Lexicon DC-1 Supplementary Manual page 7

DC-1Theory
and Design
Lateral Sound
Halls with similar RT sound very dif-
60
ferent. The best have large amounts of
sideways moving reflected sound.
4
Concert Hall Acoustics
For decades the study of concert-hall acoustics relied on certain basic
measurements to characterize halls, principally the time it takes a sound to
drop in level (decay) by 60 decibels — referred to as RT60, or "reverberation
time". Approximately the time it takes a hand clap to subside to inaudibil-
ity, reverberation time is measured as a function of frequency, usually in
bands one octave wide, over the range of audible frequencies. Although this
measurement is of fundamentalimportance in predicting the rate at which
sound decays, it does not predict how loud or how audible the sound decay
will be, and it does not predict the intelligibilty of speech or the clarity of
music. Other measures are clearly needed to determine hall quality and
suitability for a particular function or type of music.
The search for accurate and reliable measures of hall quality has been long
and contentious. Measures of intelligibility or clarity were the first to be
developed, and many have been proposed. The most common measures,
(Clarity or C80, Centertime, Deutlichkeit, etc.) have been shown to be highly
correlated in actual halls. They thus are closely related, and have common
virtues and faults. None of them appear to accurately predict intelligibility,
although they are still useful.
An accurate measure for the quality of reverberance, and for the properties
of spaciousness and envelopment, has proved much more difficult. A. H.
Marshall and Michael Barron noticed in laboratory experiments that reflec-
tions arriving from the side were important to the perception of spacious-
ness. Although this perception clearly applies equally to both early and late
arriving sound, Barron concluded that the early arriving reflections were
the most important. This conclusion (which in hindsight is misleading at
best) has dominated academic thought on hall acoustics for many years.
Fortunately, the best working acousticians gave lip service to the idea, and
continued to build halls with a combination of tradition and blind luck.
What was really needed was a method to study halls in the laboratory, but
under realistic conditions. One step toward this process was provided by
Manfred Schroeder. In an effort to answer the question of why some halls
sound so much better than others, Schroeder devised a method for compar-
ing them without transporting his subjects from hall to hall. Using a dummy
head with microphone diaphragms in place of ear drums, Schroeder made
binaural recordings in many halls. These recordings, played back through
earphones, gave excellent reproduction of spatial qualities. Unfortunately,
the stereo image tended to appear entirely inside the head, spoiling the
accuracy of such recordings for some listeners.
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