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Auditory Hazard Assessment Algorithm for Humans

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Combatants in every branch of the United States military are at risk for auditory impairments from steady state noise or impulse noises. While applying double hearing protection helps prevent auditory damage, the user is isolated from the environment and the ability to detect, identify and localize important environmental cues is impaired. With hearing protection on, a soldier is less likely to be aware of their movements, alerting the enemy to their presence. Hearing protection devices (HPD) could also require higher volume levels for communication, negating their purpose.[1]

The US Army Research Laboratory’s developed the Auditory Hazard Assessment Algorithm for Humans (AHAAH) to evaluate the potential damage to a user when exposed to high level impulse noise. The model purports to be more than 94% accurate with regards to identifying safe and hazardous exposures based upon the Blast Overpressure Walk-up study conducted by the US Army.[2][3][4] In almost every instance any error resulted in over calculation of risk. By comparison the MIL-STD-147D was deemed correct in only 38% of cases with the same data.[4] 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.[4]

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 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 decibels A-weighted (dBA) and, if wearing protection, 140 decibels (dBP) respectively.[1]

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. The bones of the middle ear are supported by ligaments and muscles in the middle ear cavity. The tensor tympani muscle attaches to the malleus bone. The stapedius muscle attaches to the top of the 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%.[5] 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."[6] The anticipatory contraction integral to the warned response is not reliable in persons with normal hearing.[7][8]

As the MIL-STD-1474 has evolved, technology and methods have improved the AHAAH model’s accuracy. 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. In a U.S. Army-funded review of auditory blast injury models, the American Institute of Biological Sciences stated that these issues be addressed before the metric is implemented. A Canadian Defence Research Council report published in Nov 2015 after the release of MIL-STD 1474E recommended against using the AHAAH metric in its current state.[9]

References

  1. ^ a b Amrein, Bruce. "NOISE LIMITS FOR WARFIGHTING Recently Revised Standard Addresses Noise from Military Operations". thesynergist. Retrieved 3 July 2018.
  2. ^ Chan, P.C.; Ho, K.H.; Kan, K.K.; Stuhmiller, J.H. (2001). "Evaluation of impulse noise criteria using human volunteer data". J. Acoust. Soc. Am. 110: 1967–1975. doi:10.1121/1.1391243.
  3. ^ Price, G.R. (2007). "Validation of the auditory hazard assessment algorithm for the human with impulse noise data". J. Acoust. Soc. Am. 122: 2786–2802. doi:10.1121/1.2785810.
  4. ^ a b c DePaolis, Annalisa; Bikson, Marome; Nelson, Jeremy; de Ru, J Alexander; Packer, Mark; Cardoso, Luis (Feb 2, 2017). "Analytical and numerical modeling of the hearing system: Advances towards the assessment of hearing damage". Hearing Research. 349: 111–118. doi:10.1016/j.heares.2017.01.015. PMID 28161584. Retrieved 3 July 2018.
  5. ^ Flamme, Gregory A.; Deiters, Kristy K.; Tasko, Stephen M.; Ahroon, William A. (21 November 2016). "Acoustic reflexes are common but not pervasive: evidence from the National Health and Nutrition Examination Survey, 1999–2012". International Journal of Audiology. 56 (sup1): 52–62. doi:10.1080/14992027.2016.1257164.
  6. ^ McGregor, Kara D.; Flamme, Gregory A.; Tasko, Stephen M.; Deiters, Kristy K.; Ahroon, William A.; Themann, Christa L.; Murphy, William J. (19 December 2017). "Acoustic reflexes are common but not pervasive: evidence using a diagnostic middle ear analyser". International Journal of Audiology. 57 (sup1): S42 – S50. doi:10.1080/14992027.2017.1416189.
  7. ^ Deiters, Kristy K.; Flamme, Gregory A.; Tasko, Stephen M.; Murphy, William J.; Greene, Nathaniel T.; Jones, Heath G.; Ahroon, William A. (November 2019). "Generalizability of clinically measured acoustic reflexes to brief sounds". The Journal of the Acoustical Society of America. 146 (5): 3993–4006. doi:10.1121/1.5132705.
  8. ^ Jones, Heath G.; Greene, Nathaniel T.; Ahroon, William A. (July 2019). "Human middle-ear muscles rarely contract in anticipation of acoustic impulses: Implications for hearing risk assessments". Hearing Research. 378: 53–62. doi:10.1016/j.heares.2018.11.006.
  9. ^ Nakashima, Ann (November 2015). "A comparison of metrics for impulse noise exposure" (PDF). Defence Research and Development Canada. Retrieved 3 July 2018.
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