Courtesy: Pattern approval for measuring instruments
Military standards
Combatants in every branch of the United States’ military are at risk for auditory impairments from steady state or 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.
- MIL-STD 1474D The first military standard (MIL-STD) on sound was published in 1984 and underwent revision in 1997 to become MIL-STD-1474D. This standard establishes acoustical noise limits and prescribes testing requirements and measurement techniques for determining conformance to the noise limits specified herein. This standard applies to the acquisition and product improvement of all designed or purchased (non-developmental items) systems, subsystems, equipment, and facilities that emit acoustic noise. This standard is intended to address noise levels emitted during the full range of typical operational conditions.
- MIL-STD 1474E In 2015, MIL-STD 1474D 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 decibels A-weighted (dBA) and, if wearing protection, 140 decibels (dBP) respectively. It establishes acoustical noise limits and prescribes testing requirements and measurement techniques for determining conformance to the noise limits specified herein. This standard applies to the acquisition and product improvement of all designed or purchased (non-developmental items) systems, subsystems, equipment, and facilities that emit acoustic noise. This standard is intended to address noise levels emitted during the full range of typical operational conditions. This standard includes two methods for assessing the impulse noise and risk to hearing.
- The Auditory Hazard Assessment Algorithm for Humans (AHAAH), a one-dimensional electro-acoustic analog of the auditory system, produced MIL-STD 1474E’s numerical guidelines. Over time the predictability of this algorithm has been claimed to have increased to 95% accuracy. US Army Research Laboratory researchers state that almost every error resulted in overcalculation of risk. By comparison, the MIL-STD-147D was deemed correct in 38% of cases with the same data. Originally developed from a cat animal model and later informed by human data, the AHAAH sums the basilar membrane displacements of 23 locations.The AHAAH model calculates the estimated displacement of the basilar membrane and summates the accumulation of the flexure of the basilar membrane. 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.
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- The AHAAH’s claimed 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’s annular ligament flexes and strongly clips the sound’s oscillation peak.[21] As the MIL-STD-1474 has evolved, technology and methods have improved the AHAAP’s accuracy. Researchers claim that the 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. 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.
- TOP-1-2-608A This Test Operations Procedure (TOP) describes procedures for measuring the sound levels transmitted through air of developmental and production materiel as a means of evaluating personnel safety, speech intelligibility, security from acoustic detection and recognition, and community annoyance. It covers tests for steady-state noise from military vehicles and general equipment, and impulse noise from weapon systems and explosive-ordnance materiel.