A New Generation of Hearing Aid Transducers
Mead C. Killion, Elmer V. Carlson · The Journal of the Acoustical Society of America · 1974
Recent developments in subminiature microphone and receiver technology make possible several new performance options for ear level hearing aids: (1) a smooth response extending to higher frequencies; (2) a drastically reduced vibration sensitivity; and (3) a cardioid directional characteristic, allowing the hearing aid to reject interfering sounds from the rear. These improvements have been brought about by (a) a new wide-band receiver to complement the wide-band piezoelectric ceramic microphone introduced several years ago; (b) the application of the wide-band tailored frequency response of the ceramic microphone to a new electret condenser microphone; and (c) a series of directional microphone configurations using both ceramic and condenser transducers. In the case of the receiver, the well-honed technology on balanced armature magnetic devices was applied to produce a device with an extended high-frequency response. In the case of the electret condenser microphone, separating the electret function from the diaphragm function made possible a device with both exceptionally low-vibration sensitivity and an improved charge storage capability. As with the ceramic microphones, devices with a flat frequency response have been designed in addition to those with a response tailored to the special needs of ear level hearing aids. The directional microphones are realized by using the R-C time constant of the damping resistance and internal air volume compliance to produce a time delay for sounds arriving at one of the two inlet ports. A reasonably accurate analog of such devices requires 25 to 30 discrete elements plus distributed lines, but permits efficient computer optimization of their frequency response and directional characteristics.