A computer simulation of hearing aid response and the effects of ear canal size

Abstract

The response of a hearing aid is affected by many factors which include the head and outer ear, the microphone, amplifier, and receiver used in the hearing aid, the properties of the ear canal and the eardrum, and acoustic feedback through the vent. This article presents a computer simulation of an in-the-ear (ITE) hearing aid that includes all of the above factors. The simulation predicts the pressure at the eardrum for a frontal free-field sound source. The computer model was then used to determine the effects on the hearing aid response due to variations in the size of the ear canal. The simulation indicates that, for an unvented hearing aid, changes in the size of the ear canal shift the overall sound-pressure level at the eardrum but have only small effects on the shape of the frequency response. The situation is more complicated when a vent is present, however, since changes in the size of the ear canal that cause apparently small perturbations in the acoustic feedback signal may, nonetheless, have large effects on the overall system response.

Waveguide model of the hearing aid earmold system

“…The length and shape of the ear canal are unique for each individual. However, a sufficiently accurate model of the ear canal valid for frequencies below 8 kHz is a cylindrical tube of 7.5 mm in length and 22.5 mm in diameter [ 13 ]. The useful frequency range in hearing aids rarely exceeds 8 kHz, mainly due to the deficiencies of the receiver, so that this model is adequate for this application.…”

Principles of Digital Dynamic-Range Compression

“…The frequency response from a simulation (Kates, 1988) of this simple hearing aid is plotted in Figure 3. At frequencies below 300 Hz, the gain is 0 dB despite the 30-dB amplification provided by the amplifier.…”

Individual Differences within and across Feedback Suppression Hearing Aids

“…Standard deviations of less than 1 dB are generally reported when the loudspeaker is placed at a 45u azimuth (e.g., Killion and Revit, 1987) as compared to 0u azimuth. However, of greater interest for the current study, a number of other factors such as individual differences in ear canal geometry and the specific hearing aid to ear coupling may affect the feedback loop resulting in differences in susceptibility to feedback (e.g., Kates, 1988;Egolf et al, 1989;Hellgren et al, 1999;Rafaely et al, 2000). Since accurate modeling of the feedback loop by the FS algorithm is crucial for optimal feedback suppression, the magnitude of AGBF is expected to differ across patients.…”