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The '''Auditory Hazard Assessment Algorithm for Humans (AHAAH)''' is a mathematical model of the [[human auditory system]] that calculates the risk to [[Hearing|human hearing]] caused by exposure to [[Impulse noise (acoustics)|impulse sounds]], such as gunfire and airbag deployment. It was developed by the [[United States Army Research Laboratory|U.S. Army Research Laboratory (ARL)]] to assess the effectiveness of [[
In 2015, the AHAAH became one of the two metrics used by the [[United States Department of Defense|U.S. Department of Defense]] to approve the [[United States Military Standard|Military Standard (MIL-STD) 1474E]] for regulating maximum noise level exposure from military systems.<ref name=":8">{{Cite journal|last=Nakashima|first=Ann|date=November 2015|title=A comparison of metrics for impulse noise exposure|url=https://cradpdf.drdc-rddc.gc.ca/PDFS/unc206/p802859_A1b.pdf|journal=Defence Research and Development Canada|id=DRDC-RDDC-2015-R243}}</ref><ref name=":2">{{Cite conference|last=Amrein|first=Bruce|date=May 2016|title=Military standard 1474E: Design criteria for noise limits vs. operational effectiveness|url=https://www.researchgate.net/publication/303538151|conference=Proceedings of Meetings on Acoustics|volume=25|pages=040005|doi=10.1121/2.0000207|via=ResearchGate|doi-access=free}}</ref> It is also used by the [[Society of Automotive Engineers]] to calculate the hazard of airbag noise and by the [[Israel Defense Forces|Israeli Defense Force]] for impulse noise analysis.<ref>{{Cite journal|last1=Price|first1=G. Richard|last2=Kalb|first2=Joel|date=2015|title=Development of the auditory hazard assessment algorithm for humans model for accuracy and power in MIL-STD-1474E's hearing analysis|journal=The Journal of the Acoustical Society of America|volume=138|issue=1774|pages=1774|doi=10.1121/1.4933615|bibcode=2015ASAJ..138.1774P}}</ref>
== Overview ==
[[Noise-induced hearing loss|Noise-induced hearing loss (NIHL)]] typically occurs when the auditory system experiences an elevation of [[
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
== 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 [[
== Operation ==
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The AHAAH model consisted of a set of proven algorithms that accounted for a variety of exposure conditions that influenced the risk of a permanent threshold risk, such as noise attenuation caused by hearing protection devices and [[Acoustic reflex|reflexive middle ear muscle (MEM)]] contractions that occur before the onset of the stimulus being received that reduce the damage to the ear in preparation of the sound.<ref name=":8" /><ref name=":10">{{Cite journal|last1=Amrein|first1=Bruce|last2=Letowski|first2=Tomasz|date=January 2011|title=Predicting and ameliorating the effect of very intense sounds on the ear: The auditory hazard assessment algorithm for humans (AHAAH)|url=https://www.researchgate.net/publication/301511369|journal=NATO|id=RTO-MP-HFM-207}}</ref> Unlike previous energy-based damage models, the AHAAH could also accurately predict the scope of the damage by analyzing the pressure-time dependence of the [[Sound|sound wave]]. Through this method, the model was able to determine why a low level of energy at the [[ear canal]] entrance was much more hazardous than a higher level of energy at the ear canal entrance of an ear protected by [[Earmuffs|ear muffs]]. The model discovered that the former featured a different pressure-time dependence than the latter that was able to more efficiently transfer energy through the middle ear.<ref>{{Cite journal|last1=Fedele|first1=Paul|last2=Kalb|first2=Joel|date=April 2015|title=Level-Dependent Nonlinear Hearing Protector Model in the Auditory Hazard Assessment Algorithm for Humans|url=https://apps.dtic.mil/docs/citations/ADA622427|archive-url=https://web.archive.org/web/20220601063815/https://apps.dtic.mil/docs/citations/ADA622427|url-status=live|archive-date=June 1, 2022|journal=Army Research Laboratory|id=ARL-TR-7271|via=Defense Technical Information Center}}</ref> The MEMC are not prevalent in the population of normal hearing persons, 95% prevalence with a 95% confidence interval.<ref name=":11" /><ref name=":12" /> The early activation of the MEMC occurred in 2 out of 50 subjects in a countdown study.<ref name="Jones" />
Depending on the presence of hearing protection devices, whether the sound came unexpectedly, and where the sound originated—whether in free field, at the ear canal entrance, or at the eardrum position—the AHAAH model could predict the displacements in the inner ear because it was conformal with the structure of the human ear.<ref name=":10" /> For free field, the model assumed that the sound arrived straight down the ear canal and calculated the pressure history at the eardrum, taking in the energy transferred to the [[stapes]] as input to the inner ear. For waves recorded at the ear canal entrance or at the eardrum, the model took into account the proper origin point of the sound in the circuit diagram. The displacement of the basilar membrane is calculated from the displacement of the stapes and the AHU is then determined by measuring the total displacement of the waves at 23 different locations on the [[organ of Corti]] in the inner ear.<ref>{{Cite web|url=https://arlinside.arl.army.mil/www/default.cfm?page=354|title=Functional description of the AHAAH mode|date=September 1, 2010|website=CCDC Army Research Laboratory|access-date=January 7, 2020}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</ref> The effect of the impulse sound can be displayed to create a visual representation of the damage process as it occurs.<ref name=":0" /><ref name=":1" />
== 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}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</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
== 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}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</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
== References ==
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