Strategies to combat auditory overload during vehicular command and control.

Strategies to combat auditory overload were studied. Normal-hearing males were tested in a sound isolated room in a mock-up of a military land vehicle. Two tasks were presented concurrently, in quiet and vehicle noise. For Task 1 dichotic phrases were delivered over a communications headset. Participants encoded only those beginning with a preassigned call sign (Baron or Charlie). For Task 2, they agreed or disagreed with simple equations presented either over loudspeakers, as text on the laptop monitor, in both the audio and the visual modalities, or not at all. Accuracy was significantly better by 20% on Task 2 when the equations were presented visually or audiovisually. Scores were at least 78% correct for dichotic phrases presented over the headset, with a right ear advantage of 7%, given the 5 dB speech-to-noise ratio. The left ear disadvantage was particularly apparent in noise, where the interaural difference was 12%. Relatively lower scores in the left ear, in noise, were observed for phrases beginning with Charlie. These findings underscore the benefit of delivering higher priority communications to the dominant ear, the importance of selecting speech sounds that are resilient to noise masking, and the advantage of using text in cases of degraded audio.

Divided listening in noise in a mock-up of a military command post.

This study investigated divided listening in noise in a mock-up of a vehicular command post. The effects of background noise from the vehicle, unattended speech of coworkers on speech understanding, and a visual cue that directed attention to the message source were examined. Sixteen normal-hearing males participated in sixteen listening conditions, defined by combinations of the absence/presence of vehicle and speech babble noises, availability of a vision cue, and number of channels (2 or 3, diotic or dichotic, and loudspeakers) over which concurrent series of call sign, color, and number phrases were presented. All wore a communications headset with integrated hearing protection. A computer keyboard was used to encode phrases beginning with an assigned call sign. Subjects achieved close to 100% correct phrase identification when presented over the headset (with or without vehicle noise) or over the loudspeakers, without vehicle noise. In contrast, the percentage correct phrase identification was significantly less by 30 to 35% when presented over loudspeakers with vehicle noise. Vehicle noise combined with babble noise decreased the accuracy by an additional 12% for dichotic listening. Vision cues increased phrase identification accuracy by 7% for diotic listening. Outcomes could be explained by the at-ear energy spectra of the speech and noise.

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

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.

Hearing and sound source identification with protective headwear.

This study investigated the effect on hearing, sound attenuation, and sound source identification of a prototype neck and two prototype mandible guards attached to a combat helmet. Ten male subjects participated. Free-field hearing thresholds were measured from 250 Hz to 8,000 Hz with the head bare and fitted with the helmet alone and with the guards. Sound source identification was assessed using a horizontal array of eight loudspeakers surrounding the subject. The stimulus was a 75-dB SPL, 300-ms noise burst. Neither the helmet worn alone or with the guards affected hearing or provided significant sound attenuation. The helmet combinations resulted in a significant decrease in sound source identification, of 11.6%. This was due to diminished accuracy for loudspeakers close to the interaural axis of the head. The neck guard induced a frontal bias for these positions. This error pattern is not likely to interfere with localization during combat.

Sound localization with an army helmet worn in combination with an in-ear advanced communications system.

Conventional hearing protection devices result in decrements mainly in the ability to distinguish front from rearward sound sources. The aim of this study was to investigate the effect of wearing an earplug with advanced communications capability, in combination with an army helmet, on horizontal plane speaker identification. Ten normal-hearing male subjects were tested in a semi-reverberant sound proof booth under eight conditions defined by combinations of two levels of ear occlusion (unoccluded and occluded by the earplug) and four levels of the helmet (head bare and fitted with the helmet modified to give no, partial and full ear coverage). Percent correct speaker identification was assessed using a horizontal array of eight loudspeakers surrounding the subject at one meter. These were positioned close to the midline and interaural axes of the head, at ear level. The stimulus was a 75-dB SPL, 300-ms broadband white noise. Both degree of ear coverage and ear occlusion significantly determined outcome. Overall percent correct ranged from 93.6% (bareheaded) to 79.7% (full ear coverage) with the ears unoccluded, and from 83.4%-77.5% with ear occlusion. Both variables affected the prevalence of mirror image confusions for positions 30 degrees apart in front and back of the interaural axis. With ear occlusion, front given back errors were more likely than back given front errors, increasing with degree of ear coverage to 49% and 25.4%, respectively. These errors also increased with ear coverage with the ears unoccluded, but were similar. Both degree of ear coverage and ear occlusion significantly impacted horizontal plane speaker identification, particularly for sources close to the interaural axis. However, overall percent correct was higher than observed in a previous study with conventional and level-dependent hearing protection devices, using the same array.

