Noise Health Home 

ARTICLE		 [Download PDF]
Year : 2011  |  Volume : 13  |  Issue : 55  |  Page : 378--384

Speech understanding in noise with integrated in-ear and muff-style hearing protection systems

Sharon M Abel, Ann Nakashima, Douglas Saunders 
 Individual Behaviour and Performance Section, Defence Research and Development Canada-Toronto, Toronto, Ontario, Canada

Correspondence Address:
Sharon M Abel
Individual Behaviour and Performance Section, Defence Research and Development Canada-Toronto, 1133 Sheppard Avenue West, Toronto,Ontario, M3K 2C9
Canada

Abstract

Integrated hearing protection systems are designed to enhance free field and radio communications during military operations while protecting against the damaging effects of high-level noise exposure. A study was conducted to compare the effect of increasing the radio volume on the intelligibility of speech over the radios of two candidate systems, in-ear and muff-style, in 85-dBA speech babble noise presented free field. Twenty normal-hearing, English-fluent subjects, half male and half female, were tested in same gender pairs. Alternating as talker and listener, their task was to discriminate consonant-vowel-consonant syllables that contrasted either the initial or final consonant. Percent correct consonant discrimination increased with increases in the radio volume. At the highest volume, subjects achieved 79% with the in-ear device but only 69% with the muff-style device, averaged across the gender of listener/talker pairs and consonant position. Although there was no main effect of gender, female listener/talkers showed a 10% advantage for the final consonant and male listener/talkers showed a 1% advantage for the initial consonant. These results indicate that normal hearing users can achieve reasonably high radio communication scores with integrated in-ear hearing protection in moderately high-level noise that provides both energetic and informational masking. The adequacy of the range of available radio volumes for users with hearing loss has yet to be determined.


How to cite this article:
Abel SM, Nakashima A, Saunders D. Speech understanding in noise with integrated in-ear and muff-style hearing protection systems.Noise Health 2011;13:378-384


How to cite this URL:
Abel SM, Nakashima A, Saunders D. Speech understanding in noise with integrated in-ear and muff-style hearing protection systems. Noise Health [serial online] 2011 [cited 2023 Mar 23 ];13:378-384
Available from: https://www.noiseandhealth.org/text.asp?2011/13/55/378/90289


Full Text

 Introduction



Military personnel are reluctant to wear conventional level-independent hearing protection devices during field operations. [1] Although these devices reduce exposure to high-level sounds that could potentially damage hearing, they may interfere with the detection and localization of auditory warnings and the ability to understand and relay orders, putting the lives of the soldiers and completion of their mission at risk. This is especially the case for individuals who have already sustained a hearing loss. [2],[3] The sound attenuation provided by the device adds to elevated hearing thresholds, increasing the severity of the impairment. Level-dependent hearing protection devices may offer a solution. [4] Passive nonelectronic types have a sharp-edged orifice or valve that allows low to moderate level noninjurious sounds to pass unimpeded. [5] The attenuation is no more than 20 dB. [6],[7] High-level impulsive sounds, such as weapon's fire, will create turbulence in the airflow surrounding the orifice, increasing the resistance and hence the sound attenuation. In contrast, active noise reduction (ANR) devices incorporate an electronic circuit which samples the ambient sound and adds it out of phase to the incoming waveform, thereby reducing the level of sound reaching the ear. ANR devices are designed for steady-state sounds below 1 kHz. The low-frequency attenuation that may be achieved varies widely across ANR devices but is typically no more than 20 dB. [8] Higher frequencies are attenuated conventionally by about 35 dB. [9]

The integrated hearing protection system, a relatively new development in hearing protection technology, offers a way to enhance free field and radio communications among soldiers in the field while at the same time protecting their hearing from the damaging and masking effects of high-level sounds. These may offer both ANR and protection against impulsive sounds. In a recently published study, Nakashima and Abel [10] investigated speech understanding in 75-dBA pink noise in listener/talker pairs fitted with either of two in-ear integrated hearing protection systems, the Nacre QUIETPRO (Nacre AS, Trondheim, Norway) and the Silynx QuietOps (Silynx Communications Inc., Rockville, Maryland). These were compared with a Canadian Forces (CF) standard issue headset which offered no hearing protection. All three were used in combination with a Personal Role Radio (PRR; Marconi Selenia Communications, Genoa, Italy). Tests of consonant discrimination and sentence intelligibility showed that performance was best with the standard headset and radio followed by the Nacre QUIETPRO and Silynx QuietOps. Percent correct ranged from 70 to 89% for consonant discrimination and 70 to 93% for intelligibility of words in sentences. User acceptance was highest for the QUIETPRO, followed closely by the standard headset.

