Auditory Hazard Assessment Algorithm for Humans: Difference between revisions

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[[Noise-induced hearing loss|Noise-induced hearing loss (NIHL)]] typically occurs when the auditory system experiences an elevation of [[Hearing threshold|hearing thresholds]] due to exposure to high-level noise, a phenomenon known as a [[Auditory fatigue|temporary threshold shift (TTS)]], and does not return to normal threshold levels.<ref>{{Cite journal|last1=Ryan|first1=Allen|last2=Kujawa|first2=Sharon|last3=Hammill|first3=Tanisha|last4=Le Prell|first4=Colleen|last5=Kil|first5=Jonathan|date=September 2016|title=Temporary and Permanent Noise-Induced Threshold Shifts: A Review of Basic and Clinical Observations|journal=Otology & Neurotology|volume=37|issue=8|pages=e271–e275|pmc=4988324|pmid=27518135|doi=10.1097/MAO.0000000000001071}}</ref> The damage to the auditory system can vary depending on the type of noise exposure. Unlike the continuous [[background noise]] often found in industrial environments, the impulse noise produced by weapons and [[Firearm|firearms]] demonstrates a very high pressure level within a very short duration of time, typically around a few milliseconds. As a result, near-field peak levels measured close to the muzzle of a weapon can range from 150 dB for handheld weapons and over 180 dB for [[heavy artillery]]. By comparison, noises from industrial settings were measured to have peak levels of 113 to 120 dB.<ref name=":3">{{Cite journal|last1=Nakashima|first1=Ann|last2=Farinaccio|first2=Rocco|date=April 2015|title=Review of Weapon Noise Measurement and Damage Risk Criteria: Considerations for Auditory Protection and Performance|journal=Military Medicine|volume=180|issue=4|pages=402–408|doi=10.7205/MILMED-D-14-00204|pmid=25826345|doi-access=free}}</ref>

In order to protect soldiers from hearing loss, the U.S. Army adhered to the [[United States Military Standard|Military Standard (MIL-STD) 1474]], which defined the maximum noise levels permitted to be produced by military systems.<ref name=":4">{{Cite journal|last1=Amrein|first1=Bruce|last2=Letowski|first2=Tomasz|date=January 2012|title=Military noise limits: How much is too much?|url=https://www.researchgate.net/publication/290297963|journal=Internoise 2012|pages=3981–3992|via=ResearchGate}}</ref><ref>{{Cite news|url=https://synergist.aiha.org/201611-noise-limits-for-warfighting|title=Noise Limits for Warfighting|last=Amrein|first=Bruce|date=December 15, 2019|work=The Synergist|access-date=January 7, 2020}}</ref> However, human volunteer studies demonstrated that the standard used since 1997, the MIL-STD-1474D, overestimated the hazard associated with impulse noise exposure.<ref name=":5">{{Cite journal|last1=Patterson|first1=James|last2=Ahroon|first2=William|date=December 2004|title=Evaluation of an Auditory Hazard Model Using Data from Human Volunteer Studies|url=https://apps.dtic.mil/docs/citations/ADA429771|archive-url=https://web.archive.org/web/20220601063500/https://apps.dtic.mil/docs/citations/ADA429771|url-status=live|archive-date=June 1, 2022|journal=U.S. Army Aeromedical Research Laboratory|id=2005-01|via=Defense Technical Information Center}}</ref> The subsequent overprotection of the ears based on inaccurate evaluations of hearing loss risk was believed to potentially hamper verbal communication between military personnel on the battlefield and reduce situational awareness.<ref name=":3" /><ref name=":4" /> The AHAAH was developed to more accurately assess the hazard to the human ear from impulse noise by incorporating the acoustic and physiological characteristics of the ear in its analysis, which were not accounted for in previous metrics.<ref name=":5" /><ref name=":6">{{Cite journal|last=Price|first=G. Richard|date=July 2011|title=The Auditory Hazard Assessment Algorithm for Humans (AHAAH): Hazard Evaluation of Intense Sounds|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a550723.pdf|archive-url=https://web.archive.org/web/20200326004945/https://apps.dtic.mil/dtic/tr/fulltext/u2/a550723.pdf|url-status=live|archive-date=March 26, 2020|journal=Army Research Laboratory|id=ARL-TR-5587}}</ref> The AHAAH wasmodel eventuallyand usedan inequivalent 2015 to completely revise the MIL-STDA-1474Eweighted andenergy establishmethod, aL_Aeq100ms new standardwere knownincluded asin the revised MIL-STD-1474E.<ref name=":2" />