Stimulus complexity and dual tasking effects on sustained auditory attention in noise.

INTRODUCTION: The effect on sustained auditory attention of a constellation of factors that characterize aviation, maritime, and land environments was investigated. The ability to detect infrequent spoken messages while performing an unrelated visual motor tracking task was studied in the presence of background noise. METHOD: Six subjects each were assigned to four conditions defined by the presence/absence of 80-dBA pink noise and presence/absence of the secondary task. Subjects were given three detection tests in which the 75-dB SPL critical signal was comprised of one, two, or three components. A button press with the left hand signified detection of the critical signal. For each test there were six consecutive 20-min vigils comprised of 150 trials. The critical signal was presented in 10 of these. For the tracking task subjects used a computer mouse controlled by the right hand to follow the sinusoidal movement of a 3-cm vertical line across a computer monitor. RESULTS AND CONCLUSIONS: Background noise resulted in a significant decrease in hits, and significant increases in false alarms and response time. Since the signal and noise were both auditory, the noise was likely effective as a masker rather than a stressor. The secondary task did not impact performance, possibly because the auditory task was too easy and the event rate too slow. As in previous studies using alphanumeric events, there was no attention decrement over time. However, response time decreased as the number of components in the critical signal increased. Possible explanations are guided attention through priming or the provision of a temporal foreperiod.

Barriers to hearing conservation programs in combat arms occupations.

INTRODUCTION: The Canadian military instituted a hearing conservation program over 45 yr ago. Yet the prevalence of noise-induced hearing loss is escalating. A focus group study involving four combat arms occupations was carried out to probe individuals' knowledge, attitudes, and behaviors relating to hearing loss prevention to find ways to improve compliance. METHODS: One group each of 4-5 Infantry Soldiers, Artillerymen, Armored Soldiers, and Combat Engineers, with the rank of Warrant Officer, Sergeant, or Master Corporal, and at least 5 yr of service participated. Discussions were led by a Moderator and recorded by an Assistant Moderator. Questions posed related to susceptibility and consequences of hearing loss, benefits and drawback of hearing protection, and preferences. RESULTS: Age range was 28-48 yr and length of service 10-30 yr. Individuals were exposed to noise from weapons, explosives, vehicles, and aircraft. Infantry Soldiers and Artillerymen had confirmed moderate to severe hearing loss. Armored Soldiers and Combat Engineers had not perceived a change in hearing. Main concerns of using hearing protection were interference with detection and localization of auditory warnings, and perception of orders. Devices were often incompatible with other gear and difficult to fit. DISCUSSION: Good hearing was critical to the occupations studied. Difference in hearing loss among groups was related to type and level of noise exposure. Loss of hearing and/or the use of hearing protection compromised situational awareness, exchange of information, and auditory task performance. Participants favored opportunities to try recommended devices, policies governing use, and sufficient funding to ensure protection for both regular and special forces.

Sound localization with communications headsets: comparison of passive and active systems.

