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Multichannel sound reproduction systems are becoming a standard component of home entertainment systems. In this section, we consider the acoustics of small rooms and some of the important issues involved with multispeaker setups within such rooms.
- In small rooms, many reflections begin arriving soon after the direct sound. Thus, early and late reflections become fused into a single reverberant sound.
- The resonances in a rectangular box with dimensions , and and without damping are
, where is the speed of sound and , and are integers.
- The only means for achieving a long reverberation time in a small room is to have low absorption at the walls. This, however, results in prominent individual resonances.
- A large room has many more resonances than a small room (within the audible range). This produces a fairly smooth frequency response. Small rooms, on the other hand, may produce considerable coloration of sound.
- The reverberant field in a large room builds up much more slowly than that in a small room, providing an important perceptual cue to room size.
- Equidistant left and right speakers playing a monophonic sound will create an ``image'' on the median plane. Increasing the signal strength of one of the speakers will move the image toward that speaker.
- If one speaker is farther away from the listener than another, the precedence effect will determine the apparent sound image. However, if the signal strength from the more distant speaker is made greater than that from the closer speaker, the image can often be moved back toward the median plane.
- The ability to trade off distance with source strength is dependent on frequency, as might be expected given the different means our auditory system uses to localize sounds of high and low frequency.
- If the spectra from two sources are different, the image appears to broaden and both speakers will appear to deliver the full spectrum (unless the crossover is very abrupt).
- A variety of microphone arrangements have been used for stereophonic recording.
- The ``favorable'' listening area is fairly close to the median plane and at a distance such that the two speakers are separated by an angle of about 40 to 90 degrees.
- Additional speakers in a stereophonic context can help add some sense of spaciousness to a room.
- Difference signals and/or high- or low-frequency emphasis applied to various speakers can also create the illusion of spaciousness.
- In order to emulate the tonal balance heard in a concert hall, low frequencies should be boosted and high frequencies decreased.
- 5.1 channel systems are now specified for various home entertainment systems (AC3, MPEG4).
- With respect to radiation directivity, the subwoofer position is usually not critical (because it radiates in a nearly omni-directional manner). However, the subwoofer can influence sound localization via source-to-listener time delay. Thus, a multi-channel sound system designed for precise sound image localization should have a separate subwoofer for each channel.