Lexicon DC-1 Supplementary Manual page 9

DC-1Theory
and Design
The confusing frontal reflections can be
absorbed, leaving the essential lateral
ones. This is better, but not ideal.
With the DC-1, confusing short reflec-
tions can be absorbed; the DC-1 will
supply the essential lateral sound —
which can simulate a much larger space.
The more absorbent the playback room,
the better it will sound, and the closer it
will sound to a real hall, or larger envi-
ronment.
6
The reason is simple — music is highly effective at masking its own reflected
energy. Most of the time, note follows note with very little space between.
It is only in the gaps between notes that our ears have time to perceive the
background sound which the hall creates. When that sound has a strong
lateral component, a sense of spaciousness is created.
The time delay of the lateral reflected energy turns out to be very important.
The ear is relatively insensitive to reflected energy in the first 150ms or so
after a note ends, and musical masking increases this dead time to over
300ms. The greater the amount of reflected energy that comes to our ears
after this time, the greater the sense of spaciousness and reverberance. Thus,
both the loudness of the reverberation (the total energy relative to the direct
sound) and the reverberation time are important to our perception. In
natural halls the loudness and the reverberation time are linked by the hall
volume. In a very large hall the reverberation level tends to be low, but the
lack of level can be compensated by an increase in reverberation time. This
effect can be clearly heard in an organ concert in a great cathedral. The
organ, although often quite distant, is completely clear, yet it is bathed in a
marvelous quiet reverberance. Similarly, small rooms with very little
absorption can be loud and muddy, but seldom seem spacious or reverber-
ant.
This understanding of the importance of late arriving lateral energy is quite
new, and its effect on hall design has yet to be seen. The best halls have high
late arriving lateral energy for obvious architectural reasons, such as a long
narrow shape with high coffered ceilings. Fan-shaped halls have better
sight lines, are more adaptible to multiple uses, and hold a greater number
of seats for their total volume than shoe-box halls. However, their greater
number of seats increases the total absorption of the audience, and de-
creases the strength of the later reverberance. These differences can some-
times be overcome. A notable example is the Boston Symphony's fan-
shaped Tanglewood music shed. Although it used to be thought that
diffraction from the edges of the overhead reflectors provided the needed
spaciousness, it is more likely that the high internal volume of the hall and
the low absorption at low frequencies combine to make the low frequency
reverberance unusually audible for such a hall. Middle and upper
frequences are dry unless one is distant from the orchestra.
In a small playback room, a spacious and enveloping sound can be achieved
only if several conditions are met. First, significant energy should reach the
listeners from the side. Second, this energy must be different on the left and
right, i.e. it should be stereo, with excellent separation. Third, at least some
of this energy should have a time delay of at least 100ms. These conditions
can only be completely satisfied by placing loudspeakers at the sides of the
listeners, and then driving those loudspeakers with a stereo signal which
contains either ambient information from the original venue, or ambient
information synthesized in the processor. Achieving all of these conditions
is the guiding principle behind the DC-1.
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