Studies have demonstrated that conventional hearing protectors interfere with sound localization. This research examines possible benefits from advanced communications devices. Horizontal plane sound localization was compared in normal-hearing males with the ears unoccluded and fitted with Peltor H10A passive attenuation earmuffs, Racal Slimgard II communications muffs in active noise reduction (ANR) and talk-through-circuitry (TTC) modes and Nacre QUIETPRO TM communications earplugs in off (passive attenuation) and push-to-talk (PTT) modes. Localization was assessed using an array of eight loudspeakers, two in each spatial quadrant. The stimulus was 75 dB SPL, 300-ms broadband noise. One block of 120 forced-choice loudspeaker identification trials was presented in each condition. Subjects responded using a laptop response box with a set of eight microswitches in the same configuration as the speaker array. A repeated measures ANOVA was applied to the dataset. The results reveal that the overall percent correct response was highest in the unoccluded condition (94%). A significant reduction of 24% was observed for the communications devices in TTC and PTT modes and a reduction of 49% for the passive muff and plug and muff with ANR. Disruption in performance was due to an increase in front-back reversal errors for mirror image spatial positions. The results support the conclusion that communications devices with advanced technologies are less detrimental to directional hearing than conventional, passive, limited amplification and ANR devices.

Hearing, communication and cognition in low-frequency noise from armoured vehicles.

An experiment was performed to study auditory perception and cognitive function in the presence of low-frequency dominant armoured vehicle noise (LAV III). Thirty-six normal hearing subjects were assigned to one of three noise backgrounds: Quiet, pink noise and vehicle noise. The pink and vehicle noise were presented at 80 dBA. Each subject performed an auditory detection test, modified rhyme test (MRT) and cognitive test battery for three different ear conditions: Unoccluded and fitted with an active noise reduction (ANR) headset in passive and ANR modes. Auditory detection was measured at six 1/3 octave band frequencies from 0.25 to 8 kHz. The cognitive test battery consisted of two subjective questionnaires and five performance tasks. The earmuff, both in the conventional and ANR modes, did not significantly affect detection thresholds at any frequency in the pink and vehicle noise backgrounds. For the MRT, there were no significant differences between the speech levels required for 60% correct responses for three ear conditions in the pink and vehicle noise backgrounds. A small but significant (4 dB) increase in speech level was required in pink noise as compared to vehicle noise. For the serial reaction time task, the mean response time in the vehicle noise background (751 ms) was significantly higher than in pink noise and quiet (709 and 651 ms, respectively). The mean response time in the pink noise background was also significantly higher than in quiet. Thus, the presence of noise, especially low-frequency noise, had a negative effect on reaction time.

Measurement of noise and vibration in Canadian forces armoured vehicles.

Noise and whole-body vibration measurements were made in the following Canadian Forces vehicles: LAV III, Bison and M113A2 ADATS (air defence anti-tank system). Measurements were made at different crew positions while the vehicles were driven at different speeds over rough terrain and paved roads. The participants completed a questionnaire at the end of each measurement session on their reactions to the noise and vibration. Noise levels were as high as 115 dBA in the ADATS, 102 dBA in the Bison and 96 dBA in the LAV III, exceeding the Canada Labour Code exposure limit of 87 dBA for 8 h(1)). A communications headset was found to be sufficient to reduce the noise exposure to safe levels in most cases. The vector sum vibration magnitudes for the LAV III and Bison were relatively low during highway driving (0.3 m/s(2) for both vehicles) compared to rough terrain (0.71 and 1.36 m/s(2), respectively). The ADATS vibration increased with driving speed (0.62 m/s(2) at 8 km/h and 1.26 m/s(2) at 32 km/h). The questionnaire responses indicated that half the crewmembers had difficulty communicating in vehicle noise, but were generally unaffected physically by vibration. The latter result may have been due to the relatively short exposure duration.

Auditory perception with ear and cold weather face protection worn in combination.

