Military Noise Research Articles

A Little Occupational Audiology for the Clinical Audiologist

A Little Occupational Audiology for the Clinical Audiologist

Development of metrics to identify military impulse noise

Urban encroachment of military bases continues to be a serious problem, affecting training exercises and overall military readiness. The military currently performs noise monitoring around bases to ensure that environmental conditions are favorable for tests or exercises. Despite refinements, current monitoring systems still suffer from occasional false positives and rely heavily on human interpretation. The long range goal of this work is to create a more accurate and autonomous noise classifier. The specific aims of the work are to create a library of recorded military impulse noise sources and to develop, test, and refine the noise classifier algorithms. Representative, high quality field measurements of various noise sources (25 mm and larger) will be conducted at a few military bases under varying environmental conditions. Measurements of wind noise and thunder will also be recorded. The software will be developed in MATLAB and will employ several common impulse noise metrics used for hearing conservation, including rise, peak, and fall times, crest factor, A- and B-durations, Lpk and kurtosis, among others. A neural network will be trained to discern between the recorded military noise sources and naturally occuring noise based on these metrics. Preliminary results from the measurements and analysis will be presented. [Work supported by SERDP.]

Protection efficiency of hearing protectors against military noise from handheld weapons and vehicles

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 95-97 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.

Distributed exhaust for jet noise reduction

Acoustic detection of low flying, jet-powered military air vehicles and takeoff noise levels from commercial jets are often driven by jet mixing noise radiated from the exhaust. Cueing provided by the noise signature results in increased opportunity for a ground-based visual observer against such a target. Significant reductions in detectability and takeoff noise levels can be achieved through reductions in jet noise. Research has been conducted over the past several years to develop innovative, quiet, distributed exhaust nozzle (DEN) concepts, which attempt to achieve revolutionary reductions in jet mixing noise while minimizing propulsion penalties. The DEN’s benefit relies on discharging the exhaust flow through many miniature nozzles rather than one or two large nozzles. Noise suppression from the DEN concept results from a favorable shift in frequency content compared to conventional jets. Significant increases in atmospheric attenuation and decreases in the ear’s sensitivity at these higher frequencies result in much reduced detection ranges and perceived noise levels. This technology is applicable to both subsonic and supersonic aircraft, and particularly to low flying fixed-wing unmanned air vehicles, special operations forces transports, and future commercial transports with especially stringent noise reduction requirements. [Work supported by NASA LaRC.]

Evaluation of the performance of the blast analysis and measurement system

In the years since the introduction of the C-weighted day–night average sound level (DNL) to assess the noise of military explosives, Army practice has evolved to incorporate linear peak sound-pressure level into the evaluation of military training noise. Although the DNL remains as the method of choice for National Environmental Policy Act (NEPA) documentation and for land-use planning, peak level is used by firing range operators for day-to-day complaint management. Several different monitoring system designs are being used at Army installations to provide range operators with real-time feedback on blast noise levels in nearby residential areas. One of these, the Blast Analysis and Measurement (BLAM) system, is a modified version of a sonic boom monitor designed by the U.S. Air Force. Data collected from two BLAM units located near a 120-mm tank gunnery range were evaluated in terms of hit rate and false-alarm rate over a range of 94 to 140 decibels linear peak. Hit- and false-alarm rates are compared with hit- and false-alarm rates reported for other blast noise monitoring system designs.

Caribou, military jets and noise: The interplay of behavioural ecology and evolutionary psychology

Whether a human activity is likely to have a negative impact on a species depends largely on how stimuli from that activity are interpreted and acted upon by individuals, within the context of their behavioural ecology. The interpretations and decisions made by individuals in response to these stimuli are largely governed by neural systems evolved by the species as adaptations to common and recurrent selective pressures. In this paper I will review previous findings concerning the responses of caribou to overflights by military jet aircraft in Labrador/Quebec and Alaska, casting them in an evolutionary psychological framework. One prediction from such an exercise is that identical stimuli (noise from jet overflights) that elicit similar responses (short-distance avoidance) can have quite different population consequences for sedentary (woodland) and migratory (barren-ground) ecotypes. For a female woodland caribou, which shares her calving range with a resident predator population, an increase in movements following disturbance may significantly increase her calf's exposure to predators. Similar movements by a female barren-ground caribou, which has fewer predators to contend with, may have only a negligible impact on her calf's predation risk. Thus woodland caribou may be more vulnerable to negative impacts of military jet noise during calving periods, dependent on predator density.

