Localization of sound in rooms, III: Onset and duration effects.

The steady-state sound field of a sine tone does not provide useful localization information in a room. Nevertheless, listeners can localize a sine tone in a room if it has an onset transient which allows the precedence effect to operate. In the present study, we made a quantitative assessment of onsets and the precedence effect by systematically varying onset duration from 0 s (impulsive), where the precedence effect is maximal, to 5 s, where there is no precedence effect at all. We also assessed listeners' sensitivity to the steady-state sound field under impulsive conditions by varying the total duration of tone pulses. Our experiments were conducted in a room with a single acoustical reflection having various directions and delays, and in an anechoic room. The results for tones of various frequencies (500 and 2000 Hz) and sound-pressure levels (65 and 40 dBA) indicate the following: Localization in rooms is facilitated by onsets even if the onsets are as long as 100 ms. The facilitation depends upon the peak intensity of the tone, as well as the onset duration, suggesting that onset rate is critical for the precedence effect; our results are most consistent with rate expressed as an increase in sound pressure per unit time. The facilitation also depends upon the reflection delay time for a room; gradual onsets take on much more importance for the precedence effect in rooms with long delays. As onsets begin to lose their effectiveness listeners become increasingly "misdirected" by invalid cues in the steady-state sound field. The pattern of misdirection suggests a perceptual averaging of cues over an interval more than an order of magnitude longer than previous estimates of the summation window for the precedence effect. The pattern of misdirection varies with the frequency of a tone, due to frequency-dependent interference effects in a room, but it is independent of signal level. Localization of an impulsive sine tone in rooms is very insensitive to the pulse duration; this suggests that binaural inhibition models of the precedence effect must be supplemented by an evaluative component that we term the "plausibility hypothesis."