Auditory Hazard Assessment Algorithm for Humans: Difference between revisions

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Combatants in every branch of the United States’ military are at risk for auditory impairments from steady state or [[Impulse_noise|impulse noises]]. While applying double hearing protection helps prevent auditory damage, it may compromise effectiveness by isolating the user from his or her environment. With hearing protection on, a soldier is less likely to be aware of his or her movements, alerting the enemy to their presence. Hearing protection devices (HPD) could also require higher volume levels for communication, negating their purpose.<ref name=":0">{{cite web |last1=Amrein |first1=Bruce |title=NOISE LIMITS FOR WARFIGHTING Recently Revised Standard Addresses Noise from Military Operations |url=http://synergist.aiha.org/201611-noise-limits-for-warfighting |website=thesynergist |accessdate=3 July 2018}}</ref>

The first 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 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 deciblesdecibels A-weighted (dBA) and, if wearing protection, 140 deciblesdecibels (dBP) respectively.<ref name=":0" />

The [[United_States_Army_Research_Laboratory|US Army Research Laboratory]]’s Auditory Hazard Assessment Algorithm for Humans (AHAAH) produced these numerical guidelines. Over time the predictability of this algorithm has increased to 95% accuracy.<ref name=":1">{{cite journal |last1=DePaolis |first1=Annalisa |last2=Bikson |first2=Marome |last3=Nelson |first3=Jeremy |last4=de Ru |first4=J Alexander |last5=Packer |first5=Mark |last6=Cardoso |first6=Luis |title=Analytical and numerical modeling of the hearing system: Advances towards the assessment of hearing damage |journal=Elsevier |date=Feb 2, 2017 |volume=349 |page=111–118 |doi=10.1016/j.heares.2017.01.015 |url=https://europepmc.org/abstract/med/28161584 |accessdate=3 July 2018}}</ref> In almost every instance any error resulted in overcalculation of risk. By comparison the MIL-STD-147D was deemed correct in only 38% of cases with the same data.<ref name=":1" /> Originally developed from a cat animal model and later informed by human data, the AHAAH sums the basilar membrane displacements of 23 locations. The user inputs their noise exposure, protection level, and whether they were forewarned of the noise, to receive their hazard vulnerability in auditory risk units (ARU). This value can be converted to compound threshold shifts and the allowed number of exposure (ANE). Compound threshold shifts is a value that integrates both temporary and permanent shifts in auditory threshold, the latter being correlated to hair cell function.<ref name=":1" />

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 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 stape’sstapes’s annular ligament flexes and strongly clips the sound’s oscillation peak.<ref name=":1" />

As the MIL-STD-1474 has evolved, technology and methods have improved the AHAAP’s accuracy. AHAAP 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=":2">{{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 |accessdateaccess-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=":2" />

== References ==