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.
[10] 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.
[1][12][13] Originally designed to function as an electro-acoustic model of the ear, AHAAH was the product of numerous noise exposure experiments which, in turn, guided the direction of future studies.
The output of the AHAAH model is auditory risk units (ARUs), which are related to summation of the upward displacements of the basilar membrane at 23 different locations.
[2][14] The AHAAH model consisted of a set of proven algorithms that accounted for a variety of exposure conditions that influenced the risk of a permanent threshold risk, such as noise attenuation caused by hearing protection devices and reflexive middle ear muscle (MEM) contractions that occur before the onset of the stimulus being received that reduce the damage to the ear in preparation of the sound.
For waves recorded at the ear canal entrance or at the eardrum, the model took into account the proper origin point of the sound in the circuit diagram.
The studies took place at Kirkland Air Force Base in Albuquerque, New Mexico, where participants were exposed to four different pressure-time signatures at seven different intensity levels and at various successions and sequences.
[7][21] The experiment consisted of exposures to free-field impulse waveforms produced by explosive charges at distances of 5, 3, and 1 meters while wearing hearing protection.
The fourth exposure condition was a reverberant environment with the participants seated at the end of a 3-meter long steel tube that opened into a concrete bunker.
Various conditions were accounted for, such as the distance of the participant's ear from the tube, the acoustics of the surrounding environment, the level of hearing protection, and the number of impulses, establishing a matrix of possible exposures.
[3] In 2003, a NATO research study on impulse noise found that the AHAAH produced unsatisfactory results for several exposure conditions, and the concluding report contained conflicting opinions from several experts.
[23] A 2010 review by the American Institute of Biological Sciences (AIBS) also concluded that while the AHAAH model was a step in the right direction in terms of incorporating factors such as the middle ear muscle contractions in its analysis, it was not yet fully developed and validated.
[24] In 2012, a review by the National Institute for Occupational Safety and Health (NIOSH) argued that the MEM contractions that were used by the AHAAH to justify increasing the recommended maximum noise levels were not present in enough people to be applied as a valid form of analysis.
The report also noted that the AHAAH did not adequately take into account the effects of secondary exposure, such as adjacent shooters and range safety personnel.