A New Method for Rating Hazard from Intense Sounds: Implications for Hearing Protection, Speech Intelligibility, and Situation Awareness

The auditory hazard assessment algorithm for the human (AHAAH), developed by the US Army Research Laboratory, is theoretically based and has been demonstrated to rate hazard from intense sounds much more accurately than existing methods. The AHAAH model operates in a PC level computer and analyses hazard in the time domain. In addition to reproducing the conductive path from air to the cochlea accurately it includes a non-linear stapes (clips large displacements) and an active middle ear muscle system. It is being written into a new US Army MIL-STD-1474(e) and is used by the Society of Automotive Engineers for the analysis of airbag noise hazard. The model shows that low-frequency energy at high levels can act to reduce the flow of energy into the inner ear, reducing the hazard. Traditional analyses tend to overrate the hazard from large calibre weapons impulses. The model also shows that impulses with little low-frequency energy, e.g. rifles, may be underrated in hazard by traditional methods. Hearing protective devices (HPDs), in order to be effective for gunfire type impulses, were shown to need most attenuation in the mid-range and less at lower frequencies, much like the attenuation curve for the non-linear combat arms plug. At the same time, speech intelligibility with such an attenuator could be much better than for an HPD with good low-frequency attenuation. Future developments of the model will include expansion to cover a wider range of intensities and an adaptive middle ear muscle system.