The effects on hearing thresholds, sound attenuation, and consonant discrimination of wearing a balaclava under hearing-protecting earmuffs were studied. This combination is commonly worn during cold weather military operations. One group of 20 normal-hearing adults (10 male and 10 female subjects) was tested. Within-subject measurements were made of diffuse-field hearing thresholds from 0.25 kHz to 8 kHz and consonant discrimination in quiet with the ears unoccluded and protected with the earmuffs alone and with the balaclava worn full face or rolled. Attenuation was derived from the protected and unoccluded thresholds at each frequency. When the balaclava was worn full face, attenuation decreased by 16 to 18 dB, relative to the muff alone, below 6.3 kHz. With the balaclava worn as a cap, there was an inverted U-shaped decrement in attenuation of 18 to 27 dB from 0.25 Hz to 4 kHz. Consonant discrimination decreased by 7% with the muffs alone. These findings underscore the importance of assessing protective equipment under the conditions in which it will be worn.

Sound attenuation from earmuffs and earplugs in combination: maximum benefits vs. missed information.

INTRODUCTION: Noise levels from military aircraft range from 100-130 dBA. Peak pressure levels from large caliber weapons may reach 180 dB SPL. To protect against hearing loss, individuals are encouraged to wear double hearing protection. This study determined ways to maximize benefit. METHOD: Hearing thresholds from 0.25-8 kHz and consonant discrimination were assessed in normal-hearing subjects with ears unoccluded and fitted with highly rated earmuffs and earplugs, singly or in combination. The earplugs were available in two sizes. Selection was based on best fit. Attenuation values were derived from the threshold measurements. RESULTS: With the muff, plug, and muff and plug in combination, thresholds ranged from 35-48 dB SPL, 40-55 dB SPL, and 44-66 dB SPL, respectively, across the frequencies tested. The combination (without regard to size of plug) resulted in attenuation values of 38-54 dB. With the smaller of the two plugs, low-frequency values as high as 53-61 dB were realized. Consonant discrimination decreased by 6-8% with the devices worn singly and by 22% with the devices in combination, relative to unoccluded listening. DISCUSSION: Sufficient low-frequency attenuation may be achieved with muffs and plugs in combination to prevent hearing loss from operational noise. Attenuation may be maximized by choosing a smaller earplug to achieve a better fit. Possible downsides are reduced detection of warning sounds and speech intelligibility. To be heard warning sounds should surpass protected thresholds by at least 5 dB. Choosing devices which provide somewhat less attenuation may be necessary to preserve communication capability.

Hearing loss in military aviation and other trades: investigation of prevalence and risk factors.

INTRODUCTION: Continuous increase in the cost of claims for noise-induced hearing loss in Canadian Forces personnel prompted a review of hearing conservation practices. Investigations comprised retrospective analyses of hearing test results and a prospective survey of risk factors for hearing loss. METHOD: There were 1057 individuals working in 107 air, land, and sea trades who contributed their current and first hearing test results. Subjects completed a 64-item questionnaire relating to demographics, occupational and non-occupational noise exposure history, training in and use of hearing protection, and non-noise risk factors, including head injury, ear disease, medications, and solvent exposure. Subject recruitment, hearing assessment, and questionnaire distribution were carried out by military personnel. Apparatus and protocols for hearing testing conformed to current clinical practice. RESULTS: Prevalence of moderate to severe hearing loss progressed with age, with hearing thresholds in those over 45 yr ranging broadly from normal to over 70 dB HL. Unprotected exposure to noise and solvents appeared to be the likely determinates of adverse outcome. Head injury, ear disease, and the use of ototoxic medications had minimal impact. The data for individuals in 17 selected trades in the 3 environments modeled that of the total sample. DISCUSSION: Responses to the questionnaire suggested that training on noise hazard and selection and use of hearing protection were inadequate. Hearing protection was reported to be incompatible with other gear, uncomfortable, and an impediment to communication. Approaches for upgrading hearing conservation strategies, including the creation of end-user focus groups, were considered.

Hearing and performance during a 70-h exposure to noise simulating the space station environment.

