Probably we can obtain a better mental picture of reverberation by using optical analogy again. Let us imagine that we have a room with mirrored walls and both a source of sound and light at the point S as in Figure 111. If an observer is stationed at A he will see the object and hear the direct sound, not only at S but he will see the images and hear the reflected sounds also from the image sources S1, S2, S3, and S4. These images form the first reflections and are reflected again, and likewise the second images are reflected, forming a new set of images. Obviously the successive images are farther and farther away so that the image the farthest away would be the faintest in loudness. Optically, one stationed at A would see many images of S in all directions.

The effect of reverberation is to cause blurred speech and music, due to the overlapping of the successive syllables in speech. Up to a certain point this overlapping is apparently beneficial because it increases the loudness of sound, but beyond this optimum point overlapping of successive syllables is detrimental. Thus, there is a period of reverberation, depending on the size of the auditorium, which will give optimum results. Although reverberation greater than the optimum value is detrimental, it is tolerable if not excessive. Thus, there is a time of reverberation greater than the optimum period which will give acceptable results. In general, the reverberation time should never be much greater than two seconds.

The audience in any theatre causes the time of reverberation to vary, for each auditor absorbs a large quantity of sound. It is therefore desirable to have optimum results obtained for the average size audience and acceptable results for the minimum size audience entertained in the auditorium.

The reverberation time of a room, which is the time required for a sound of given initial intensity to die away to the point where it is just barely audible, depends directly on the loudness of sound and the size of the room, and depends inversely on the absorption in the room. The standard reverberation time of a room is the reverberation time obtained when using an initial intensity of 1,000,000 times the intensity of the faintest sound which can be heard. This represents a sound volume of six times the loudness of a sound barely audible.

Increasing the size of a room increases the reverberation period due to the fewer number of reflections which occur in it during a given space of time, although the total number of reflections remains approximately the same.

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