Lexicon 960L Owner's Manual page 62

Using the Reverb Programs
The perception of envelopment requires that the interaur-
al time delay and level differences should be fluctuating.
In a small relatively dead room the only way these differ-
ences can fluctuate is if you have at least two sound
sources, and the relative phase between these sources is
fluctuating. (Ideally these sound sources should be repro-
ducing decorrelated reverberation.) The optimum direc-
tion for these two sound sources (loudspeakers) depends
on frequency. Once again below 700Hz the optimum
direction is from the side, and as the frequency goes up
the optimum angle moves toward the medial plane – clos-
er to the front, or closer to the rear.
Although envelopment can be perceived at all frequen-
5-6
cies, psychologically the low frequencies are particularly
important. For example, even in a dead room a standard
two channel loudspeaker system (+/-30 degrees from the
front) can produce envelopment if the sound source is
decorrelated and contains primarily frequencies above
1000Hz – such as strings or applause. But low frequency
envelopment, the life blood of lower strings and basses, is
missing. Add low frequency uncorrelated reverberation to
loudspeakers at the sides, and the life magically returns.
(For a more thorough overview of this subject, see David
Griesinger's web page at www.lexicon.com/links.)
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