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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 [[Hearing protection device|hearing protection devices]] and aid the design of machinery and weapons to make them safer for the user.<ref name=":0">{{Cite web|url=https://arlinside.arl.army.mil/www/default.cfm?page=343|title=Auditory Hazard Assessment Algorithm for Humans (AHAAH)|last=|first=|date=September 24, 2015|website=CCDC Army Research Laboratory|url-status=live|archive-url=|archive-date=|access-date=January 6, 2020}}</ref><ref name=":1">{{Cite journal|last=Fedele|first=Paul|last2=Binseel|first2=Mary|last3=Kalb|first3=Joel|last4=Price|first4=G. Richard|date=December 2013|title=Using the Auditory Hazard Assessment Algorithm for Humans (AHAAH) With Hearing Protection Software, Release MIL-STD-1474E|url=https://apps.dtic.mil/docs/citations/ADA592047|journal=Army Research Laboratory|volume=|pages=|id=ARL-TR-6748|via=Defense Technical Information Center}}</ref>
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|format=PDF|journal=Defence Research and Development Canada|volume=|pages=|id=DRDC-RDDC-2015-R243|via=}}</ref><ref name=":2">{{Cite journal|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_Military_standard_1474E_Design_criteria_for_noise_limits_vs_operational_effectiveness|journal=Proceedings of Meetings on Acoustics|volume=25|pages=|doi=10.1121/2.0000207|via=ResearchGate}}</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|last=Price|first=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|url=https://asa.scitation.org/doi/10.1121/1.4933615|journal=The Journal of the Acoustical Society of America|volume=138|issue=1774|pages=|via=}}</ref>
== Overview ==
[[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|last=Ryan|first=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|url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4988324/|journal=Otology & Neurotology|volume=37|issue=8|pages=e271-e275|via=NCBI}}</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|last=Nakashima|first=Ann|last2=Farinaccio|first2=Rocco|date=April 2015|title=Review of Weapon Noise Measurement and Damage Risk Criteria: Considerations for Auditory Protection and Performance|url=https://academic.oup.com/milmed/article/180/4/402/4160429|journal=Military Medicine|volume=180|issue=4|pages=402-408|via=Oxford Academic}}</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|last=Amrein|first=Bruce|last2=Letowski|first2=Tomasz|date=January 2012|title=Military noise limits: How much is too much?|url=https://www.researchgate.net/publication/290297963_Military_noise_limits_How_much_is_too_much|journal=Internoise 2012|volume=|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|url-status=live}}</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|last=Patterson|first=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|journal=U.S. Army Aeromedical Research Laboratory|volume=|pages=|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|format=PDF|journal=Army Research Laboratory|volume=|pages=|id=ARL-TR-5587|via=}}</ref> The AHAAH was eventually used in 2015 to completely revise the MIL-STD-1474E and establish a new standard known as the MIL-STD-1474E.<ref name=":2" />
== Development ==
The AHAAH 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|last=Kalb|first=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|format=PDF|journal=Army Research Laboratory|volume=|pages=|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|url-status=live|archive-url=|archive-date=|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 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 ==
The AHAAH calculated the auditory hazard of impulse sounds by modelling their transmission based on how it interacted with an electroacoustic model of the [[basilar membrane]] in the [[cochlea]]. This wave motion analysis relied on the [[Wentzel-Kramers-Brillouin approximation|Wentzel-Kramers-Brillouin wave dynamics method]], which was normally used to solve wave function problems in [[quantum mechanics]] and [[solid-state physics]] for several decades. The computer program could calculate the acoustic hazards from the free-field, through hearing protection, and through the middle ear where the hearing damage typically occurred. The AHAAH represented the output in auditory risk units (ARUs), which related to the damage caused by displacements of the basilar membrane in the inner ear at 23 different locations. According to the model, the recommended limit for daily occupational exposures were 200 ARUs, while any dose greater than 500 ARUs were predicted to produce permanent hearing loss.<ref name=":1" /><ref name=":9">{{Cite journal|last=De Paolis|first=Annalisa|last2=Bikson|first2=Marom|last3=Nelson|first3=Jeremy|last4=de Ru|first4=J. Alexander|last5=Packer|first5=Mark|last6=Cardoso|first6=Luis|date=June 2017|title=Analytical and numerical modeling of the hearing system: Advances towards the assessment of hearing damage|url=https://www.sciencedirect.com/science/article/pii/S0378595516302787?via%3Dihub|journal=Hearing Research|volume=349|pages=111-128|via=ScienceDirect}}</ref>
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|last=Amrein|first=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/profile/Tomasz_Letowski5/publication/301511369_Predicting_and_ameliorating_the_effect_of_very_intense_sounds_on_the_ear_The_auditory_hazard_assessment_algorithm_for_humans_AHAAH/links/5716ec1e08aeefeb022c3f3b/Predicting-and-ameliorating-the-effect-of-very-intense-sounds-on-the-ear-The-auditory-hazard-assessment-algorithm-for-humans-AHAAH.pdf|journal=NATO|volume=|pages=|id=RTO-MP-HFM-207|via=}}</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 Wave|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|last=Fedele|first=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|journal=Army Research Laboratory|volume=|pages=|id=ARL-TR-7271|via=Defense Technical Information Center}}</ref>
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|last=|first=|date=September 1, 2010|website=CCDC Army Research Laboratory|url-status=live|archive-url=|archive-date=|access-date=January 7, 2020}}</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|url-status=live|archive-url=|archive-date=|access-date=January 7, 2020}}</ref> During the experiment, the participants were seated at the end of a 3-meter long steel tube, where at the other end, various explosive materials were detonated. 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 data obtained from the Albuquerque Studies, the AHAAH 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. For all three approaches, the errors mainly stemmed from the methods overpredicting the danger of the hazard.<ref name=":9" />
== Controversy ==
The AHAAH was the subject of controversy in regards to its use as the universal metric for 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|last=|first=|date=April 2003|title=Reconsideration of the Effects of Impulse Noise|url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.214.6990&rep=rep1&type=pdf|format=PDF|journal=NATO|volume=|pages=|isbn=92-837-1105-X|id=TR-017|via=}}</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|first=|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|volume=|pages=|via=}}</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>{{Cite journal|last=Murphy|first=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|format=PDF|journal=U.S. Department of Health and Human Services|volume=|pages=|id=EPHB 209-05h|via=}}</ref><ref>{{Cite journal|last=Murphy|first=William|last2=Kardous|first2=Chucri|date=January 10, 2012|title=A Case for Using A-Weight Equivalent Energy as a Damage Risk Criterion|url=https://www.cdc.gov/niosh/surveyreports/pdfs/350-11a.pdf|journal=CDC Workplace Safety and Health|volume=|pages=|via=}}</ref> As of 2015, the AHAAH model has not been adopted by the NATO community.<ref name=":3" />
== References ==
{{reflist}}
[[Category:Military of the United States standards]]
[[Category:Hearing loss]]
[[Category:Audiology]]
[[Category:Occupational safety and health]]
[[Category:Deafness]]
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