The present study compared the effectiveness of increasing the radio volume on radio communications in speech babble noise with integrated in-ear and muff-style hearing protection systems. Speech babble provides both informational and energetic masking. Energetic masking depends on the spectral overlap and thus the speech-to-noise ratio (SNR) of the speech and masking sounds. In informational masking, the listener has to disentangle elements of the two that sound similar. [11] The candidate systems that were selected for inclusion in this study were the Nacre QUIETPRO (Quietpro) in-ear device and the Peltor PowerCom Plus (Powercom) headset (3M, St. Paul, MN). Choice of in-ear or muff-style hearing protection for use in military operations or industrial settings will depend on how well they can be fit with other head and face equipment, worn in combination. [12]

Experimental design

Ten males and ten females of working age (military and/or civilian) participated in the experiment. To control for the effect of fluency on interpersonal communication in English, all subjects were required to be proficient in speaking and understanding unaccented North American English. While accented speech may be intelligible in quiet, it is more difficult to understand in background noise. [13] Prior to inclusion in the study, subjects were also screened for a history of ear disease, including excess wax build up, hearing loss and tinnitus, claustrophobia and difficulty concentrating over a 2-hour period, factors which could influence outcome. Those who passed these screening criteria underwent a hearing test conducted by a trained technician to ensure that pure-tone air conduction thresholds were no greater than 20 dB HL (decibels, hearing level) at sound frequencies from 0.5 to 8 kHz. This represents no more than a slight hearing loss. [14]

Subjects were tested in same gender pairs in a sound-proof room. Some studies have shown that speech spoken by females is more intelligible than speech spoken by males, although to a relatively small degree. [15],[16] Bradlow et al. [15] reported that intelligibility was positively correlated with fine-grained acoustic-phonetic talker characteristics that favored female talkers. In contrast, Nixon et al. [17] found relatively small gender differences favoring males at some of the high levels of the aircraft background noises, in which they presented their speech materials. They acknowledged that this outcome may have been due to their military voice communications system which was designed for the male speech spectrum. Testing subjects in same gender pairs in the present study avoided a potential confound that gender differences might be due either to the gender of the speaker or the listener. [16]

Pair mates were fitted binaurally, in turn, with the Quietpro earplugs and the Powercom headset. The order of the two devices was counterbalanced across the 10 subject pairs. In each of these two ear conditions, measurements were made of the ability to understand radio communications in a background of speech babble noise. The level of the speech babble was adjusted to provide a safe at-ear value of 85 dBA. [18] Based on pilot testing, this level was sufficient to mask normal voice levels without the devices fitted, ensuring that performance depended solely on the audibility and clarity of the speech presented over the radios. Measurements were made of the ability to discriminate consonants presented at five radio volume settings which included the minimum and maximum available for each device. These were presented in randomized order, determined for each pair, within device.

Members of each pair served in turn as the talker and listener. One test of speech understanding, the Four Alternative Auditory Feature Test (FAAF), [19] was used. Subjects were given a typewritten list of 80 sets of four common monosyllabic words in the form of consonant-vowel-consonants (CVCs). Half the sets contrasted the initial and half the final consonant (e.g., wet, bet, get, yet or bad, bag, bat, back). The order of these two types was random throughout each list. Published studies have demonstrated that initial consonants are significantly more distinctive than final consonants. [20] One word from each set, the designated target, was spoken by the talker at a comfortable voice level and the listener was required to circle the alternative heard on an answer sheet. The target word was embedded in the phrase "Can you hear….clearly", spoken as a statement rather than a question. Midway through the list, the pair reversed roles. Five different lists were available. These were counterbalanced across the 100 (2 × 5 × 10) possible device by radio volume by subject pairs.

 Methods



Subjects

The study protocol was approved in advance by the Human Research Ethics Committee of our institution. Subjects were recruited with the aid of an email sent to all employees. Those who passed the screening criteria signed a Consent Form that described the study prior to participating.

