The quest for a damage risk criterion for hazardous noise exposure.

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Effects of unilateral eye closure on middle ear muscle contractions.

Middle ear muscle contractions (MEMCs) are most commonly considered a response to high-level acoustic stimuli. However, MEMCs have also been observed in the absence of sound, either as a response to somatosensory stimulation or in concert with other motor activity. The relationship between MEMCs and non-acoustic sources is unclear. This study examined associations between measures of voluntary unilateral eye closure and impedance-based measures indicative of middle ear muscle activity while controlling for demographic and clinical factors in a large group of participants (N=190) with present clinical acoustic reflexes and no evidence of auditory dysfunction. Participants were instructed to voluntarily close the eye ipsilateral to the ear canal containing a detection probe at three levels of effort. Orbicularis oculi muscle activity was measured using surface electromyography. Middle ear muscle activity was inferred from changes in total energy reflected in the ear canal using a filtered (0.2 to 8 kHz) click train. Results revealed that middle ear muscle activity was positively associated with eye muscle activity. MEMC occurrence rates for eye closure observed in this study were generally higher than previously published rates for high-level brief acoustic stimuli in the same participant pool suggesting that motor activity may be a more reliable elicitor of MEMCs than acoustic stimuli. These results suggest motor activity can serve as a confounding factor for auditory exposure studies as well as complicate the interpretation of any impulsive noise damage risk criteria that assume MEMCs serve as a consistent, uniform protective factor. The mechanism linking eye and middle ear muscle activity is not understood and is an avenue for future research.

Population-based age adjustment tables for use in occupational hearing conservation programs.

Objective: In occupational hearing conservation programmes, age adjustments may be used to subtract expected age effects. Adjustments used in the U.S. came from a small dataset and overlooked important demographic factors, ages, and stimulus frequencies. The present study derived a set of population-based age adjustment tables and validated them using a database of exposed workers.Design: Cross-sectional population-based study and retrospective longitudinal cohort study for validation.Study sample: Data from the U.S. National Health and Nutrition Examination Survey (unweighted n = 9937) were used to produce these tables. Male firefighters and emergency medical service workers (76,195 audiograms) were used for validation.Results: Cross-sectional trends implied less change with age than assumed in current U.S. regulations. Different trends were observed among people identifying with non-Hispanic Black race/ethnicity. Four age adjustment tables (age range: 18-85) were developed (women or men; non-Hispanic Black or other race/ethnicity). Validation outcomes showed that the population-based tables matched median longitudinal changes in hearing sensitivity well.Conclusions: These population-based tables provide a suitable replacement for those implemented in current U.S. regulations. These tables address a broader range of worker ages, account for differences in hearing sensitivity across race/ethnicity categories, and have been validated for men using longitudinal data.

Generalizability of clinically measured acoustic reflexes to brief sounds.

Middle ear muscle contractions (MEMC) can be elicited in response to high-level sounds, and have been used clinically as acoustic reflexes (ARs) during evaluations of auditory system integrity. The results of clinical AR evaluations do not necessarily generalize to different signal types or durations. The purpose of this study was to evaluate the likelihood of observing MEMC in response to brief sound stimuli (tones, recorded gunshots, noise) in adult participants (N = 190) exhibiting clinical ARs and excellent hearing sensitivity. Results revealed that the presence of clinical ARs was not a sufficient indication that listeners will also exhibit MEMC for brief sounds. Detection rates varied across stimulus types between approximately 20% and 80%. Probabilities of observing MEMC also differed by clinical AR magnitude and latency, and declined over the period of minutes during the course of the MEMC measurement series. These results provide no support for the inclusion of MEMC as a protective factor in damage-risk criteria for impulsive noises, and the limited predictability of whether a given individual will exhibit MEMC in response to a brief sound indicates a need to measure and control for MEMC in studies evaluating pharmaceutical interventions for hearing loss.

Hearing Loss Among World Trade Center Firefighters and Emergency Medical Service Workers.

