Experimental study of the vibration analysis of the receivers of the bone conduction hearing aids by holographic interferometry

Abstract

A bone conduction hearing aid is considered to be useful in aiding the patients with conductive hearing loss or partially mixed hearing loss. However, the bone conduction hearing aid is inferior to a air conduction hearing aid in sound characteristics. The standard for calibrating these hearing aids has not yet established. Little is known about the standard. The vibratory mode of the receiver of the bone conduction hearing aid was analyzed by holographic interferometry. The technique used was time averaged holography with the light source of an argon laser, the wave length of which was 5145 A. In the present study, 6 types of bone conduction hearing aid were used. The input voltage of each receiver was 0.5 to 2.5V. The frequency was 250 to 8, 000Hz. The vibration of the receiver was recorded and reproduced. Further, the vibration was gained and recorded in a free sound field of sound pressure of 80 to 105dB SPL and frequency of 250 to 2, 000Hz and following results were obtained. 1) The frequency of the bone conduction hearing aid was characterized by the peak around 1, 000Hz and a rapid reduction in the region of high frequency. 2) In the region of low frequency of 250Hz or 500Hz, an increase in the output will lead to improvement in the properties of the hearing aid to some extent. However, improvement in the region of high frequency around 8, 000Hz seemed to be difficult. 3) In respect to the vibration pattern of the bone conduction hearing aid, two patterns (hinge-like movement and distortion movement) were observed according to the type of the hearing aid. However, the difference in the pressure on the skull seemed practically insignificant among the hearing aids.