Apparatus

Pair mates were seated side by side at the center of a double-walled, semi-reverberant sound proof booth (Series 1200, IAC, Winchester, UK) with inner dimensions of 3.5 metres (L) × 2.7 metres (W) × 2.3 metres (H) that met the requirements for hearing protector testing specified in American National Standard S12.6-2008. [21] A room divider placed between them prevented visual contact. The ambient noise in the test facility was less than the maximum permissible for audiometric test rooms specified in American National Standard S3.1-1999 (R2008). [22] The speech babble noise was taken from the Speech Perception in Noise (SPIN) test. [23] Speech babble noise is commercially available on audio cassette (Auditec, Inc., St. Louis, MO). It was presented free field over a triad of loudspeakers (DL10, Celestion, Maidstone, Kent, UK) positioned near the corners of the room to create a uniform sound field. [24] The 85 dBA at-ear level of the speech spectrum noise was the same for pair mates.

The Quietpro and Powercom devices used in the study are battery-powered and can be used for two-way radio communications [Figure 1]. Both have OFF and ON modes of operation. According to the manufacturers' specifications, in the OFF mode the Quietpro affords users passive (conventional) attenuation of the ambient sound of 24-42 dB from 0.125 to 8 kHz. The Powercom attenuates surround sounds similarly by 19-39 dB, in the same frequency range. When turned ON, both devices have a push-to-talk (PTT) feature for voice communications, and both allow for independent volume control of the surround and radio communications. When the Quietpro is in the ON mode, the ambient sound is maintained at a level no greater than 85 dBA. If the surround becomes too intense, talk through capability will be shut down, and active noise reduction (ANR) will be activated to provide a total attenuation (active plus passive) of 33-42 dB from 0.125 to 8 kHz. Intense impulsive sounds (e.g., from a gunshot or explosion) are compressed instantaneously. In the case of the Powercom, the surround must be turned off manually to protect hearing by the passive attenuation provided by the muff. The Quietpro requires connection to an external radio such as the CF standard issue personal role radio (PRR). The Powercom has a built in radio with a boom microphone for sending messages. For both devices, the subject hears the messages in the ear.{Figure 1}

In order to obtain an at-ear level of 85 dBA for the present experiment, the speech babble was set at 75 dBA at source with the Quietpro surround feature set at the eighth of 11 possible levels and the Powercom surround feature set at the fifth of five possible levels. These selections were based on calibrations made using an acoustic test fixture. [25] The Quietpro has nine radio volume settings. Settings 1, 3, 5, 6 and 9 were tested. The Powercom has five radio volume settings. Settings 1, 2, 3, 4 and 5 were tested.

Procedure

Subjects participated in three experimental sessions. During the first session, pure tone thresholds were measured in each ear by a trained technician to ensure that the hearing screening criteria were met. They were then shown the two devices and given verbal instructions for fitting each before doing so themselves. Foam ear canal tips that attach to the transducer housing of the Quietpro [Figure 1]a are available in five sizes. The system conducts an automatic fit check to ensure that a good seal has been achieved. The Powercom is available in only one size. The headband is adjustable to allow for a good seal of the ear cups to the external ears. The fits of both devices were checked by a trained technician to ensure that they were well seated. This is a variation of Method A (Experimenter-Supervised Fit) described in ANSI Standard 12.6-2008. [21]

During the first session, subjects were also trained individually to use the PTT feature when performing as talkers. The Powercom was equipped with a PTT finger button [Figure 1]c. The PTT button on the Quietpro is located on the control unit [Figure 1]a. Subjects were instructed to pause for a count of five after depressing the button and again before releasing it, before and after the utterance, respectively, to avoid cutting off the beginning or ending. They paused for a count of 10 between presentations to allow the listener sufficient time to respond. The surround and radio volumes were set by the experimenter. Subjects were advised not to change the settings. For the Quietpro, subjects were also instructed not to adjust the volume of the PRR which was set to maximum.

Following the training session, subjects were scheduled to participate in two sessions on separate days to test each of the two devices, respectively. For these, they were paired with another subject of the same gender, randomly chosen by the experimenters. During each session, the FAAF lists were presented binaurally to the listener by the live voice of the talker over the radio of the communication device being tested. Listeners were encouraged to guess if uncertain as to the word that was presented. No feedback was given about the correctness of the responses. Subjects were given a brief rest after each list of 80 words had been presented, during which time the experimenter entered the booth and changed the volume of the radios. Throughout the experiment, communications were monitored by the experimenter seated outside the booth, using either a PRR radio in combination with a PRR headset for the Quietpro trials [Figure 1]b or a family radio service (FRS) walkie talkie for the Powercom trials. At the conclusion of the experiment subjects were invited to comment on what they liked and disliked about the two devices they had worn.