OBJECTIVE: To determine if World Trade Center (WTC) exposure is associated with hearing loss. METHODS: Logistic regression to evaluate the immediate impact of WTC exposure and parametric survival analysis to assess longitudinal outcomes. RESULTS: Those arriving on the morning of September 11, 2001 had elevated odds of low-frequency (odds ratio [OR]: 1.24; 95% confidence interval [CI]: 1.04 to 1.47) and high-frequency (OR: 1.16; 95% CI: 1.02 to 1.31) hearing loss at their first post-September 11, 2001 examination. Longitudinally, participants arriving before September 13, 2001 and spending more than or equal to 6 months at the WTC-site had greater risk of hearing loss in the low frequencies (risk ratio [RR]: 1.31; 95% CI: 1.05 to 1.60) and high frequencies (RR: 1.37; 95% CI: 1.22 to 1.54). By 2016, 3194 (37%) had abnormal hearing sensitivity in either ear and 1751 (20%) in both ears. CONCLUSIONS: More heavily WTC-exposed workers were at increased risk of hearing loss, and group differences persisted for at least 15 years. Those with abnormal hearing sensitivity may benefit from interventions such as hearing aids and other rehabilitation.

Persistent Hearing Loss among World Trade Center Health Registry Residents, Passersby and Area Workers, 2006-2007.

BACKGROUND: Prior studies have found that rescue and recovery workers exposed to the 9/11 World Trade Center (WTC) disaster have evidence of increased persistent hearing and other ear-related problems. The potential association between WTC disaster exposures and post-9/11 persistent self-reported hearing problems or loss among non-rescue and recovery survivors has not been well studied. METHODS: We used responses to the World Trade Center Health Registry (Registry) enrollment survey (2003-2004) and first follow-up survey (2006-2007) to model the association between exposure to the dust cloud and persistent hearing loss (n = 22,741). RESULTS: The prevalence of post-9/11 persistent hearing loss among survivors was 2.2%. The adjusted odds ratio (aOR) of hearing loss for those who were in the dust cloud and unable to hear was 3.0 (95% CI: 2.2, 4.0). Survivors with persistent sinus problems, headaches, PTSD and chronic disease histories had an increased prevalence of reported hearing problems compared to those without symptoms or chronic problems. CONCLUSIONS: In a longitudinal study, we observed an association between WTC-related exposures and post-9/11 self-reported hearing loss among disaster survivors.

Auditory risk of exposure to ballistic N-waves from bullets.

OBJECTIVE: Assessment of the auditory risk associated with sound from ballistic N-waves produced by a rifle bullet. DESIGN: Acoustical recordings of ballistic N-waves passing through a microphone array at 6.4 metres down range were analysed to determine (a) the trajectory of the bullet, (b) the distance between the trajectory and each microphone (less than 1.3 m), and (c) the numbers of permissible exposures according to both damage-risk criteria for impulsive noise in the current U.S. military standard (MIL-STD-1474E). STUDY SAMPLE: The gun was an AR-15 style semiautomatic rifle configured to fire a 0.50 calibre Beowulf00AE cartridge. Four sample shots were recorded for each of four microphone spacing conditions and five kinds of ammunition (80 shots in total). RESULTS: The ballistic N-waves recorded in this study would constitute a significant auditory risk to unprotected listeners at all distances sampled. The numbers of permissible exposures decreased as the distance to the bullet trajectory decreased, decreased with increased bullet length, and departed from linear increases as the bullet velocity increased. CONCLUSIONS: Unprotected exposure to a ballistic N-wave from a supersonic 0.50 calibre bullet presents a significant risk to hearing at distances of 6.4 metres down range and through trajectories within 1.2 metres of an ear.

Kids Nowadays Hear Better Than We Did: Declining Prevalence of Hearing Loss in US Youth, 1966-2010.

