Bose Panaray MA12 Technical Foundation & Discussion - Page 13
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THREE KINDS OF SOURCES
A
explained, the hang-and-tilt approach evolved because of a fundamental property of the
S
speakers used. Namely, the speakers have sound waves that radiate in all directions, in and
out, up and down, left and right. And that means that the intensity of sound leaving the
sources falls off by 6 dB per doubling of distance. We call these spherical-wave sources. To
provide consistent sound levels over a widely distributed audience, listeners must be nearly
equidistant to the speaker, and hence the need to raise the speaker into the air over the heads
of the listeners.
The obvious question is whether this fundamental 6 dB per doubling of distance property is
true for all sources. The answer is no. There are other kinds of sources that produce different
kinds of sound waves with very different behavior.
Plane waves
F
example, we generate plane waves to measure the performance of compression drivers
OR
in the lab. These waves diverge in only one dimension: out, but not up and down or left and
right. As a result, the intensity of a plane wave does not fall off at all with distance
in Figure 5. In other words, the distance can be doubled and the sound level is the same. Such
a source, therefore, could produce the same sound pressure level in all seats – what would be
considered ideal coverage.
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This is an ideal description. In reality, any sound wave, whether spherical or planar, is affected by
environmental effects, such as humidity, temperature and wind.
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Bose
MA12™ Modular Array: Technical Foundation & Discussion
April 2002, © Bose Corporation, All Rights Reserved
Page 13 of 36
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, as shown