Effects of center frequency and bandwidth on echo threshold and buildup of echo suppression

Echo threshold is the delay of a reflected sound relative to the direct sound at which a subject can no longer hear the reflection as a separate event. Echo threshold was measured for a single 4‐ms noise burst as a joint function of center frequency (500–4000 Hz) and bandwidth (0.08–1.00 octave) of the burst. The direct burst came from a loudspeaker at −45° azimuth, and the simulated echo from a loudspeaker at 0° azimuth. A single‐interval, forced‐choice procedure was employed, in which the subject had to say on each trial whether one (direct sound alone) or two (direct plus reflected sound) events occurred. Thresholds ranged from 3–61 ms, with marked differences among individual subject’s functions; however, there was some tendency for threshold to increase with decreasing center frequency and decreasing bandwidth. Buildup of echo suppression [R. L. Freyman et al., J. Acoust. Soc. Am. 90, 874–884 (1991)] was measured in the same conditions by determining echo threshold for the noise burst when it followed a train of 9 like‐filtered bursts. Elevation in threshold ranged from 0–49 ms, with staggering inter‐subject differences across conditions. These results support the conclusions from earlier studies that the suppression of echos and the buildup of this suppression is mediated at a high level in the auditory system. [Work supported by NIDCD.]