OBJECTIVES/HYPOTHESIS: To investigate factors associated with hearing impairment (HI) in adolescent youths during the period 1966-2010. STUDY DESIGN: Cross-sectional analyses of US sociodemographic, health, and audiometric data spanning 5 decades. METHODS: Subjects were youths aged 12 to 17 years who participated in the National Health Examination Survey (NHES Cycle 3, 1966-1970; n = 6,768) and youths aged 12 to 19 years in the Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994; n = 3,057) and NHANES (2005-2010; n = 4,374). HI prevalence was defined by pure-tone average (PTA) ≥ 20 dB HL for speech frequencies (0.5, 1, 2, and 4 kHz) and high frequencies (3, 4, and 6 kHz). Multivariable logistic models were used to estimate the odds ratio (OR) and 95% confidence interval (CI). RESULTS: Overall speech-frequency HI prevalence was 10.6% (95% CI: 9.7%-11.6%) in NHES, 3.9% (95% CI: 2.8%-5.5%) in NHANES III, and 4.5% (95% CI: 3.7%-5.4%) in NHANES 2005 to 2010. The corresponding high-frequency HI prevalences were 32.8% (95% CI: 30.8%-34.9%), 7.3% (95% CI: 5.9%-9.0%), and 7.9% (95% CI: 6.8%-9.2%). After adjusting for sociodemographic factors, overall high-frequency HI was increased twofold for males and cigarette smoking. Other significant risk factors in NHANES 2005 to 2010 included very low birth weight, history of ear infections/otitis media, ear tubes, fair/poor general health, and firearms use. CONCLUSIONS: HI declined considerably between 1966 to 1970 and 1988 to 1994, with no additional decline between 1988 to 1994 and 2005 to 2010. Otitis media history was a significant HI risk factor each period, whereas very low birth weight emerged as an important risk factor after survival chances improved. Reductions in smoking, job-related noise, and firearms use may partially explain the reduction in high-frequency HI. Loud music exposure may have increased, but does not account for HI differences. LEVEL OF EVIDENCE: NA Laryngoscope, 129:1922-1939, 2019.

The reduction of gunshot noise and auditory risk through the use of firearm suppressors and low-velocity ammunition.

OBJECTIVE: This research assessed the reduction of peak levels, equivalent energy and sound power of firearm suppressors. DESIGN: The first study evaluated the effect of three suppressors at four microphone positions around four firearms. The second study assessed the suppressor-related reduction of sound power with a 3 m hemispherical microphone array for two firearms. RESULTS: The suppressors reduced exposures at the ear between 17 and 24 dB peak sound pressure level and reduced the 8 h equivalent A-weighted energy between 9 and 21 dB depending upon the firearm and ammunition. Noise reductions observed for the instructor's position about a metre behind the shooter were between 20 and 28 dB peak sound pressure level and between 11 and 26 dB LAeq,8h. Firearm suppressors reduced the measured sound power levels between 2 and 23 dB. Sound power reductions were greater for the low-velocity ammunition than for the same firearms fired with high-velocity ammunition due to the effect of N-waves produced by a supersonic bullet. CONCLUSIONS: Firearm suppressors may reduce noise exposure, and the cumulative exposures of suppressed firearms can still present a significant hearing risk. Therefore, firearm users should always wear hearing protection whenever target shooting or hunting.

Acoustic reflexes are common but not pervasive: evidence using a diagnostic middle ear analyser.

OBJECTIVE: The objective of this study is to determine whether acoustic reflexes are pervasive (i.e. known with 95% confidence to be observed in at least 95% of people) by examining the frequency of occurrence using a friction-fit diagnostic middle ear analyser. DESIGN: Adult participants with very good hearing sensitivity underwent audiometric and middle ear testing. Acoustic reflexes were tested ipsilaterally and contralaterally in both ears across a range of elicitor frequencies. Reflex elicitors were 700 ms tones presented at maximum level of 100 dB HL. Two automated methods were used to detect the presence of an acoustic reflex. STUDY SAMPLE: A group of 285 adult volunteers with normal hearing. RESULTS: There were no conditions in which the proportion of participants exhibiting acoustic reflexes was high enough to be deemed pervasive. Ipsilateral reflexes were more likely to be observed than contralateral reflexes and reflexes were more common at 0.5 and 1 kHz elicitor frequencies as compared with 2 and 4 kHz elicitor frequencies. CONCLUSIONS: Acoustic reflexes are common among individuals with good hearing. However, acoustic reflexes are not pervasive and should not be included in damage risk criteria and health hazard assessments for impulsive noise.

Prevention of Noise-Induced Hearing Loss from Recreational Firearms.

