Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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Figure 4.13 Sound radiation from a loudspeaker, viewed from top
4.1.7.1 Absorption, Reflection, Refraction, and Diffraction
In the real world, there are any number of things that can get in the way of sound, changing its
direction, amplitude, and frequency components. In enclosed spaces, absorption plays an
important role. Sound absorption is the conversion of sound’s energy into heat, thereby
diminishing the intensity of the sound. The diminishing of sound intensity is called attenuation.
A general mathematical formulation for the way sound attenuates as it moves through the air is
captured in the inverse square law, which shows that sound decreases in intensity in proportion
to the square of the distance from the source. (See Section 4.2.1.6.) The attenuation of sound in
the air is due to the air molecules themselves absorbing and converting some of the energy to
heat. The amount of attenuation depends in part on the air temperature and relative humidity.
Thick, porous materials can absorb and attenuate the sound even further, and they're often used
in architectural treatments to modify and control the acoustics of a room. Even hard, solid
surfaces absorb some of the sound energy, although most of it is reflected back. The material of
walls and ceilings, the number and material of seats, the number of persons in an audience, and
all solid objects have to be taken into consideration acoustically in sound setups for live
performance spaces.
Sound that is not absorbed by objects is instead reflected from, diffracted around, or
refracted into the object. Hard surfaces reflect sound more than soft ones, which are more
absorbent. The law of reflection states that the angle of incidence of a wave is equal to the angle
of reflection. This means that if a wave were to propagate in a straight line from its source, it
reflects in the way pictured in Figure 4.15. In reality, however, sound radiates out spherically
from its source. Thus, a wavefront of sound approaches objects and surfaces from various
angles. Imagine a cross-section of the moving wavefront approaching a straight wall, as seen
from above. Its reflection would be as pictured in Figure 4.15, like a mirror reflection.
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