Factors affecting the loudness of modulated sounds

Loudness matches were obtained between unmodulated carriers and carriers th at were amplitude modulated either periodically (rates between 2 and 32 Hz, modulation sinusoidal either on a linear amplitude scale or on a dB scale; the latter is called dB modulation) or with the envelope of the speech of a single talker. The carrier was a 4-kHz sinusoid, white noise, or speech-s haped noise. Both normally hearing subjects and subjects with cochlear hear ing loss were tested. Results were expressed as the root-mean-square (rms) level of the modulated carrier minus the level of the unmodulated carrier a t the point of equal loudness. If this difference is positive, this indicat es that the modulated carrier has a higher rms level at the point of equal loudness. For normally hearing subjects, the results show: (1) For a 4000-H z sinusoidal carrier, the difference was slightly positive (averaging about 0.7 dB). There was no significant effect of modulation rate or level over the range 20-80 dB SL. (2) For a speech-shaped noise or white noise carrier , the difference was close to zero, although for large modulation depths it tended to be negative. There was no clear effect of level (over the range 35-75 dB SPL) or modulation rate. For the hearing-impaired subjects, the di fferences were small, but tended to be slightly negative for both the 4000- Hz carrier and the noise carriers, when the modulation rate was above 2 Hz. Again. there was no clear effect of overall level. However, for dB modulat ion, the differences became more negative with increasing modulation depth. For modulation rates in the range 4-32 Hz, the results could be fitted rea sonably well using the assumption that the loudness of modulated sounds is based on the rms value of the time-varying intensity of the response of the basilar membrane (taking into account the compression that occurs in the n ormal cochlea). The implications of the results for the fitting of multi-ba nd compression hearing aids and for the design of loudness meters are discu ssed. (C) 1999 Acoustical Society of America. [S0001-4966(99)04705-0].