In the United States and other parts of the world, recreational firearm shooting is a popular sport that puts the hearing of the shooter at risk. Peak sound pressure levels (SPLs) from firearms range from ∼140 to 175 dB. The majority of recreational firearms (excluding small-caliber 0.17 and 0.22 rifles and air rifles) generate between 150 and 165 dB peak SPLs. High-intensity impulse sounds will permanently damage delicate cochlear structures, and thus individuals who shoot firearms are at a higher risk of bilateral, high-frequency, noise-induced hearing loss (NIHL) than peer groups who do not shoot. In this article, we describe several factors that influence the risk of NIHL including the use of a muzzle brake, the number of shots fired, the distance between shooters, the shooting environment, the choice of ammunition, the use of a suppressor, and hearing protection fit and use. Prevention strategies that address these factors and recommendations for specialized hearing protectors designed for shooting sports are offered. Partnerships are needed between the hearing health community, shooting sport groups, and wildlife conservation organizations to develop and disseminate accurate information and promote organizational resources that support hearing loss prevention efforts.

Persistent Post-9/11 Hearing Problems Among World Trade Center Health Registry Rescue and Recovery Workers, 2001 to 2007.

OBJECTIVE: To examine the association between 9/11-related exposures and self-reported hearing problems among 16,579 rescue/recovery workers in the World Trade Center (WTC) Health Registry. METHODS: Using Registry Waves 1 (2003 to 2004) and 2 (2006 to 2007), we modeled the association between two metrics of 9/11-related exposures and hearing difficulties. RESULTS: The prevalence of incident, persistent hearing problems was 4.4%. In a fully adjusted model, workers with higher environmental hazards scores were twice as likely (interquartile range OR 2.1; 95% confidence interval [CI] 1.8, 2.5) to report hearing problems. Based on the same fully adjusted model, workers unable to hear in the dust cloud were 2.3 (95% CI 1.8, 3.0) times more likely to report hearing problems as compared with workers not in the dust cloud. CONCLUSIONS: We observed a consistent association between WTC-related exposures and self-reported hearing problems among rescue/recovery workers.

Declining Prevalence of Hearing Loss in US Adults Aged 20 to 69 Years.

Importance: As the US population ages, effective health care planning requires understanding the changes in prevalence of hearing loss. Objective: To determine if age- and sex-specific prevalence of adult hearing loss has changed during the past decade. Design, Setting, and Participants: We analyzed audiometric data from adults aged 20 to 69 years from the 2011-2012 cycle of the US National Health and Nutrition Examination Survey, a cross-sectional, nationally representative interview and examination survey of the civilian, noninstitutionalized population, and compared them with data from the 1999-2004 cycles. Logistic regression was used to examine unadjusted, age- and sex-adjusted, and multivariable-adjusted associations with demographic, noise exposure, and cardiovascular risk factors. Data analysis was performed from April 28 to June 3, 2016. Interventions: Audiometry and questionnaires. Main Outcomes and Measures: Speech-frequency hearing impairment (HI) defined by pure-tone average of thresholds at 4 frequencies (0.5, 1, 2, and 4 kHz) greater than 25 decibels hearing level (HL), and high-frequency HI defined by pure-tone average of thresholds at 3 frequencies (3, 4, and 6 kHz) greater than 25 decibels HL. Results: Based on 3831 participants with complete threshold measurements (1953 men and 1878 women; mean [SD] age, 43.6 [14.4] years), the 2011-2012 nationally weighted adult prevalence of unilateral and bilateral speech-frequency HI was 14.1% (27.7 million) compared with 15.9% (28.0 million) for the 1999-2004 cycles; after adjustment for age and sex, the difference was significant (odds ratio [OR], 0.70; 95% CI, 0.56-0.86). Men had nearly twice the prevalence of speech-frequency HI (18.6% [17.8 million]) as women (9.6% [9.7 million]). For individuals aged 60 to 69 years, speech-frequency HI prevalence was 39.3% (95% CI, 30.7%-48.7%). In adjusted multivariable analyses for bilateral speech-frequency HI, age was the major risk factor (60-69 years: OR, 39.5; 95% CI, 10.5-149.4); however, male sex (OR, 1.8; 95% CI, 1.1-3.0), non-Hispanic white (OR, 2.3; 95% CI, 1.3-3.9) and non-Hispanic Asian race/ethnicity (OR, 2.1; 95% CI, 1.1-4.2), lower educational level (less than high school: OR, 4.2; 95% CI, 2.1-8.5), and heavy use of firearms (≥1000 rounds fired: OR, 1.8; 95% CI, 1.1-3.0) were also significant risk factors. Additional associations for high-frequency HI were Mexican-American (OR, 2.0; 95% CI, 1.3-3.1) and other Hispanic race/ethnicity (OR, 2.4; 95% CI, 1.4-4.0) and the combination of loud and very loud noise exposure occupationally and outside of work (OR, 2.4; 95% CI, 1.4-4.2). Conclusions and Relevance: Adult hearing loss is common and associated with age, other demographic factors (sex, race/ethnicity, and educational level), and noise exposure. Age- and sex-specific prevalence of HI continues to decline. Despite the benefit of delayed onset of HI, hearing health care needs will increase as the US population grows and ages.

