Hearing Aid Design Criteria

Abstract

The restricted low frequency and exaggerated peaky high frequency response characteristics of some modern transistor hearing aids, coupled with their very high output levels are considered unsatisfactory for use by many perceptively deaf children. Experiment has shown that providing sufficient amplification is given in the 250-750 c/s frequency range, very high output levels are seldom required, even by severely deaf children. No aid known to the author will perform according to the design criteria outlined which propose suggestions not usually accepted in hearing aid design. These are considered likely to satisfy many children who at present are not making full use of their residual hearing.

Depending on the type and severity of the deafness a flat frequency response characteristic should be incorporated over the range of 250-4000 c/s. The maximum output levels should be restricted for example to either 125 dB or 110 dB spl. with associated maximum air to air gain figures of 60 dB or 45-50 dB respectively, and the circuit controlling the restriction on the output levels should be as distortionless as possible. A receiver which attenuates at the rate of about 12 dB/octave above 600 c/s is recommended for use to obtain both a modified frequency response and to reduce acoustic feedback created by a poorly fitting earmould.