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After sound has been produced in, or enters, an enclosed space, it is reflected repeatedly by the boundaries of the enclosure, even after the source ceases to emit sound. This prolongation of sound after the original source has stopped is called reverberation. A certain amount of reverberation adds a pleasing characteristic to the acoustical qualities of a room. However, excessive reverberation can ruin the acoustical properties of an otherwise well-designed room. A typical record representing the sound-pressure level at a given point in a room plotted against time, after a sound source has been turned off, is given in the decay curve shown in the illustration. The rate of sound decay is not uniform but fluctuates about an average slope. See Sound
the process by which a sound in an enclosed space gradually decays after its source ceases to emit. The volume of air in the enclosure constitutes an oscillatory system with many natural frequencies. Each natural oscillation is characterized by a decay rate; the absorption of a sound during its reflection from the boundaries of an enclosure and during its propagation determine the decay rate of the sound. Consequently, the natural oscillations of various frequencies that are excited by the same source do not decay simultaneously.
Reverberation has an important effect on the audibility of speech and music in an enclosure because the listener perceives direct sound against a background of oscillations that have been previously excited in the volume of air; the spectra of these oscillations vary in time because of the gradual decay of the natural oscillations. Reverberation is all the more important when oscillations decay slowly. When the dimensions of a room are large compared with the wavelengths, the spectrum of natural oscillations can be regarded as continuous and reverberation can be represented as the result of both a direct sound and a series of delayed repetitions with decreasing amplitudes, which reflect from the boundary surfaces.
The duration of reverberation is characterized by reverberation time, during which the intensity of a sound decreases by a factor of 106 and the level decreases by 60 decibels. Reverberation time is a very important factor in determining the acoustics of a room. Reverberation time is longer when a room is larger, the time of a free sound path is longer, and there is less absorption by the boundary surfaces. Reverberation time is measured by recording a sound level as it decreases after the source of the sound has been cut off; the recording devices used for this purpose have a logarithmic scale. Reverberation time is calculated from the average slope of the recorded level graph.
Reverberation is also applied to the echoes produced in the ocean by the reflection and scattering of an original sound from the bottom (bottom reverberation), from the agitated surface (surface reverberation), or from a heterogeneous aqueous medium, fish, and other biological objects (volume reverberation).
REFERENCESBeranek, L. Akuslicheskie izmereniia. Moscow, 1952. (Translated from English.)
Furduev, V. V. Akusticheskie osnovy veshchaniia. Moscow, 1960.