Objective Measures of Spaciousness and Envelopment

This paper will discuss current knowledge of spaciousness and envelopment (assumed to be the same perception) with particular attention to measurement techniques that might be applicable to virtual audio systems. We find that all frequencies contribute to the perception of envelopment. At frequencies below 2000Hz envelopment arises from fluctuations in the Interaural Time Delay (ITD) and the Interaural Intensity Difference (IID) at the ears of the listener. Fluctuations that follow the ends of sounds by at least 160ms are the most effective. These fluctuations can be produced by decorrelated sound sources on opposite sides of the listener. The optimum angle for such sources varies with frequency. For frequencies below 700Hz sources should be at 90 degrees from the front. Above this frequency the optimum angle moves toward the medial plane. For broadband signals above 2000Hz, spectral cues characteristic of sound sources behind the listener increase the perceived envelopment. The optimum angle for broadband sound sources with most of their energy above 2000Hz is approximately 150 degrees from the front. We conclude that multiple sound sources (either real or virtual) are necessary for optimal envelopment, consisting of at least a pair of sources at 90 degrees from the front, and a pair at 150 degrees. The effectiveness of these sources will depend on the shape and reflectivity of the room. For measuring the envelopment delivered by the loudspeaker/room system, the Diffuse field Transfer Function (DTF) is recommended.