 Results



The dataset for each of the 20 listeners consisted of the percentages (percent) of correctly identified initial and final consonants for the two devices (Quietpro and Powercom), at each of five radio volumes. The mean percent correct, averaged across listeners, was calculated for each of the ten combinations of device and radio volume, for the two consonant positions (initial vs final) taken separately. The outcomes are presented in [Table 1] and [Table 2] for two types of group classification, gender (male vs female) and military status (civilian vs military), respectively. Previous studies have shown that female talkers may result in higher scores than male talkers in communication tests. The gender of the listener has not been found to be significant. [15],[16] In the present study, both the listener and the talker were the same gender. Many of the military subjects had prior experience as radio operators, and thus were more experienced as listeners than the civilians, especially in noisy surroundings. There were an equal number of males and females in each of the civilian and military groups. However, while gender was a condition for the pairing of subjects, military status was not. Thus a military listener might be paired with a civilian talker or the reverse.{Table 1}{Table 2}

The results displayed in [Table 1] for the gender grouping indicate that for each device, the percent correct generally increased with increases in the radio volume over the range of settings investigated. Standard deviations for initial and final consonant positions at the minimum and maximum volumes for the two devices ranged widely from 8.1 to 27%. They were not systematically related to device, volume or gender. The median standard deviation was 17.6%, and 50% of the values fell between 11.5% and 19.7%. A repeated measures analysis of variance (ANOVA) [26] was applied to the dataset to assess the effects of device, radio volume and consonant position, by gender. Since the range of radio volumes differed for the two devices, only the data for the minimum and maximum settings were included in this analysis. The ANOVA showed significant effects for device, radio volume, consonant position, and the interaction of consonant position and gender (P<0.02 or better). The main effects are shown in [Figure 2] and the interaction in [Figure 3]. The difference due to the device, averaged across the minimum and maximum radio volumes, consonant position and gender, was 8%, with the Quietpro receiving the higher score of 70%. The effect of the increase in the radio volume, averaged across device, consonant position and gender, was 16% (74% for the maximum setting vs 58% for the minimum setting). Final consonants resulted in a 4% higher score than initial consonants (68% vs 64%). With respect to the interaction, the difference due to initial versus the final consonant position was only 1% for males (64% vs 63%, respectively) favoring the initial consonant but 10% for females (63% vs 73%, respectively), favoring the final consonant.{Figure 2}{Figure 3}

An ANOVA applied to the dataset for device, radio volume (minimum and maximum settings) and consonant position, by military status [Table 2] showed significant main effects of device and radio volume (P<0.02 or better). The observed differences due to these variables were the same as those observed for the ANOVA conducted for the gender groups. Military status was not significant nor were there any significant interactions. Consonant position was borderline significant (P<0.054). Averaged across device, minimum and maximum radio volumes and civilian and military groups, initial consonants resulted in a score of 64%, compared with 68% for the final consonant. The difference is similar to that observed for the gender ANOVA. [Table 3] shows the median percent correct achieved by male and female listeners, in each of the military and civilian groups. Medians were calculated in preference to means since there were only five subjects in each of the four gender by military status categories. These data show that, regardless of military status, device or radio volume, the median percent correct for the females was always relatively greater for the final than for the initial consonant. For males in the civilian subgroup, percent correct was relatively higher for the initial consonant, regardless of device or volume. For the male listeners in the military subgroup, a difference favoring the initial consonant was only evident for the Quietpro device at the lowest volume.{Table 3}

Eighteen of the 20 subjects commented on their experiences with the two devices tested. Responses obtained from males and females were similar. Regardless of the device, most subjects reported that the task became easier as the radio volume increased. However, a few noted that the difference in audibility was not appreciable for the Powercom. With respect to the Quietpro, most experienced discomfort when using the PTT button on the control unit. The button was difficult to see and to push, resulting in fatigue and soreness of the thumb by the end of the session. Five of the 18 subjects reported discomfort with the plugs. All preferred the finger PTT button used with the Powercom. It should be noted that a finger PTT is also available for the Quietpro. It was not used in the present study because pilot subjects found it too cumbersome to hold the control unit and the finger PTT, along with the clipboard and pencil required for responding. Five of the 18 subjects reported uncomfortable pressure of the Powercom muffs on the ears or head.

