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Side sound must be within this angle.
Theory
and
Design
Understanding the importance of lateral reflections enabled more accurate
evaluation of architectural acoustics. The best halls were usually found to
have high lateral energy for obvious architectural reasons, such as a long
narrow shape with high ceilings. Fan-shaped halls, although they have
better sight lines, are more adaptible to multiple uses and hold a greater
number of seats for their total volume than a shoe-box hall, tend to have
fewer attractive, sideways reflections and more of the unpleasant, monau-
ral overhead and rear-wall kind. These differences can be overcome by
design. A notable example is the Boston Symphony’s fan-shaped Tangle-
wood music shed. Tanglewood sounds good because the reflectors above
and in front of the orchestra (known as clouds) have angled sides that reflect
energy sideways onto the audience instead of just downward.
Lateral Sound in your Listening Environment
In an ordinary listening room, conventional stereo set-ups (with loudspeak-
ers separated by 60 degrees or less) do not excite enough SI to sound
pleasant without some help from the room. (A similar speaker arrangement
in the artificial environment of an anechoic chamber is exceedingly detailed
and precise, but unpleasant.) Some lateral reflections are needed to make
the sound musical.
Ordinary two-speaker stereo works as well as it does because sideways-
moving reflections can be excited at low frequencies by two loudspeakers
if they are placed asymmetrically in the room or if they are driven with out-
of-phase low-frequency information.
3,4
(Out-of-phase bass is intentionally
provided in the best stereo recordings.) Another reason is that most
listening rooms have reflective surfaces to the sides of the listener. A
popular listening room treatment puts absorptive material at the front of the
room, leaving the walls by the listener reflective. This improves the clarity
by removing the front reflections, while retaining those from the side. This
also explains the appeal of loudspeakers that produce lots of sideways-
reflected energy.
Unfortunately, in most two-speaker set-ups the mid- and high-frequency
lateral sound is reduced unless the speakers are unusually widely placed.
The listener can hear a little of the original hall, stretched between the stereo
loudspeakers, but never really becomes a part of it. What is worse, the
lateral sound that exists in most playback rooms has so little delay that the
ear can not separate it from the direct sound. The reflections generate some
SI but they also cause coloration and muddiness. Small rooms usually
sound better if these reflections are broken up (with wall hangings, furni-
ture or bookcases) or absorbed (with curtains or sound-absorbent panels).
When this is done the room becomes quieter and clearer but not in any way
like the original hall.
The Lexicon CP-1 resolves this deficiency by supplying appropriate signals
to loudspeakers at the sides of the listener or by modifying signals to the
main loudspeakers to fool the ear into thinking there are loudspeakers at the
Sound from the side is vital to lis-
tener comfort and involvement....It
must really be from the side!
In an ordinary room, the room sup-
plies these directions, and the sound
is cramped, but tolerable. The overall
impression, however, may be muddy
due to unwanted frontal reflections.
No Sound
In an anechoic chamber, this side-
ways sound is missing...and music
sounds unpleasant.