing loss (87 %) among
Finnish conscripts occur in conjunction with combat exercises but outside actual events in which weapons are handled. In this environment, communication ear protectors could be used instead of the ear plugs currently used. One common problem with earplugs is that they are easily lost (Toivonen et al. 2002; Toivonen et al. 2003).
Combat exercises in the field
In this study, the noise exposure of the trainers in the combat exercises was 95-97 dB, and under the earplug the mean exposure was less than 85 dB, which is considered the limit for noise exposure (Toivonen et al. 2003; EEC 1986). In addition, combat exercises seldom last intensively over 8 hours, instead they generally continue for 1-3 hours, and, therefore, the noise exposure is even less. According to this reasoning, military trainers should not be at risk in combat exercises. Military personnel also have mixed exposure to high-level impulses and high-level steady noise in transportation. In the latter case good low-frequency noise attenuation is also needed. For this purpose, active noise cancellation hearing protectors are becoming important.
Under field conditions, the studied hearing protectors did not attain the levels of the laboratory results. However, when active noise reduction protectors are used, the daily exposure limit (85 dB(A)) is not exceeded if the noise exposure corresponds to the typical exposure profile. In field conditions noise attenuation is always worse than in laboratory conditions, and communication increases noise exposure. Therefore, the measured noise exposures were 91-101 dB(A) in field tests although communication was reduced to the minimum. If the laboratory and field results are compared, the noise exposure varied from 73 to 86 dB in laboratory conditions and from 91 to 101 dB in field conditions with a difference of 5-28 dB between the calculations and practical field measurements. Even though there are inaccuracies in the results, the difference is significant.
The averaged values for the pilots varied in the cockpit from 96 dB to 100 dB and in the ear canal from 88 dB to 95 dB. The highest values were measured in the Mig 21 Bis fighters that is not used any more in Finland. The technicians received surprisingly similar noise exposures over a day, varying from 93 to 97 dB. The starting values were the highest, 108 dB (L Aeq 15min ), for the Hornet and Hawk planes, which use an auxiliary starting motor. If one estimates that a Silenta Super earmuff can reduce fighter noise by 30 dB (Paakkonen et al. 1996), the daily exposure of technicians varies from 63 to 67 dB, and, if the A-weighted instantaneous sound pressure levels vary from 110 to 120 dB, then the maximum noise levels entering the ear canal would range from 80 to 90 dB.
The analyses of the cockpit noise and the helmet noise indicate that the average radio noise level is 4-10 dB higher than the background noise inside the helmet when measured as equivalent noise. The instantaneous noise level difference was significantly higher, 10-20 dB.
The average noise exposure inside the flight helmet varied by 88-95 dB over the flight depending on the aircraft [Table 4]. If a pilot flies four flight rounds or sorties daily, then the daily A-weighted equivalent level (L Aeq 8h ) varies by 85-92 dB, depending on the type of plane (-3 dB reduction of the energy equivalent based on time). These levels are often interpreted as potential hearing risks. Another, perhaps more important parameter in this evaluation is flight safety and its deterioration. If the cockpit noise penetration into the helmet is considered, the equivalent noise levels varied by 80-94 dB.
Active noise cancellation has been a subject of intensive research, and currently several manufacturers apply the active noise cancellation principle (Bose Corporation in the USA, Sennheiser in Germany, Helmet Integrated Systems in UK, Racal Corporation in the UK, etc.). Active noise cancellation can improve speech intelligibility, but the upper frequency limit of action is restricted typically to 500-1000 Hz. The analyses of frequency spectra versus time indicated indirectly that, for radio communication, the most important frequency area is 300-3000 Hz. Therefore, the helmet should attenuate in this frequency range as much as possible. If the flight helmets are compared with a specially developed noise helmet (Paakkonen et al 1991), it is apparent that classical noise enclosure principles offer possibilities for development, especially for frequencies of over 500 Hz.
Different types of earplugs and earmuffs seem to offer effective protection against noise from handheld weapons. The problem with plugs is that, for some persons, they probably provide no protection at all. On the other hand, typical hearing protectors do not allow enough communication under military conditions. Currently, the best protection for soldiers seems to be provided by active ear muffs that are equipped for communication purposes and are worn during the entire military exercise.
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