General overview of spatial impression, envelopment, localization, and externalization

This paper will provide an overview of the current knowledge on the problems of spatial impression, envelopment, and localization in enclosed spaces. The emphasis will be on small spaces, but the approach draws heavily on concert acoustic studies. The literature on this subject has been contradictory and controversial. In the last few years a consensus has been developing that envelopment is primarily determined by lateral reflected energy arriving at least 80ms after the direct sound. Localization is traditionally linked to the “apparent source width” (ASW) which has been associated with the interaural correlation (IACC) of the first 80ms of the impulse response of a space. Although ASW and IACC may be useful in concert hall measurement, they do not work well in small spaces. This paper will present two new measures, the Diffuse Field Transfer function (DFT), and the Average Interaural Time Delay (AITD). The DFT is a measure of envelopment, and is useful both in small rooms and large rooms. The AITD is a measure for “externalization” a sonic property unique to small rooms. A closely related measure, the Net Interaural Time Delay (NITD), is useful in understanding localization in small spaces.