== Development ==

The AHAAH model was first developed in 1987 by the U.S. Army Human Engineering Laboratory (HEL), which later became part of the [[United States Army Research Laboratory|U.S. Army Research Laboratory (ARL)]], to investigate the complex interactions between the [[Outer ear|outer]], [[Middle ear|middle]], and [[Inner ear|inner ears]] and understand the process behind hearing loss on the level of the [[cochlea]].<ref name=":0" /><ref>{{Cite journal|last1=Kalb|first1=Joel|last2=Price|first2=G. Richard|date=April 2015|title=Mathematical Model of the Ear's Response to Weapons Impulses|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a617009.pdf|archive-url=https://web.archive.org/web/20200325153332/https://apps.dtic.mil/dtic/tr/fulltext/u2/a617009.pdf|url-status=live|archive-date=March 25, 2020|journal=Army Research Laboratory|id=ARL-RP-0521|via=Defense Technical Information Center}}</ref><ref name=":7">{{Cite web|url=https://arlinside.arl.army.mil/www/default.cfm?page=344|title=Executive Summary of the Development and Validation of AHAAH|last=Price|first=G Richard|date=September 1, 2010|website=CCDC Army Research Laboratory|access-date=January 7, 2020}}</ref> Originally designed to function as an electro-acoustic model of the ear, the AHAAH was the product of numerous noise exposure experiments which, in turn, guided the direction of future studies. <ref name=":7" /> The first version of the AHAAH was modeled after pre-existing, available data on the cat ear since much of the physiological and acoustic characteristics and values for the cat were more well-known at the time compared to that of humans and could be studied more directly. Additionally, the ears of mammals were similar enough that only modest adaptations to the model were required to adjust for human ear anatomy.<ref name=":6" /> By 1997, the AHAAH was modified into a human model that accounted for the structure of the human ear. In subsequent years, the AHAAH underwent several validation tests, including The Albuquerque Studies, which was one of the largest early studies of human impulse noise exposure and led to the creation of a large systematic database that documented the effects of impulse noise on humans.<ref name=":5" /><ref name=":7" /> Results from these studies according to the developer have demonstrated that the AHAAH was correct in 95 percent of the tests with protective hearing and 96 percent of the instances for all tests. In contrast, the MIL-STD-1474D method of hazard prediction was shown to have been correct only 38 percent of the time in protected hearing tests.<ref name=":7" />

== Operation ==

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== The Albuquerque Studies ==

