As already pointed out in Chapter I the reflection of sound is in many ways analogous to the
reflection of light and water waves, and therefore obeys the law of equal angle of incidence
and reflection. Thus in Figure 109 angle "i" equals angle "r." Also by analogy to optics the
reflected sound R appears to come from its image source S. The rebound of a billiard ball off
the table cushion resembles the reflected ray of sound from a flat surface. In this case, the
angle which the path of the ball makes with the cushion as it bounds away is of the same
degree as the angle which the path of the ball made with the cushion as the ball approached it.
158. Reverberation.-The action of sound when confined in an enclosure is much more
complex as compared to its action in free air. In free air, only the direct sound from the source
can be heard. In a room, however, the sound one hears is composed of both the direct and the
reflected waves. Consider the travel of a single ray of sound from the source S in Figure 110.
As it proceeds as part of the expanding spherical sound wave, it first meets with a surface
where it suffers partial reflection, absorption and transmission. The reflected portion now
continues until it strikes a second surface, where it again suffers partial reflection, absorption
and transmission. This process is repeated until the sound ray is completely dissipated.
Likewise, all other portions of the spherical wave undergo this same process of multiple
reflection. In an ordinary room with plaster walls and ceiling, due to these successive
reflections the sound energy is rather quickly diffused throughout the entire room. In such a
room the sound will suffer from 200 to 300 reflections before its energy is completely
dissipated. If we recall that sound travels 1120 feet per second, it is readily seen that the
duration of this prolonged "after-sound," called the reverberation time of the room, will be
several seconds for the average theatre.