Characteristics and Affecting Factors of Tinnitus in Noise Exposed Workers

Objectives: Tinnitus is a relatively common complaint of workers who are exposed to noise. The aim of the study was to investigate the prevalence and characteristics of tinnitus as well as the factors related to the prevalence of tinnitus. Methods: A cross-sectional audiologic survey was combined with a questionnaire on tinnitus in a stratified random sample of 246 shipyard workers who had long-term exposure to noise. Results: Forty seven had tinnitus, giving a prevalence of 19.1 percent. The tinnitus was the here and now in 44.7 percent of cases, bilateral in 51.1 percent, and caused sleep disturbances in 12.8 percent. The prevalence of tinnitus was highest in shipfitters and those with longer exposure duration. The workers with tunnitus had consistently higher hearing thresholds at both high and low frequencies than those with no tinnitus. The workers with tinnitus had a significantly higher prevalence of hearing disorders such as conductive hearing loss, a C5-dip, and NIHL. Multivariative logistic regression indicated that past history of ear disease, working and military service exposure to noise were independently associated with tinnitus. The adjusted odds ratio estimates for tinnitus were 3.0 times greater (95% CI 1.4-6.4) for the group with a past history of ear diseases, 2.3 times greater (95% CI 1.0-5.0) for the military noise exposed group, and 4.5 times greater (95% CI 1.8-11.1) for the working noise exposed group. Conclusions: The results provide evidence that reports of tinnitus at the time of the annual audiometric testing may be useful in identifying workers at greater risk of developing significant shifts in their hearing thresholds. An awareness of the possible occurrence of tinnitus may encourage workers to cooperate more actively in a company’s hearing conservation program.

Hereditary Hearing Loss - the Role of Environmental Factors

A large family with mid onset sensorineural hearing loss (HL) was used to study the effect of environmental factors on progression HL. Data from five-generations of one family were traced and 104 living members were included in the study. Audiograms were measured for 60 family members. We used an expert program to study the individual risk factors, which included the known risk factors for sensorineural HL. The pattern of inheritance in this HL family was autosomal dominant and 22 individuals were affected. The HL among affected individuals was symmetrical and varied from mild to severe. The mean age at onset of HL was 22 years. The mean deterioration of hearing at 2 kHz frequency was 1 dB/year. In the risk analysis the use of NSAID-analgetics and military noise exposure correlated with HL. There was no correlation between occupational and free time noise-exposure and HL. Analgetics may aggravate the HL in subjects with inherited HL. This type of genetic lesion is new and is not in those chromosomal areas already documented in the literature.

Evaluating noise induced hearing loss with distortion product otoacoustic emissions.

This study assessed the clinical efficacy of screening for noise induced hearing loss (NIHL) with distortion product otoacoustic emissions (DPOAE). DPOAEs were recorded from 76 military personnel (137 ears) aged between 17 and 41 years in response to equilevel 70 dB SPL primary stimulating tones. The 2f1-f2 DPOAE levels were correlated with audiometric thresholds at frequencies close to f2. Ears with normal audiograms, but with a history of military noise exposure, had DPOAEs that were significantly decreased in amplitude as compared to the ears of normal hearing non-exposed to noise subjects. These ears also had an increased absence of DPOAEs as compared with the ears of the normal hearing non-exposed to noise subjects. Although, in general, the DPOAE amplitudes and spectral frequency ranges reflected the audiometric NIHL configurations, in a number of cases DPOAEs were present for hearing losses up to 75 dB HL. As a consequence, DPOAEs correlated moderately and negatively with the audiometric thresholds. Applying test criteria designed to logically reflect NIHL, DPOAE sensitivity and specificity levels ranged between 0.51-0.90 and 0.63-0.25, respectively. These findings indicate that DPOAEs, recorded and analysed as described, are not sufficiently sensitive to serve as a single test to identify NIHL.