Conducted in the 1990s and sponsored by the [[U.S. Army Medical Research and Materiel Command]], the Albuquerque Studies were a series of human volunteer studies that aimed to establish new limits on the acceptable level of exposure to impulse noise produced by heavy weapons. The studies took place at [[Kirtland Air Force Base|Kirkland Air Force Base]] in Albuquerque, New Mexico, where participants were exposed to four different pressure-time signatures at seven different intensity levels and at various successions and sequences. The data collected from these studies formed a large database used to evaluate the performance of the AHAAH model.<ref name=":3" /><ref>{{Cite web|url=https://arlinside.arl.army.mil/www/default.cfm?page=353|title=The uniqueness of the Albuquerque data set and "Evaluation of impulse noise criteria using human volunteer data"|last=Price|first=G. Richard|date=September 1, 2010|website=CCDC Army Research Laboratory|access-date=January 7, 2020}}</ref> The experiment consisted of exposures to free-field impulse waveforms produced by explosive charges at distances of 5, 3, and 1 meters while wearing hearing protection. The 5m exposure was performed with a bare charge suspended above the ground and the subjects wore an unmodified earmuff with the left ear towards the charge. The 5m exposure was repeated with a modified earmuff that included a series of small tubes inserted through the earmuff cushion to simulate a poorly fit earmuff. The 3m and 1m exposures used the modified earmuff and the charges were detonated at the base of a tube pointed vertically. The left ears of the subjects were positioned 1m or 3m from the lip of the tube and 1 inch (2.54 cm) or 3 inches (7.62 cm) above the top edge of the tube. The fourth exposure condition was a reverberant environment with the participants seated at the end of a 3-meter long steel tube that opened into a concrete bunker. The explosive charged were detonated outside the end of the 3m tube. Various conditions were accounted for, such as the distance of the participant’s ear from the tube, the acoustics of the surrounding environment, the level of hearing protection, and the number of impulses, establishing a matrix of possible exposures. An [[audiogram]] was used before and after each exposure to measure the threshold and the resulting threshold shift. The pressure-time signatures were measured using bare gauges for all exposure conditions.<ref name=":3" /> According to the dataanalysis obtained fromof the Albuquerque Studies by AHAAH's developer, the AHAAH model model correctly predicted the acoustic hazards in 95 percent of the cases, while the MIL-STD-1474D was correct in only 38 percent of the cases and the A-weighted energy method was correct in only 25 percent of the cases. With different selection of the exposure limits, the equivalent A-weighted energy can yield similar accuracy.<ref name=":13" /> For all three approaches, the errors mainly stemmed from the methods overpredicting the danger of the hazard.<ref name=":9" />

== Controversy ==

The AHAAH is the subject of controversy in regards to its use to assess acoustic hazards.<ref name=":8" /> In 2003, a [[NATO]] research study on impulse noise found that the AHAAH produced unsatisfactory results for several exposure conditions, and the concluding report contained conflicting opinions from several experts.<ref>{{Cite journal|date=April 2003|title=Reconsideration of the Effects of Impulse Noise|journal=NATO|isbn=92-837-1105-X|id=TR-017|citeseerx=10.1.1.214.6990}}</ref> A 2010 review by the [[American Institute of Biological Sciences|American Institute of Biological Sciences (AIBS)]] also concluded that while the AHAAH model was a step in the right direction in terms of incorporating factors such as the middle ear muscle contractions in its analysis, it was not yet fully developed and validated. According to the AIBS, there were concerns as to whether the AHAAH model was capable of modeling the acoustic hazard of a complex military environment with continuous noise from various different machinery and weapons being produced simultaneously.<ref>{{Cite journal|last=American Institute of Biological Sciences|date=November 9, 2010|title=Peer Review of Injury Prevention and Reduction Research Task Area Injury Models|url=https://arlinside.arl.army.mil/www/pages/343/AHAAH_AIBS_revew_Public_Release_11Aug14.pdf|journal=Army Research Laboratory}}</ref> In 2012, a review by the [[National Institute for Occupational Safety and Health|National Institute for Occupational Safety and Health (NIOSH)]] argued that the MEM contractions that were used by the AHAAH to justify increasing the recommended maximum noise levels were not present in enough people to be applied as a valid form of analysis. The report also noted that the AHAAH did not adequately take into account the effects of secondary exposure, such as adjacent shooters and range safety personnel.<ref name=":13">{{Cite journal|last1=Murphy|first1=William|last2=Khan|first2=Amir|last3=Shaw|first3=Peter|date=December 3, 2009|title=An Analysis of the Blast Overpressure Study Data Comparing Three Exposure Criteria|url=https://www.cdc.gov/niosh/surveyreports/pdfs/309-05h.pdf|journal=U.S. Department of Health and Human Services|id=EPHB 209-05h}}</ref><ref>{{Cite journal|last1=Murphy|first1=William|last2=Kardous|first2=Chucri|date=January 10, 2012|title=A Case for Using A-Weighted Equivalent Energy as a Damage Risk Criterion|url=https://www.cdc.gov/niosh/surveyreports/pdfs/350-11a.pdf|journal=CDC Workplace Safety and Health}}</ref> As of 2015, the AHAAH model has not been adopted by the NATO community.<ref name=":3" />

