Surround Sound: Psychoacoustics - Part 2

Localization, Spaciousness, and Envelopment
The discussion of localization so far has centered on locating a sound at a point in space. Real-world sources may be larger than point sources, and reverberation and diffuse ambiences are meant to be the opposite of localized, that is, diffuse. Some ideas of how to make a simple monaural or 2-channel stereo source into 5 channels are given in Chapter 4.

There are two components to describe spatial sound: spaciousness and envelopment. These terms are often used somewhat interchangeably, but there is a difference. Spaciousness applies to the extent of the space being portrayed, and can be heard over a 2-channel or a 5-channel system. It is controlled principally by the ratio of direct sound to reflections and reverberation. On a 2-channel system, the sound field is usually constrained to being between the loudspeakers, and spaciousness applies to the sense that there is a physical space portrayed between the loudspeakers. The depth dimension is included, but the depth extends only to the area between the loudspeakers.

Envelopment, on the other hand, applies to the sensation of being surrounded by sound, and thus being incorporated into the space of the recording, and it requires a multichannel sound system to reproduce. Two-channel stereo can produce the sensation of looking into a space beyond the loudspeakers; multichannel stereo can produce the sensation of being there.

Lessons from Concert Hall Acoustics
A number of factors have been identified in concert hall acoustics that are useful in understanding the reproduction of sound over multichannel systems:

• The amount of reverberation, and its settings such as reverberation time, time delay before the onset of reflections, amount of diffusion, and so forth are very important to the perception of envelopment, which is generally a desirable property of a sound field.
• Early reflections from the front sides centered on ±55° from straight ahead (with a large tolerance) add to auditory source width (ASW) and are heard as desirable.
• All directions are helpful in the production of the feeling of envelopment, so reverberation returns and multichannel ambiences should apply to all of the channels, with uncorrelated sources for each channel, although some work shows that a difference at the two ears is preferred for contributions to envelopment. Thus the most important 2 channels for reverberation are LS and RS, the next most important are L and R, and C has little importance.
• Research has shown that 5 channels of direct sound are the minimum needed to produce the feeling of envelopment in a diffuse sound field, but the angles for such a feeling do not correspond to the normal setup. They are ±36°, ±108°, and +180° referenced to center at 0°. Of these, the ±36° corresponds perceptually with ±30° and ±108° with ±110°, but there is no back channel in the standard setup. Thus the sensation of complete diffuse envelopment with the standard 5.1 channel setup is problematic.
• Dipole surrounds are useful to improve the sensation of envelopment in reproduction, and are especially suitable for direct/ambient style recording/reproduction.
• Person-to-person differences in sound field preferences are strong. Separable effects include listening level, the initial time delay gap between the direct sound and the fi rst reflection, the subsequent reverberation time, and the difference in the sound field at the two ears.
  

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