INTELLIGIBILITY OF FREQUENCY-LOWERED SPEECH PRODUCED BY A CHANNEL VOCODER

Frequency lowering is a form of signal processing designed to match sp eech to the residual auditory capacity of a listener with a high frequ ency hearing loss. A vocoder-based frequency-lowering system similar t o one studied by Lippmann was evaluated in the present study. In this system, speech levels in high frequency bands modulated one-third-octa ve bands of noise at low frequencies, which were then added to unproce ssed speech. Results obtained with this system indicated, in agreement with Lippmann, that processing improved the recognition of stop, fric ative, and affricate consonants when the listening bandwidth was restr icted to 800 Hz. However, results also showed that processing degraded the perception of nasals and semivowels, consonants not included in L ippmann's study. Based on these results, the frequency-lowering system was modified so as to suppress the processing whenever low frequency components dominated the input signal. High and low frequency energies of an input signal were measured continuously in the modified system, and the decision to process or to leave the signal unaltered was base d on their relative levels. Results indicated that the modified system maintained the processing advantage for stops, fricatives, and affric ates without degrading the perception of nasals and semi-vowels. The r esults of the present study also indicated that training is an importa nt consideration when evaluating frequency-lowering systems.