INTRODUCTION: Elevated hearing thresholds have been documented in some astronauts after long-term spaceflights although noise levels were lower than those normally associated with noise-induced hearing loss in ground-based operations. The present study was conducted to determine whether prolonged exposure (70 h) to levels (72 dBA) recorded on the International Space Station (ISS) service module would impact diverse measures of auditory function, as well as cognition and memory, motivation, and cardiovascular function. METHOD: Five mixed gender subgroups of five normal-hearing subjects, aged 20-50 yr, were sequestered for 70 h in an environment that modeled conditions on the ISS. They were assigned to one of three background conditions: quiet (n = 5), continuous noise from the ISS service module (n = 10), or continuous noise during the day only (n = 10). Subjects were tested repeatedly within and across days as individuals or pair mates. RESULTS: There were no negative effects of the noise on any of the outcome measures. Introduction of a delay or noise in a communication channel used in the assessment of speech communicability significantly affected the time taken for joint problem solving by partners. DISCUSSION: The results of this study were not consistent with the observation of hearing loss measured after spaceflights. Nor were changes evident in cognition, motivation, or cardiovascular function. Factors which might account for the discrepancy are discussed.

Sound attenuation of the indoor/outdoor range E-A-R plug.

The sound attenuation provided by the AOSafety Indoor/Outdoor Range E-A-R Plug was examined. This device, currently used in military operations in several countries, is comprised of two plugs that provide conventional ("indoor" plug) and level-dependent sound reduction (the "outdoor" plug), respectively. The effects of the user's gender and repeated fittings were explored. Eight men and eight women were tested on two separate occasions. Unoccluded and protected hearing thresholds were measured for each of nine one-third octave noise bands centered at 0.125 to 8 kHz. Attenuation was calculated as the difference between these two measures. The indoor plug provided 21 to 40 dB of sound reduction across the frequencies tested, closely matching the manufacturer's specification. The outdoor plug provided 5 to 22 dB of conventional attenuation, suggesting that it might serve as a safe means of conventional and level-dependant attenuation in hearing-impaired users. No differences were found in relation to gender or repeated fittings.

A comparison of the audiometric performance of bone anchored hearing aids and air conduction hearing aids.

OBJECTIVE: To compare the function of bone anchored hearing aids (BAHA) with conventional air conduction hearing aids (ACHA) by means of objective audiometric tests focusing on temporal acuity and consonant discrimination in quiet and noise, as well as subjective quality-of-life questionnaires. SETTING: Tertiary referral center. STUDY DESIGN: Prospective. SUBJECTS: Patients using BAHAs because of profuse drainage from chronic suppurative otitis media, and a comparison group of healthy volunteers. MAIN OUTCOME MEASURES: Objective measures: sound field audiograms, duration discrimination, gap discrimination, and final and initial consonant discrimination in quiet and in noise. Subjective measures: Sanders' Profiles, MOS SF-36 questionnaire. RESULTS: Normal-hearing subjects always performed better than hearing-impaired patients in all tests. When the BAHA was compared with the ACHA, there were no significant differences in any of the measures. CONCLUSIONS: The BAHA and the ACHA provided similar audiometric functioning in audiometric tests. The BAHA, although using a nonphysiologic sound conduction route, did not sacrifice temporal processing ability or speech perception in noise, and should be considered for patients with profuse ear drainage.

The effect of blindness on horizontal plane sound source identification.

The effect of blindness on sound source identification was studied. Four groups of normally-hearing adults, two sighted and two blind, participated. Subjects were tested using arrays of four and eight loudspeakers, surrounding them in the horizontal plane. One sighted group was tested in quiet. The other groups were tested in continuous 60-dB SPL white noise. Three 75-dB SPL 300-ms stimuli were localized: one-third octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. Broadband noise was easiest to localize (both binaural and spectral cues available), and the 0.5-kHz noise band was the most difficult (primarily interaural temporal difference cue available). Subjects with late-onset blindness achieved significantly higher scores than the early blind and blindfolded sighted subjects. The percentage correct decreased with an increase in the number of speakers, but background noise had no effect. The results attest to the benefit of early visual experience for spatial hearing in adulthood, and demonstrate the negative impact of sudden loss of sight.