On reducing auditory feedback in hearing aids
Richard E. C. White · The Journal of the Acoustical Society of America · 1979
An inherent feature of a conventional magnetic earphone is that the diaphragm motion which generates acoustic output also gives rise to unbalanced displacing forces acting on the earphone's case. Although the displacements of the case are small, the resulting acoustic radiation may be sufficient to cause feed back, particularly with high gain hearing aids. Following a suggestion by Mead Killion, an experimental dynamically balanced earphone system was constructed. Two conventional miniature earphones were coupled back-to-back so that their diaphragms always moved in opposite directions, thus reducing the net out-of-balance force on their cases. The acoustic outputs of the earphones were delivered to the hearing aid tubing via a simple manifold. The success of the scheme was confirmed both by the reduced vibration amplitude of the coupled cases and by a lower level of radiated sound. Preliminary results suggest that in the absence of other limitations this system could allow hearing aid gain to be increased by about 10 dB before the onset of feedback. [Work supported by NINCDS.]