Magnesium concentration and susceptibility to noise-induced hearing loss

Joachims et al. [J. Acoust. Soc. Am. 74, 104–108 (1983)] reported that magnesium (Mg) intake influenced susceptibility to noise-induced hearing loss (NIHL), with noise-exposed rats fed a Mg-deficient diet incurring significantly greater NIHL than animals fed a Mg-rich diet. Subsequently, Joachims et al. [Mag. Bull. 9, 130–131 (1987)] reported a highly significant negative correlation between serum Mg concentration and NIHL in 24 Israeli Air Force pilots who had been frequently exposed to high-level noise. If reliable, these findings have far-reaching implications for predicting susceptibility and preventing NIHL. This study sought to replicate Joachims et al. (1987) results. Sixty-eight U.S. Army soldiers who had been routinely exposed to potentially hazardous military noise over a 10–15 year period provided pure-tone audiograms following a noise-free period of ≳15 h. Blood samples were obtained following a fasting period of 6–12 h. Samples were analyzed via atomic absorption spectrophotometry to determine the concentrations of Mg in serum, mononuclear cells, and erythrocytes. Results revealed no significant correlation between audiometric thresholds and any of the Mg measures. [Work supported by the Department of Clinical Investigation, Walter Reed Army Medical Center.]

Effect of ‘‘conditioning’’ on hearing loss from military noise exposures

It has been shown with chinchillas as subjects that prophylactic (‘‘conditioning’’) exposures to an interrupted OBN centered at 0.5 kHz for 10 days provide protection against permanent threshold shift (PTS) from high level impulse noise in chinchillas [L. W. Henselman et al., Hear. Res. 78, 1–10 (1994)]. The present investigation studied the effect of ‘‘conditioning’’ with helicopter noise on PTS resulting from high level impulse noise. Thirty‐eight chinchillas were randomly assigned to one of four experimental groups (helicopter noise exposure prior to impulse noise exposure) or a control group (impulse noise exposure only). It was shown that (a) interrupted exposures over a 10‐day period to helicopter noise presented at 112 dB SPL for 1.5 h/day caused TTSs to decrease as exposure days continued at the test frequencies of 0.5 to 8 kHz, and (b) after 4 weeks of recovery, subjects were protected from PTS after ‘‘conditioning’’ with helicopter noise (except the group with prolonged ‘‘conditioning’’). Histological results were consistent with audiological findings and revealed significantly less hair cell loss in the experimental groups that were protected from PTS. The results are discussed in terms of possible application to hearing conservation programs. [Work supported by the U.S. Army Medical Command.]

Penalties for small arms noise assessment.

Proper assessment of the noise created by Army testing and training remains a question that is not fully answered. The most difficult noises to assess are the impulsive noises generated by large weapons, small arms, and helicopters. Currently, general community noise is assessed using the A-frequency weighting and some form of ‘‘energy equivalent’’ level. In the United States, the day–night average sound level is used. For clearly impulsive sound, adjustments or ‘‘penalties’’ are sometimes added to the formulation to account for the increase in annoyance generated by the impulsive character of the sound. One purpose of the present test was to further define and develop adjustments or ‘‘penalties’’ that can be used to assess military noise vis-á-vis normal, urban noises. The study was designed as a paired comparison test where the subjects were presented pairs of sounds and asked, for each pair, which was more annoying, the first or the second sound. The test sounds came from one of three categories of military sources that were: (1) tracked vehicles, (2) small arms fire, or (3) large blasts. The other sound in a pair was one of two control sounds that were: (1) the sound of a wheeled vehicle passing by, or (2) a computer-generated noise. This paper concentrates on small arms noise. The results are for measurements made outdoors−even when the subjects are situated indoors. This is considered to be the proper location since common noise such as traffic is normally assessed on the basis of outdoor levels. The small-arms penalty averages about 8–10 dB across conditions of windows closed or open or subjects outdoors.

Low-Frequency Noise Impulses from Explosions

For military noise evaluation, noise impulses from explosions of 60 g – 2 20 kg equivalents of TNT (trinitrotoluene) were measured at distances of 10 – 300m. Noie and air-blast sensors (an LC-33 pencil gauge, a Kistler 6103 detonation sensor, B&K 8103 and B&K 8105 hydrophones and a B&KI 4136 microphone) were used for the measurements. The peak values, A-durations, impulse energies and frequency transforms were usually analysed from the tape-recorded signals (waveform analyser DATA 6000). At a distance of 100m, the means of the 60 g and 20 kg explosions were as follows: 144 dB and 163 dB for the peak values, 4.0 ms and 13.0 ms for the A-durations, 250 Pa 2s and 51 kPa 2s for the impulse energies, and 1200 Hz and 15 Hz for the most prominent frequencies, respectively. The peak levels decreased 5–8 dB when the distance was doubled. A graphic nomogram was formed from the peak values for training, safety, and noise control purposes.

