LISTENERS EXPECTATIONS ABOUT ECHOES CAN RAISE OR LOWER ECHO THRESHOLD

Echo threshold increases with exposure to redundant trains of stimuli. Three experiments were conducted to test the hypothesis that a change in the ongoing train would affect listeners' perception of the echo, but only if it signified an unusual change in room acoustics. The stimulus train was composed of 4-ms narrow-band noise bursts, with the leading sound from a loudspeaker placed 45 degrees left of midline and the lagging sound or simulated echo from 45 degrees right, delivered in an anechoic chamber. The lagging sound in the test noise, which followed the train after a 750-ms pause, came randomly from loudspeakers at 35 degrees or 55 degrees right, and the listener's task was to choose which position the echo came from on each trial. In experiment 1 the delay between onsets of the leading and lagging bursts was varied between train and test bursts, which simulated a sudden movement of the reflecting surface either toward the listener (if the delay of the test burst was shorter than the train) or away (if the delay was longer). In both cases listeners detected the echo's direction more easily, compared to trials when there was no change between train and test burst delays. In order to check whether any change between train and test bursts would increase echo discriminability, experiment 2 varied frequency and experiment 3 varied intensity. These variations were not expected to affect the echo's detectability because such changes signify that the original sound changed in these characteristics and the echo reflected these changes. These events are highly probable in the listener's everyday experience because sound sources (and their reflections) typically vary in frequency and intensity content from moment to moment. As predicted, echo detectability in experiments 2 and 3 was not affected by whether the test noise bursts' frequency or intensity was the same as the train's or was varied. The results from all three experiments were interpreted in terms of Listeners' expectations about echoes. It is proposed that echoes provide information about room acoustics, which the listener picks up during the ongoing sound and uses to form expectations about what will be heard. When expectations are violated by changes in the echo, this disruption can be seen in a lowering of echo threshold, relative to the ''built-up'' threshold when expectations are fulfilled.