Foundation for Hear. Aid Res., P.O. Box 306, Woodstock, NY 12498
Harvey Fletcher described the effect of a transmission channel on speech recognition in terms of the channel's ``articulation'' plotted against channel gain. For normal listeners, articulation increases with gain to a maximum and then levels off or decreases. For hearing-impaired listeners, however, Fletcher noted: ``... the gain (alpha)[sub t] corresponding to the tolerable level is reached before it is large enough (to bring articulation to a maximum).'' This occurs in cochlear impairment because the dynamic range of hearing is reduced by recruitment. It is the problem to which the design of modern compression hearing aids is addressed. Compression aids compensate for recruitment by providing increased relative gain for weak speech sounds; thus they allow the articulation to reach higher values before tolerance levels are exceeded. An analysis of compression/equalization signal processing is presented. The essential elements of this processing are: (1) more than one channel of full-dynamic-range compression (the K-AMP design achieves similar results in a different way), (2) post-compression frequency-response shaping, and (3) adjustability of the compression in each channel, and of the equalization, to fit the amplified speech into the residual dynamic and frequency range of hearing of the individual patient.