• Acoustics of Small Rooms
• Multiple Source Sound Images
• Stereophonic Sound
• The Sound Field in Listening Rooms
• Multichannel Sound Systems

High-Fidelity Sound-Reproducing Systems

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.

Acoustics of Small 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 $a, b$, and $c$ and without damping are $f_{lmn} = \frac{v}{2} \sqrt{ (l/a)^{2} + (m/b)^{2} + (n/c)^{2}}$, where $v$ is the speed of sound and $l, m$, and $n$ 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.

Multiple Source Sound Images

• 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).

Stereophonic Sound

• 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.

The Sound Field in Listening Rooms

• 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.

Multichannel Sound Systems

• 5.1 channel systems are now specified for various home entertainment systems (AC3, MPEG4).
• The subwoofer position is usually not critical, due to its omni-directional character.