Spectro-temporal modulation transfer and speech intelligibility for multiband amplitude compression
Joost Μ. Festen · The Journal of the Acoustical Society of America · 1998
For ears with sensorineural hearing loss the growth of loudness with stimulus level is often larger than in normal ears. This phenomenon is called recruitment. To restore normal loudness perception and to fit speech in the limited dynamic range of an impaired ear many modern hearing aids apply some form of multiband amplitude compression. However, within the cochlea nonlinearity and frequency selectivity are closely linked and therefore recruitment often goes together with reduced frequency selectivity. While compression may correct the recruitment, it does not improve the frequency selectivity. The opposite is even more correct; multichannel compression reduces the spectro-temporal variations in speech. Therefore in terms of speech intelligibility, the net effect of compression is difficult to predict. In an experiment speech intelligibility after multiband compression in quiet and in noise for 16 hearing-impaired listeners was measured. For each individual listener the speech spectrum was shaped and presented monaurally halfway in the ear’s dynamic range in all conditions. Results show a decrease in intelligibility with both increasing compression ratio and increasing number of channels. These results can be fitted with a single curve when intelligibility is plotted as a function of the transfer of phase-locked modulations in time and frequenc.