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.

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