Auditory Hazard Assessment Algorithm for Humans

The Auditory Hazard Assessment Algorithm for Humans (AHAAH) is a mathematical model of the human auditory system that calculates the risk to human hearing caused by exposure to impulse sounds, such as gunfire and airbag deployment. It was developed by the U.S. Army Research Laboratory (ARL) to assess the effectiveness of hearing protection devices and aid the design of machinery and weapons to make them safer for the user.

In 2015, the AHAAH became one of the two metrics used by the U.S. Department of Defense to approve the Military Standard (MIL-STD) 1474E for regulating maximum noise level exposure from military systems. It is also used by the Society of Automotive Engineers to calculate the hazard of airbag noise and by the Israeli Defense Force for impulse noise analysis.

Overview

Noise-induced hearing loss (NIHL) typically occurs when the auditory system experiences an elevation of hearing thresholds due to exposure to high-level noise, a phenomenon known as a temporary threshold shift (TTS), and does not return to normal threshold levels. 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 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.

In order to protect soldiers from hearing loss, the U.S. Army adhered to the Military Standard (MIL-STD) 1474, which defined the maximum noise levels permitted to be produced by military systems. However, human volunteer studies demonstrated that the standard used since 1997, the MIL-STD-1474D, overestimated the hazard associated with impulse noise exposure. 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. 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. 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.

Development

The AHAAH was first developed in 1987 by the U.S. Army Human Engineering Laboratory (HEL), which later became part of the U.S. Army Research Laboratory (ARL), to investigate the complex interactions between the outer