Acoustic diffraction and resonance effects for the aided versus the unaided listener

Abstract

The acoustic pressure at the hearing aid microphone differs from the incident pressure field, be it partially or totally progressive or diffuse, primarily as a result of diffraction by the head and/or torso of the aided listener. However, for the normally hearing subject the incident pressure field, before reaching the eardrum, is additionally transformed by the pinna and the ear canal, which together produce additional resonance amplification and directional effects; neither of these effects is produced at the hearing aid microphone. Results showing the separate effects of the head, torso, and external ear on the pressure transformations of acoustic waves for incidence in the horizontal—and median vertical—planes, as well as for random incidence, will be presented. The frequency response, the horizontal—and median vertical—plane directivities and the available localization cues for aided and unaided listeners will be compared. Some schemes to restore the localization cues for the aided listener to those of the unaided listener will be discussed. Furthermore, a physical mechanism which may cause speech in noisy environments to be less intelligible for aided listeners than for unaided listeners will be proposed.