 Discussion



The experiment described above was designed to assess the effect of increasing the radio volume on the intelligibility of radio communications in moderately high-level speech babble noise for exemplars of in-ear and muff-style integrated hearing protection systems. Choice of these types for military operations or industrial settings will depend on requirements for use in combination with other head worn equipment. The results showed that radio volume affected the outcome significantly, although not significantly differently for the two devices. Averaged across the 20 subjects and consonant position, the highest mean score was observed for the highest volume setting, 79% for the Quietpro, compared with 69% for the Powercom. The mean score for the lowest volume setting was 61% for the Quietpro, compared with 55% for the Powercom. The difference between devices may been due to either differences in the range of at-ear radio volume levels or differences in clarity. Studies reported in the literature indicate that normal-hearing subjects can achieve 85% consonant discrimination for items mixed with speech spectrum noise presented over earphones at a speech-to-noise ratio of −4 dB. [27] Speech-to-noise ratios could not be computed in the present study since changes in at-ear levels with changes in radio volume were not available for the two devices. However, the published data suggest that for the highest radio volume setting of the in-ear device the at-ear level would have had to be about 75-80 dBA. Gender of the listener (or talker since these were the same) was not a significant determinant of outcome as a main effect. This finding is in line with results reported by Nixon et al. [17]

Averaged across the two devices, minimum and maximum radio volumes, and gender, final consonants were significantly more easily recognized than initial consonants, although by a relatively small amount, 4%. However, statistical analysis also showed a significant gender by consonant interaction. Averaged across the two devices and the minimum and maximum radio volumes, females showed a 10% advantage for the final consonant, compared with 1% advantage for the initial consonants for the males. The mean scores obtained by males for the initial and final consonants and by the females for the initial consonant were similar at 63-64%, compared with 74% for the final consonant observed for the females. When the subjects were grouped by military versus civilian status of the listener, the group by consonant contrast interaction was only borderline statistically significant. Both groups showed relatively higher scores for the final consonant, with civilians at 3% and the military at 6%. This outcome was mainly due to the results for the female listeners (or talkers) in each group.

The observed benefit for final consonants in the present study is at odds with the previously reported finding of less difficultly perceiving changes in the initial consonant. [20] In research by Redford and Diehl, [20] CVCs were presented in a background of pink noise at an SNR of +15 dB to the open ears of male and female college students The talkers were two males and two females. An initial consonant advantage was observed for all four talkers. By contrast, in the present study, subjects were listening to speech materials in a background of speech babble noise presented at a moderately high level over two different radios, each with its own unique acoustic properties. The observed final consonant advantage for the females may have been due to the spectral characteristics of the female talkers against the spectrum of the babble noise, which was presented at an adverse speech-to-noise ratio.

In summary, the results of this study showed that in moderately high levels of speech babble noise, and at the highest radio volume setting, subjects could achieve relatively high intelligibility scores with an in-ear integrated hearing protection communications device. However, one or other of the two styles of device tested might have a practical advantage depending on other head gear worn. For both the in-ear and muff-style devices, females proved to be either more understandable or better listeners than males, for final consonants. Previous research suggests that the gender of the talker rather than the listener is the more important variable. [16] Drawbacks of the present study were that only subjects with normal hearing were tested and with relatively simple messages. Given the prevalence of noise-induced hearing loss among military personnel, [28] the adequacy of the range of available radio volumes for users with hearing loss should be investigated, as well as message complexity.

