Reverberation is a result of multiple reflections. A sound wave in an enclosed or semi-enclosed environment will be broken up as it is bounced back and forth among the reflecting surfaces. Reverberation is, in effect, a multiplicity of echoes whose speed of repetition is too quick for them to be perceived as separate from one another. W.C. Sabine established the official period of reverberation as the time required by a sound in a space to decrease to one-millionth of its original strength (i.e. for its intensity level to change by -60 dB). However minimal, the reverberant quality of any space, whether enclosed or not, helps to define the way in which it is perceived. Although it may not be realised consciously, reverberation is one of many cues used by a listener for orientation in a given space. The ratio of direct to reverberated sound is also an important cue for the perception of depth and distance. Reverberation will also increase the ambient noise level and apparent loudness of sounds within a space, an important factor to consider in acoustic design.
Artificial reverberation is traditionally produced by means of a reverberation chamber or echo chamber, multiple tape echo, or more commonly, by exciting a metal spring or plate at one end, and picking up the delayed signal at another point. However, digital processing devices and computer techniques have been developed in recent years that allow a good simulation of naturally produced reverberation. These systems allow for a variable ratio of direct to reflected sound, and some include both global reverberation (i.e. reflected sound from all directions) and local reverberation (i.e. that coming from the direction of the sound source). (Source: Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)