Karen B. Snell
Dept. of Audiol., Rochester Inst. of Technol., Rochester, NY 14618
Ajit Janardan
Boston Univ., Boston, MA 02215
Recent studies suggest that fluctuations in the envelopes of narrow-band signals influence the detection of temporal gaps. In this study, the effects of amplitude modulation on gap detection using wideband noisebursts were measured by varying depth of modulation and phase angle. Gap detection thresholds were obtained for four subjects with normal hearing. The digitized noisebursts were 6 kHz in bandwidth and 150 ms in duration. The level of each noiseburst was jittered (70--75 dB SPL) and modulated with a 20-Hz sinusoid. Gaps were placed 100 ms after stimulus onset. Gap detection thresholds were estimated at six modulation depths (0 to 100%) and at one random and three fixed phase angles. A continuous white noise (45 dB SPL) was mixed with the signals. Thresholds were obtained in an adaptive, 2IFC procedure using 100 trial runs. Twenty-four conditions were completed in random order in 2 h. Several replications were completed by subjects. Mean gap detection thresholds increased with depth of modulation and varied with phase angle. Mean gap detection thresholds were smallest (2.5--4.0 ms) for the noisebursts which were least modulated (0--40%). Mean thresholds were largest (20 ms or more) for the noisebursts modulated 100% at a random phase. These results are consistent with recent findings using narrowband signals [B. R. Glasberg and B. C. J. Moore, Hear. Res. 64, 81--92 (1992)].