The relation between spatial impression and the precedence effect
This paper reviews results of listening tests on auditory spatial impression (ASI) that describe the relation between individual characteristics of spatial impression and the precedence effect. ASI is a general concept defined as the spatial extent of the sound image, and is comprised at least two components. One is auditory source width (ASW), defined as the width of a sound image fused temporally and spatially with the direct (preceding) sound image; the other is listener envelopment (LEV), defined as the degree of fullness of the sound image surrounding the listener, and which excludes the direct sound image for which ASW is judged. Listeners can perceive separately these two components of ASI, and their subjective reports demonstrate that they can distinguish between them. The perception of ASW and LEV has close connection with The precedence effect (the law of the first wave front). Acoustic signal components that arrive within the time and amplitude limits of the effect contribute to ASW, and those beyond the upper limits contribute to LEV. It is possible to control ASW and LEV independently by controlling physical factors that influence each of the components. It is well-known, for example, that the degree of interaural cross-correlation (ICC) is an important physical factor in the control of ASI. ASW can be predicted from ICC (and thereby controlled by the manipulation of ICC) regardless of the number and directions of arrival of sound sources. But measurements of ICC within 1/3-octave bands are preferred for estimating ASW, whereas the use of wide band and 1-octave band signals, as described in the ISO standard, are not. On the other hand, LEV cannot be controlled only through manipulation of ICC, as LEV is also affected by the spatial distribution of sounds (e.g., front/back energy ratio).