Both NIOSH and the US Army Aeromedical Research Laboratories funded research to investigate the classical conditioning that has been integral to the warned AHAAH model. In the warned mode, the middle ear muscles are assumed to be already contracted. In the unwarned mode, the middle ear muscles are contracted after a loud sound exceeds a threshold of about 134 dB peak SPL. Several studies conducted between 2014 and 2020 have examined the prevalence and reliability of the MEMC. According to a nationally representative survey of more than 15,000 persons, the prevalence of the acoustic reflex measured in persons aged 18 to 30 was less than 90%.<ref name=":11">{{cite journal |last1=Flamme |first1=Gregory A. |last2=Deiters |first2=Kristy K. |last3=Tasko |first3=Stephen M. |last4=Ahroon |first4=William A. |title=Acoustic reflexes are common but not pervasive: evidence from the National Health and Nutrition Examination Survey, 1999–2012 |journal=International Journal of Audiology |date=21 November 2016 |volume=56 |issue=sup1 |pages=52–62 |doi=10.1080/14992027.2016.1257164|pmid=27869511 |s2cid=26258703 }}</ref> A follow-on study that carefully assessed 285 persons with normal hearing concluded that "acoustic reflexes are not pervasive and should not be included in damage risk criteria and health assessments for impulsive noise."<ref name=":12">{{cite journal |last1=McGregor |first1=Kara D. |last2=Flamme |first2=Gregory A. |last3=Tasko |first3=Stephen M. |last4=Deiters |first4=Kristy K. |last5=Ahroon |first5=William A. |last6=Themann |first6=Christa L. |last7=Murphy |first7=William J. |title=Acoustic reflexes are common but not pervasive: evidence using a diagnostic middle ear analyser |journal=International Journal of Audiology |date=19 December 2017 |volume=57 |issue=sup1 |pages=S42–S50 |doi=10.1080/14992027.2017.1416189|pmid=29256642 |pmc=6719315 }}</ref> The anticipatory contraction integral to the warned response is not reliable in persons with normal hearing.<ref name=Deiters>{{cite journal |last1=Deiters |first1=Kristy K. |last2=Flamme |first2=Gregory A. |last3=Tasko |first3=Stephen M. |last4=Murphy |first4=William J. |last5=Greene |first5=Nathaniel T. |last6=Jones |first6=Heath G. |last7=Ahroon |first7=William A. |title=Generalizability of clinically measured acoustic reflexes to brief sounds |journal=The Journal of the Acoustical Society of America |date=November 2019 |volume=146 |issue=5 |pages=3993–4006 |doi=10.1121/1.5132705|pmid=31795698 |pmc=7043895 |bibcode=2019ASAJ..146.3993D }}</ref><ref name=Jones>{{cite journal |last1=Jones |first1=Heath G. |last2=Greene |first2=Nathaniel T. |last3=Ahroon |first3=William A. |title=Human middle-ear muscles rarely contract in anticipation of acoustic impulses: Implications for hearing risk assessments |journal=Hearing Research |date=July 2019 |volume=378 |pages=53–62 |doi=10.1016/j.heares.2018.11.006|pmid=30538053 |s2cid=54445405 }}</ref> The completion of the USAARL live fire exposure study demonstrated that the early activation of the MEMC was not present in 18 of 19 subjects during tests with an M4-rifle using live ammunition. Experienced shooters according to the hypothesis of the AHAAH developers would exhibit an early contraction that precedes the trigger pull. The warned hypothesis was demonstrated to be insufficiently prevalent to merit including the MEMC in subsequent damage risk criteria.<ref>Gregory A. Flamme, Kristy K. Deiters, Stephen M. Tasko, Madeline V. Smith, Heath G. Jones, William J. Murphy, Nathaniel T. Greene, William A. Ahroon SASRAC Technical Report #1909_0 Pervasiveness of early Middle Ear Muscle Contraction, (SASRAC, Loveland OH)</ref>