Cardiac responses caused by shots of tanks during sleep

Heart rate responses evoked by military noise were registered in 17 subjects. The subjects slept in the laboratory during 13 consecutive nights each. Prerecorded shots of tanks with peak levels from 78 to 82 dB(A) were played back every three nights, either during the first three hours after lights off or during the last three hours before awakening. The shots provoked a brief acceleration of the heart rate. The extent of the response was larger during sleep than when awake, and larger in women than in men; it was larger in the early morning than in the first hours after sleep onset. Within one night no adaptation took place; a limited habituation, however, was observed during the test series.

Differences in measured microphone responses at different distances from artificial mouth

When calibrated and operated at different distances, available artificial voice/mouth frequency response measuring systems yield significantly different results. These differences are greater for different distances than for different voice/mouths. Virtually all reported voice measurements are made at the CCITT recommended distance of 25 mm from the lips, or 25 mm in front of the “lip ring” of the artificial voice/mouths. Yet, all high-noise environment military microphones are tested 1/4 in. (6 mm) from the voice/mouth opening. This is, however, several millimeters behind the lip plane of the voices. At this distance, noise canceling microphones show minimal cancellation effects. Historically, the use of 1/4-in. distance began with a laboratory artificial voice/driver sound system, which predated the first commercially available, relatively large scale production artificial voice (B&K 4216). This paper shows the feet(s) on measured microphone response when military noise canceling microphones (M-138, H-250, M-162) and a calibrated laboratory microphone are tested, on axis, at different distances from several calibrated voices (6 mm from opening, 6 and 25 mm in front of lip ring). Real world implications are discussed.

Military Noise Induced Hearing Loss: Incidence and Management

Military Noise Induced Hearing Loss: Incidence and Management

An historical perspective of noise control

Annoyance due to noise or noise pollution is, perhaps, the oldest recognized form of pollution. Although the ancients (Egyptians, Greeks, and Romans) had a surprising knowledge of acoustics and a recognition of industrial and urban noise, there appears to have been little application of technology to the problem. In more modern times, the power of noise as an annoyance has been intuitively understood and fancifully exploited in writing and the motion pictures. In the same era, the military noise problem has been recognized, and considerable technological effort has been expended to reduce noise and exploit its generation under battle conditions. Community noise limits have been legislated in some communities for a considerable time. Similarly, industrial and environmental noise control has been practiced on a specialty basis for a relatively long time; however, it remained for the Second World War and the period following it to bring an ordered technological approach to the problem of noise. This paper attempts to present a reasonable view of the historical development of the understanding of noise control technology and its impact on the education process.

Human Factors and Environmental Design in Military Food Service Facilities; a Survey Approach

More than 2000 enlisted military personnel from three Air Force bases and one Army post were surveyed in order to elicit customer opinions about human factors and environmental design considerations in the food service facilities. It was discovered that speed of service, which includes both delay at a headcount signature station and in the serving line itself, was a major problem. Other problem areas were military atmosphere present, temperature, noise, crowding, and decor. The relevance of consumer opinion survey information in channeling human factors and environmental design efforts is emphasized.

Tentative Audiometric Threshold-Level Standards from 8 through 18 kHz

Both ears of 100 young healthy men with no previous military noise exposure, aged 17–23 yr and normal by otoscopy but otherwise unselected, were given Békésy-type audiometry through 18 kHz with the Rudmose high-frequency ear-canal insertion transducer. Minimum audible pressures (MAP) were obtained in approximate agreement with published studies on children and on adolescent males. There is also correspondence with the British 1954 Standard MAP up to 15 kHz. It is suggested that the time is approaching for standardizing audiometry in the octave 10–20 kHz by using all available data.

Military noise induced hearing loss: Problems in conservation programs

Military noise induced hearing loss: Problems in conservation programs