When similar
sounds come to a listener from two different directions with the same level and small time difference (0-2 ms), he/she perceives a single
sound image and its position changes from the middle of two
sound sources for no time difference between two
sounds to the direction of the leading
sound with increasing time difference. This phenomenon is well known as a change from the summing
localization to the precedence effect, but the process for such a change has not been sufficiently elucidated. In order to make use of the precedence effect for designing a public address system, the effect needs to be quantified.
In this paper, we describe two hearing experiments carried out with the same subjects and conditions. We then analyzed the relation between the perception of a sound image and the behavior of the summing localization and the precedence effect. The difference between two experiments was in the instruction that the subjects were asked to answer. The instruction in the first experiment was the perceived direction of the sound image, while in the second experiment the instruction was the position of the sound source nearest to the perceived sound image. The boundary time difference separating the summing localization from the localization based on the precedence effect was calculated from the results of both experiments. Consequently we found that: (a) the absolute value of the boundary time difference varies in the range of 0.4-0.8 ms with the small level difference (0, +3 dB) between two sound sources and (b) the perceived image at the boundary time difference is localized around 10¡ã from the left/right-side sound source to the middle of two sound sources.
