Reverberation

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).

See: Canyon Effect, Decay, Diffuse Sound Field, Phasing, Volume, diagram under Envelope. Compare: Damping, Eigenton, Free Field, Resonance.

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 realized consciously, reverberation is one of many cues used by a listener for orientation in a given space. However, see also the precedence effect.

The ratio of direct to reverberated sound is also an important cue for the perception of depth and distance. In larger spaces, the intensity of the direct sound falls off more sharply with distance than that of the reverberated sound, and thus the ratio shifts in favour of the latter. In an enclosed space, the reverberation time is proportional to the volume of the space and inversely proportional to the sum of each surface area multiplied by its absorption coefficient.

See also: Acoustic Space, Anechoic Chamber, Projicience.

Reverberation will also increase the Ambient Noise Level and apparent loudness of sounds within a space, an important factor to consider in the acoustic design of classrooms, daycare areas, office and industrial spaces (see lo-fi). Reverberation will also make speech indistinct by masking the onset transients, but with many types of music, particularly symphonic, reverberation adds to the blend of the individual sounds when the reverberation time is 1 - 2 seconds. Longer times tend to blur the sounds and require slower tempi to avoid indistinctness. Reverberation times of less than a second are necessary for speech comprehension. See: Articulation Index.

Architectural acousticians stress the importance of early reflections (arriving within the first 80 ms) which reinforce the direct sound as long as the angle of reflection is not too wide. Reflections arriving after 80 ms add reverberant energy which is often described as giving the sound spatiousness, warmth and envelopment. The acoustic design of such spaces usually involves creating a balance between clarity and definition on the one hand, and spatiousness on the other. Listeners often have different preferences as to this balance.

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, these units tend to have very uneven frequency response, falling off sharply at high frequencies, with the result that the sound is characteristically coloured or blurred. As well, the echo density (i.e. the number of reflected repetitions per second) is often not high enough to avoid a 'fluttering' of the sound, particularly with very short percussive sounds.

However, digital processing devices and computer techniques (such as the Schroeder model) 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 (such as Chowning's at Stanford University) 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). Others allow the frequency spectrum of the reverberation to be controlled (e.g. to simulate 'bright' or 'dark' rooms with greater or lesser high frequencies, respectively), or the reverberation to be gated (i.e. attenuated at the end of the direct sound) or even reversed.

See: Sound Synthesizer.

Ref.: M.R. Schroeder, "Natural Sounding Artificial Reverberation," Journal of the Audio Engineering Society, vol. 10, no. 3, 1962, pp. 219-223; John Chowning, "The Simulation of Moving Sound Sources," Journal of the Audio Engineering Society, vol. 19, no. 1, Jan. 1971, pp. 1-6.