References

1Abel SM. Barriers to hearing conservation programs in combat arms occupations. Aviat Space Environ Med 2008;79:591-8.
2Abel SM, Alberti PW, Haythornthwaite C, Riko K. Speech intelligibility in noise: Effects of fluency and hearing protector type. J Acoust Soc Am 1982;71:708-15.
3Abel SM, Kunov H, Pichora-Fuller MK, Alberti PW. Signal detection in industrial noise: Effects of noise exposure history, hearing loss, and the use of ear protection. Scand Audiol 1985;14:161-73.
4Abel, SM, Armstrong NM, Giguère C. Auditory perception with level-dependent hearing protectors: The effects of age and hearing loss. Scand Audiol 1993;22:71-85.
5Allen CH, Berger EH. Development of a unique passive hearing protector with level-dependent and flat attenuation characteristics. Noise Control Eng J 1990;34:97-105.
6Abel SM, Lam Q. Sound attenuation of the Indoor/Outdoor Range E-A-R® plug. Mil Med 2004;169:551-5
7Abel SM, Nakashima A. An investigation of the attenuation provided by the Surefire EP3 Sonic Defender earplug. Technical Report, DRDC Toronto TR 2008-040; May 2008.
8McKinley RL, Steuver JW, Nixon CW. Estimated reductions in noise-induced hearing loss by application of ANR headsets. In: Axelsson A, Borchgrevink H, Hamernik RP, Hellstrom PA, Henderson D, Salvi RJ, editors. Scientific basis of noise-induced hearing loss. New York (NY): Thieme; 1996. p. 347-60.
9Berger EH. Hearing protection devices. In: Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, editors. The noise manual. 5 th ed. Fairfax (VA): American Industrial Hygiene Association; 2000. p. 391-4.
10Nakashima A, Abel SM. Effects of integrated hearing protection headsets on the quality of radio communications. Technical Report, DRDC Toronto TR 2009-074, Defence Research and Development Canada; Jul 2009.
11Brungart DS. Informational and energetic masking effects in the perception of two simultaneous talkers. J Acoust Soc Am 2001;109:1101-9.
12Abel SM. The effect on earmuff attenuation of other safety gear worn in combination. Noise Health 2002;5:1-13.
13van Wijngaarden SJ, Steeneken HJ, Houtgast T. Quantifying the intelligibility of speech in noise for non-native talkers. J Acoust Soc Am 2002;112:3004-13.
14Yantis PA. Puretone air-conduction testing. In: Katz J, editor. Handbook of clinical audiology. 3 rd ed. Baltimore (MD): Williams and Wilkins; 1985. p. 153-69.
15Bradlow AR, Torretta GM, Pisoni DB. Intelligibility of normal speech I: Global and fine-grained acoustic-phonetic talker characteristics. Speech Commun 1996;20:255-72.
16Markham D, Hazan V. The effect of talker- and listener-related factors on intelligibility for a real-world, open-set perception test. J Speech Lang Hear Res 2004;47:725-37.
17Nixon CW, Morris LJ, McCavitt AR, McKinley RL, Anderson TR, McDaniel MP, et al. Female voice communications in high levels of aircraft cockpit noises-Part I: Spectra, levels, and microphones. Aviat Space Environ Med 1998;69:675-83.
18Humes LE, Joellenbeck LM, Durch JS. Noise and military service: Implications for hearing loss and tinnitus. Washington (DC): The National Academies Press; 2006. p. 54-64.
19Foster JR, Haggard MP. FAAF-An efficient analytic test of speech perception. In Proc. of the Inst. of Acoustics. Nottingham, England: MRC Institute of Hearing Research; 1979. p. 9-12.
20Redford MA, Diehl RL. The relative perceptual distinctiveness of initial and final consonants in CVC syllables. J Acoust Soc Am 1999;106:1555-65.
21American National Standards Institute. Methods for measuring the real-ear attenuation of hearing protectors, ANSI/ASA S12.6-2008. Melville (NY): Standards Secretariat, Acoustical Society of America; 2008.
22American National Standards Institute. Maximum permissible ambient noise levels for audiometric test rooms, ANSI Standard S3.1-1999 (R2008). Melville (NY): Standards Secretariat, Acoustical Society of America; 1999.
23Kalikow DN, Stevens KN, Elliott LL. Development of a test of speech intelligibility in noise using sentence materials with controlled word predictability. J Acoust Soc Am 1977;61:1337-51.
24Giguère C, Abel SM. A multi-purpose facility for research on hearing protection. Appl Acous 1990;31:295-311.
25Giguère C, Laroche C. Research on modelling the effects of hearing protection and communications devices on speech intelligibility in noise. Progress report for Contract No. W7711-088145/00/TOR. University of Ottawa; March 2010.
26Daniel WW. Biostatistics: A Foundation for Analysis in the Health Sciences. 3 rd ed. New York (NY):Wiley;1983.
27Abel, SM, Krever EM, Alberti PW. Auditory detection, discrimination and speech processing in ageing, noise-sensitive and hearing-impaired listeners. Scand Audiol 1990;19:43-54.
28Abel SM. Hearing loss in military aviation and other trades: Investigation of prevalence and risk factors. Aviat Space Environ Med 2005;76;1128-35.

 

 
Thursday, March 23, 2023
 Site Map | Home | Contact Us | Feedback | Copyright and Disclaimer