Lexicon CP-1 Theory Of Operation page 10

Multi-Channel Music & Cinema Systems
Theory and Design
Lateral Sound in your Listening Environment
In an ordinary listening room, conventional stereo set-ups (with loudspeak-
ers separated by 60° or less) provide a feeling of spaciousness only at upper
frequencies. It is the room reflections excited by the speakers which provide
low frequency lateral energy. A standard 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. (Out-of-phase bass is intentionally
provided in the best stereo recordings.) Another reason is that most listen-
ing 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 not optimal 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 room sound but they also cause coloration and muddiness. Small
rooms usually sound better if these reflections are broken up (with wall
hangings, furniture 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.
Lexicon processors resolve this deficiency by supplying appropriate signals
to loudspeakers at the sides of the listener. Our processors give the listener
a choice: the sideways energy cues of the original recording can be extracted
by our processor, or a different hall sound can be generated and supplied
from the correct directions.
Low Frequency Management in a Multichannel System
Recent research into low frequency propagation in listening rooms has
shown that the use of a single low frequency driver to cover the frequency
range of 40Hz to 120Hz is not optimal. Modeling of rectangular rooms and
measurements in several rooms both indicate that a single subwoofer (in the
front corner of the room) can create a high level of low frequency energy in
the area immediately around the subwoofer, but this level can drop sub-
stantially and non uniformly for standard listening positions.
7