3Com CP-1 owner manual Theory Design, Lateral Sound, Concert Hall Acoustics

Theory

and 6

Design

CP-1 Digital Audio Environment Processor

Lateral Sound

Concert Hall Acoustics

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 RT60 and is approximately the same as the time it takes a hand clap to subside to inaudibility. The RT60 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 RT60 curves, different halls with similar RT60 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.

Halls with similar RT60 sound very different. The best have large amounts of sideways moving re- flected sound.

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