Lexicon CP-1 - Manual (Page 39)

Page 35
CP-1 Digital Audio Environment Processor
Lateral Sound
For decades the study of concert-hall acoustics relied on certain basic
measurements to characterize halls, the main one being the time it takes a
sound to drop in level (decay) by 60 decibels. This is called the reverberation
time or RT
60
and is approximately the same as the time it takes a hand clap
to subside to inaudibility. The RT
60
is measured as a function of frequency,
usually in bands one octave wide, over the range of audible frequencies. The
resulting curve forms a frequency contour for the hall.
While it was found that most good halls have comparable RT
60
curves,
different halls with similar RT
60
measurements can sound very different
from each other and listener reactions to them can vary widely. Clearly,
other important factors in the sound field were not being measured.
Through the work of many people (including Manfred Schroeder, A. H.
Marshall, Michael Barron and others) some of these factors were identified.
In an effort to answer the question of why some halls sound so much better
than others, Schroeder devised a method for comparing 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 ear-
phones, gave excellent reproduction of spatial qualities. Unfortunately, the
stereo image tended to appear entirely inside the head, spoiling the accu-
racy of such recordings as a test of concert halls.
To overcome this problem, Schroeder played his recordings through a pair
of loudspeakers in an anechoic chamber, using a special electro-acoustic
technique (developed by Atal, Schroeder, Damaske and Mellert) to elimi-
nate crosstalk between the listener’s ears. Normally each speaker is heard
by both ears but Schroeder’s system canceled the sound reaching the right
ear from the left speaker and vice-versa. Provided that the listener held his
head in exactly the right spot, the sound had all the excellent localization
properties of earphones but was properly located outside the listener’s
head.
This technique allowed the first direct comparisons of specific halls. 1 From
these studies it was found that the best halls were all characterized by
having large differences in the sound between the two ears in the dummy
head. Very simply, the best halls gave the most stereo.
Michael Barron defined this characteristic in halls as Spatial Impression (SI)
and found that it was created by sideways-moving reflected sound.
2 Only
reflections which move from side to side produce SI, because only they give
rise to sound differences between the two ears. Reflections from the front,
ceiling, floor or back wall add loudness and muddiness to the sound but it
is the lateral reflections that draw the listener into the music.
Theory
and
Design
Concert Hall Acoustics
Halls with similar RT
60
sound very
different. The best have large
amounts of sideways moving re-
flected sound.
6