About The Reverberation Algorithm - Lexicon LEXICON480LV4 Owner's Manual

Lexicon 480L Owner's Manual
Before we jump into detailed descriptions of
programs and parameters, let's take a look at the
philosophy behind the reverberation algorithm's radi-
cal new structure.
About the
Reverberation Algorithm
The 480L incorporates the results of a great deal of
research into acoustics and reverberation. It produces
four general classes of sounds: ambience, room simu-
lations, plates, and gated sounds.
In Search of Ambience
Ambience is the use of reverberation or reflected
sound energy to give recorded music a sense of being
performed in a real acoustic location. Ideally, am-
bience gives warmth, spaciousness and depth to a
performance without coloring the direct sound at all.
Recent research into ambience has shown that this
phenomenon depends most critically on the shape of
the initial reverberation build-up and decay. Ambience
is perceived and has benefit while the music is running
(which is most of the time). But once the reverberation
has decayed 15 dB it is no longer audible in the
presence of the direct sound. So the time it takes for the
sound to build up and decay 15 dB determines the
perceived reverb time, regardless of what the decay
time to -60 dB is. Some very good halls for recording
have a rather uneven initial build-up and decay, giving
a much longer effective reverb time than their -60 dB
reverb time might suggest.
It has become common practice to use predelay in an
attempt to emulate the sound of these halls. Adding
delay to the reverb sends definitely increases the
effective audible reverb time and the apparent size of
the hall, but the result sounds unnatural.
If we make echograms of real halls, we find that there
is usually a gradual buildup of energy between the
arrival of the direct sound and the time at which the
reverberation reaches maximum loudness. The sharp
attack of added predelay in most reverberation devices
sounds entirely different.
In the 480L, the SIZE, SPREAD and SHAPE controls
allow adjustment of the buildup and decay of the initial
part of the reverberation envelope. SHAPE controls
the shape of the envelope, while SPREAD and SIZE
3-2
the set the time over which this shape is active.
In the hall and room programs, SIZE acts as a master
control for the apparent size of the space being created
by the 480L. Both SPREAD and RT MID varylinearly
with the setting of SIZE. Thus maximum reverb time
and spread require high settings of SIZE. To find an
appropriate reverb sound, start with a preset with a
similar sound to what you want to end up with. Simply
varying SIZE is often sufficient to arrive at the exact
sound you are seeking.
Once a size has been selected, SPREAD and SHAPE
are used to adjust the shape and duration of the initial
reverb envelope, which together provide the major
sonic impression of room size.
When SHAPE is at minimum, the reverberation env-
elope builds up very quickly to a maximum amplitude,
and then dies away quickly at a smooth rate. This
envelope is characteristic of small reverberation cham-
bers and reverberation plates. There are few (if any)
size cues in this envelope, so it is ineffective in creating
ambience. With this SHAPE setting, SPREAD has no
effect. The density is set by the size control, and the
rate of decay is set by RTMID. This reverberation
envelope is typical of many of the popular digital
reverberators of the last few years.
As SHAPE is raised to 32 (about 1/8th of the way up)
the initial sharp attack of the reverberation is reduced,
and reverberation builds more slowly. The envelope
then sustains briefly before it begins to die away at the
rate set by RTMID. SPREAD has little or no effect on
this shape.
When SHAPE is at 64 (1/4 of the way up) buildup is
even slower and the sustain is longer. Now SPREAD
affects the length of both the buildup and the sustain.
As a rough estimate, the sustain will be approximately
the time value indicated by the SPREAD display (in
milliseconds).
As SHAPE is raised further, the buildup and sustain
remain similar, but now a secondary sustain appears in
the envelope, at a lower level than the first. This
secondary plateau simulates a very diffused reflection
off the back wall of a hall, and is effective in creating a
sense of size and space. This reflection becomes
stronger and stronger, reaching an optimal loudness at
a SHAPE value of about 128 (1/2 way up).
The highest SHAPE settings are typically used for
effects. Near the top of the scale the back wall reflection