Page 261 - The Master Handbook Of Acoustics

236 CHAPTER TEN

Source Image

Wall

FIGURE 10-1
Reflection of sound from a point source from a flat surface (incident sound, solid
lines; reflected sound, broken lines). The reflected sound appears to be from a vir-
tual image source.

reflector for 10 kHz sound (wavelength about an inch). At the low end
of the audible spectrum, 20 Hz sound (wavelength about 56 ft) would
sweep past the book and the person holding it as though they did not
exist, and without appreciable shadows.
Below 300–400 Hz, sound is best considered as waves (chapter 15
expounds on this). Sound above 300–400 Hz is best considered as trav-
eling in rays. A ray of sound may undergo many reflections as it
bounces around a room. The energy lost at each reflection results in
the eventual demise of that ray. Even the ray concept is an oversimpli-
fication: Each ray should really be considered as a “pencil” of diverg-
ing sound with a spherical wavefront to which the inverse square law
applies.
The mid/high audible frequencies have been called the specular
frequencies because sound in this range acts like light rays on a mir-
ror. Sound follows the same rule as light: The angle of incidence is
equal to the angle of reflection, as in Fig. 10-2.