Acoustic reflexes are common but not pervasive: evidence from the National Health and Nutrition Examination Survey, 1999-2012.

OBJECTIVE: To determine whether acoustic reflexes are pervasive (i.e. sufficiently prevalent to provide 95% confidence of at least 95% prevalence) and might be invoked in damage-risk criteria (DRC) and health hazard assessments (HHA) for impulsive noise. DESIGN: Cross-sectional analyses of a nationally-representative study. STUDY SAMPLE: National Health and Nutrition Examination Survey (NHANES) data collected between 1999 and 2012 were used. Over 60 thousand reflex traces obtained from 15,106 NHANES participants were used in the study, along with demographic, audiometric, health and exposure variables obtained in that study. RESULTS: Acoustic reflexes were not sufficiently prevalent to be deemed pervasive by any detection method or in any subgroup defined by age or audiometric characteristics. The odds of observing acoustic reflexes were greater for women, young adults, and people with better hearing sensitivity. Abnormally high tympanometric admittance and "Other" race/ethnicity (i.e. people who do not self-identify as exclusively Non-Hispanic White, Non-Hispanic Black, Mexican-American, or Hispanic) were associated with lower odds. CONCLUSIONS: Acoustic reflexes are not sufficiently prevalent to be included in DRC and HHA for impulsive noise.

Auditory risk of air rifles.

OBJECTIVE: To characterize the impulse noise exposure and auditory risk for air rifle users for both youth and adults. DESIGN: Acoustic characteristics were examined and the auditory risk estimates were evaluated using contemporary damage-risk criteria for unprotected adult listeners and the 120-dB peak limit and LAeq75 exposure limit suggested by the World Health Organization (1999) for children. STUDY SAMPLE: Impulses were generated by nine pellet air rifles and one BB air rifle. RESULTS: None of the air rifles generated peak levels that exceeded the 140 dB peak limit for adults, and eight (80%) exceeded the 120 dB peak SPL limit for youth. In general, for both adults and youth, there is minimal auditory risk when shooting fewer than 100 unprotected shots with pellet air rifles. Air rifles with suppressors were less hazardous than those without suppressors, and the pellet air rifles with higher velocities were generally more hazardous than those with lower velocities. CONCLUSION: To minimize auditory risk, youth should utilize air rifles with an integrated suppressor and lower velocity ratings. Air rifle shooters are advised to wear hearing protection whenever engaging in shooting activities in order to gain self-efficacy and model appropriate hearing health behaviors necessary for recreational firearm use.

Stimulus and transducer effects on threshold.

OBJECTIVE: This study examined differences in thresholds obtained under Sennheiser HDA200 circumaural earphones using pure tone, equivalent rectangular noise bands, and 1/3 octave noise bands relative to thresholds obtained using Telephonics TDH-39P supra-aural earphones. DESIGN: Thresholds were obtained via each transducer and stimulus condition six times within a 10-day period. STUDY SAMPLE: Forty-nine adults were selected from a prior study to represent low, moderate, and high threshold reliability. RESULTS: The results suggested that (1) only small adjustments were needed to reach equivalent TDH-39P thresholds, (2) pure-tone thresholds obtained with HDA200 circumaural earphones had reliability equal to or better than those obtained using TDH-39P earphones, (3) the reliability of noise-band thresholds improved with broader stimulus bandwidth and was either equal to or better than pure-tone thresholds, and (4) frequency-specificity declined with stimulus bandwidths greater than one equivalent rectangular band, which could complicate early detection of hearing changes that occur within a narrow frequency range. CONCLUSIONS: These data suggest that circumaural earphones such as the HDA200 headphones provide better reliability for audiometric testing as compared to the TDH-39P earphones. These data support the use of noise bands, preferably ERB noises, as stimuli for audiometric monitoring.

