Auditory Hazard Assessment Algorithm for Humans: Difference between revisions

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The first military standard ([[United States Military Standard|MIL-STD]]) on sound was published in 1984 and underwent revision in 1997 to become MIL-STD-1474D. In 2015, this evolved to become MIL-STD-1474E which, as of 2018, remains to be the guidelines for United States’ military defense weaponry development and usage. In this standard, the [[United States Department of Defense]] established guidelines for steady state noise, impulse noise, aural non-detectability, aircraft and aerial systems, and shipboard noise. Unless marked with warning signage, steady state and impulse noises are not to exceed 85 [[Decibel|decibels]] A-weighted (dBA) and, if wearing protection, 140 decibels (dBP) respectively.<ref name="Amrein" />

The AHAAH model incorporates two unique features, the activation of the middle ear muscle contraction (MEMC) and the limitation of the motion of the stapes via the [[Annular ligament of stapes|annular ligament]]. The bones of the middle ear are supported by ligaments and muscles in the middle ear cavity. The [[Tensor tympani|tensor tympani]] muscle attaches to the malleus bone. The [[Stapedius|stapedius]] muscle attaches to the top of the [[Stapes|stapes]]. When these muscles contract, the transmisdion of sound from the ear canal to the cochlea is reduced. The AHAAH model analyzes the response of the ear in two modes: warned and unwarned. In the warned mode, the muscles are assumed to be already contracted. In the unwarned mode, the 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>{{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}}</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>{{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}}</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}}</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}}</ref>

The AHAAH’s improvements in accuracy are often attributed to its sensitivity to the flexing of the middle ear muscle (MEM) and [[Annular ligament of stapes|annular ligament of the stapes]]. When someone is forewarned of a sound, the MEM flexes, which is associated with reduced ability of the sound waves to reverberate. When an impulse sound is produced, the stapes’s annular ligament flexes and strongly clips the sound’s oscillation peak.<ref name=Chan />

As the MIL-STD-1474 has evolved, technology and methods have improved the AHAAH model’s accuracy. AHAAH has been proven to be more accurate in cases of double protection but not always in unwarned impulse noise instances relative to the competitive metric LAeq8hr.<ref name="Nakashima 2015">{{cite web |last1=Nakashima |first1=Ann |title=A comparison of metrics for impulse noise exposure |url=http://cradpdf.drdc-rddc.gc.ca/PDFS/unc206/p802859_A1b.pdf |publisher=Defence Research and Development Canada |access-date=3 July 2018 |date=November 2015}}</ref> Some suggestions for further development focus on creating a more user-friendly software, the placement of the microphone in data collection, the absence of the MEM reflex in populations, and the reevaluation of free-field conditions in calculations. Agencies such as NATO, the American Institute of Biological Sciences, and the [[National Institute for Occupational Safety and Health]] agreed that these suggestions be attended to before the metric is implemented. This shared conclusion was made prior to the development of MIL-STD-1474E.<ref name="Nakashima 2015" />