pleasantness of the sound field, and enlarge the listening area. This is particularly true
when speakers are placed along the sides of the room.
The primary improvement is in the perception of spaciousness. To see how multiple
speakers can help, we can look at the perceptual origin of spaciousness. Spaciousness in
concert halls and in small rooms is primarily determined by the spatial diffusion of
sounds that arrive at least 160ms after the ends of strong foreground sound events. It is
the spatial properties of the background sounds between notes, which determines how
much we feel involved with and enveloped by music.
In a small room the spatial properties of low frequencies are primarily determined by the
ratio of the lateral room modes to the back/front and the vertical modes. It is the
interference between these two modal types that determines how spacious the sound is.
The ratio of the two modal types is strongly influenced by the location and spacing of the
loudspeakers in the room, and in how the recording was made. At higher frequencies the
spatial properties are determined by the frequency of interest and the spacing between the
front loudspeakers. At some frequencies even a relatively close spacing can produce
substantial spaciousness, which is why ordinary stereo works at all.
However at all frequencies it is primarily the REVERBERANT portion of the recorded
sound which is gives rise to the sensation of spaciousness. Thus it is possible to make a
recording in which most of the instruments are pan-potted to positions near the center,
and still have it sound spacious after stereo reverberation is added. In popular music such
recordings are perhaps the rule rather than the exception. It is easy to show that two
ordinary stereo loudspeakers in a small room cannot reproduce the spatial diffusion of a
large hall or concert space. However, if the reverberant portion of the stereo signal can be
reproduced through an array of loudspeakers at the sides of the listener(s) a far more
satisfactory diffuse field can be created. Multiple speakers, if they are driven with
independent decorrelated signals, can create a diffuse sound field both at low frequencies
and at high frequencies.
A multichannel matrix audio system has two goals. First, the system should duplicate as
best as possible the sound balances and localization created by the sound mixer and the
producer. This should be true both for recordings that were mixed for multichannel
surround and for normal stereo recordings. Second, both with five-channel material and
with ordinary two-channel material the playback system should maximize the spatial
diffusion of the background sound field. (A system which was compatible with the
existing Pro-Logic standard would also be plus.)
The first requirement, that localization and balance should be preserved, is decidedly
tricky. For example, stereo recordings are routinely mixed for reproduction without a
center loudspeaker. The sound mixer mixes the center channel information equally into
the left and right channels of the recording. When we reproduce such a recording over a
matrix system that includes a center speaker, we want exactly the same balance as the
original, but we want a substantial amount of the sound power to come from the center
loudspeaker. To achieve this goal we must drive the center speaker with a signal derived