Auditory risk estimates for youth target shooting.

OBJECTIVE: To characterize the impulse noise exposure and auditory risk for youth recreational firearm users engaged in outdoor target shooting events. The youth shooting positions are typically standing or sitting at a table, which places the firearm closer to the ground or reflective surface when compared to adult shooters. DESIGN: Acoustic characteristics were examined and the auditory risk estimates were evaluated using contemporary damage-risk criteria for unprotected adult listeners and the 120-dB peak limit suggested by the World Health Organization (1999) for children. STUDY SAMPLE: Impulses were generated by 26 firearm/ammunition configurations representing rifles, shotguns, and pistols used by youth. Measurements were obtained relative to a youth shooter's left ear. RESULTS: All firearms generated peak levels that exceeded the 120 dB peak limit suggested by the WHO for children. In general, shooting from the seated position over a tabletop increases the peak levels, LAeq8 and reduces the unprotected maximum permissible exposures (MPEs) for both rifles and pistols. Pistols pose the greatest auditory risk when fired over a tabletop. CONCLUSION: Youth should utilize smaller caliber weapons, preferably from the standing position, and always wear hearing protection whenever engaging in shooting activities to reduce the risk for auditory damage.

Short-term variability of pure-tone thresholds obtained with TDH-39P earphones.

OBJECTIVE: To estimate the short-term variability and correlates of variability in pure-tone thresholds obtained using audiometric equipment designed for occupational use, and to examine the justification for excluding 8 kHz as a mandatory threshold in occupational hearing conservation programs. METHOD: Pure-tone thresholds and other hearing-related tests (e.g. noise dosimetry, otoscopy, middle-ear assessment) were conducted with a group of 527 adults between 20 and 69 years of age. Five measurement visits were completed by participants within 14 days. RESULTS: The 50% critical difference boundaries were - 5 and 0 dB at 4 kHz and below and - 5 and 5 dB at 6 and 8 kHz. The likelihood of spurious notches due to test-retest variability was substantially lower than the likelihood of failing to detect a notched configuration when present. Correlates of variability included stimulus frequency, baseline threshold, acoustic reflectance of the ear, average noise exposure during the previous eight hours, age, and the tester's level of education in audiology. CONCLUSION: The short-term variability in 8-kHz pure-tone thresholds obtained with the TDH-39P earphone was slightly greater than at other frequencies, but this difference was not large enough to justify the disadvantages stemming from the inability to detect a 6-kHz notch.

Impulse noise generated by starter pistols.

OBJECTIVE: This study describes signals generated by .22 and .32 caliber starter pistols in the context of noise-induced hearing loss risk for sports officials and athletes. DESIGN: Acoustic comparison of impulses generated from typical .22 and .32 caliber starter pistols firing blanks were made to impulses generated from comparable firearms firing both blanks and live rounds. Acoustic characteristics are described in terms of directionality and distance from the shooter in a simulated outdoor running track. Metrics include peak sound pressure levels (SPL), A-weighted equivalent 8-hour level (L(eqA8)), and maximum permissible number of individual shots, or maximum permissible exposures (MPE) for the unprotected ear. RESULTS: Starter pistols produce peak SPLs above 140 dB. The numbers of MPEs are as few as five for the .22-caliber starter pistol, and somewhat higher (≤ 25) for the .32-caliber pistol. CONCLUSION: The impulsive sounds produced by starter pistols correspond to MPE numbers that are unacceptably small for unprotected officials and others in the immediate vicinity of the shooter. At the distances included in this study, the risk to athletes appears to be low (when referencing exposure criteria for adults), but the sound associated with the starter pistol will contribute to the athlete's overall noise exposure.