Updated performance requirements for hearing aids.

This article updates the design specifications for a family of hearing aids required by the National Acoustic Laboratories (NAL) in order to meet the needs of its clients. The required range of coupler gains and frequency responses of behind-the-ear (BTE), in-the-ear (ITE) and body-level (BL) hearing aids has been recalculated on the assumption that the aids will be fitted using the new Byrne-Dillon procedure for selecting gain and frequency response. In the light of recently acquired data, the specification of maximum allowable internal noise has been changed slightly and no relaxation of the equivalent input noise criterion for high gain aids is allowed. Since the Byrne-Dillon procedure demands a somewhat smaller (though still quite substantial) range of hearing aid response shapes, the subdivision of the total response requirements has also been repeated. The number of hearing aid models specified is still 18, but the number of fitter-adjusted controls has been reduced from 6 to 4. The fitter-adjusted controls comprise a maximum power output (MPO) level control, an MPO shape control, a low-tone cut control and a high-tone cut control, with MPO control being achieved by means of low-distortion compression limiting.