LOUDNESS OF DYNAMIC STIMULI IN ACOUSTIC AND ELECTRIC HEARING

Traditional loudness models have been based on the average energy and the critical band analysis of steady-state sounds. However, most envir onmental sounds, including speech, are dynamic stimuli, in which the a verage level [e.g., the root-mean-square (rms) level] does not account for the large temporal fluctuations. The question addressed here was whether two stimuli of the same rms level but different peak levels wo uld produce an equal loudness sensation. A modern adaptive procedure w as used to replicate two classic experiments demonstrating that the se nsation of ''beats'' in a two-or three-tone complex resulted in a loud er sensation [E. Zwicker and H. Fastl, Psychoacoustics-Facts and Model s (Springer-Verlag, Berlin, 1990)]. Two additional experiments were co nducted to study exclusively the effects of the temporal envelope on t he loudness sensation of dynamic stimuli. Loudness balance was perform ed by normal-hearing listeners between a white noise and a sinusoidall y amplitude-modulated noise in one experiment, and by cochlear implant listeners between two harmonic stimuli of the same magnitude spectra, but different phase spectra, in the other experiment. The results fro m both experiments showed that, for two stimuli of the same rms level, the stimulus with greater temporal fluctuations sometimes produced a significantly louder sensation, depending on the temporal frequency an d overall stimulus level. In normal-hearing listeners, the louder sens ation was produced for the amplitude-modulated stimuli with modulation frequencies lower than 400 Hz, and gradually disappeared above 400 Hz , resulting in a low-pass filtering characteristic which bore some sim ilarity to the temporal modulation transfer function. The extent to wh ich loudness was greater was a nonmonotonic function of level in acous tic hearing and a monotonically increasingly function in electric hear ing. These results suggest that the loudness sensation of a dynamic st imulus is not limited to a 100-ms temporal integration process, and ma y be determined jointly by a compression process in the cochlea and an expansion process in the brain. A level-dependent compression scheme that may better restore normal loudness of dynamic stimuli in hearing aids and cochlear implants is proposed. (C) 1997 Acoustical Society of America.