When sound is produced in an enclosed space multiple reflections build up and blend together creating reverberation. This is most noticeable when the sound stops but the reflections continue, decreasing in amplitude, until they can no longer be heard. The time it takes for the level of the reverberation to decay 60 decibels is known as the reverberation time, or RT(60).
Cathedrals, gymnasium, indoor swimming pools, large caves etc. are examples of spaces where the reverberation can clearly be heard.
Reverberation can make it difficult to hear speech. If the reverberation from one syllable overlaps the next syllable it may make it impossible to hear. Cat, cab, and cap, will all sound the same.
Different types of music tend to sound best with different reverberartion times during a live performance, classical music and choral music tend to require longer reverberation times than modern rock and Popular music, for example.
Reverberation times are ususally different at different frequencies, depending on the acoustics of the space.
Reverberation can be created artificially for both acoustical and recording purposes.
There are several different electronic mechanisms used to create a reverberation effect:
- Plate Reverberators use large (approx. 1 m x 2 m) metal plates, suspended with springs within dampened cases to create a reverb effect. One or two transducers apply a signal to the plate by vibrating, and electronic pickup convert the vibration of the plates into an electric signal. Reverberation time can be controlled by applying a mechanical damper to the plate.
- Spring Reverberators use coiled springs to reverberate a signal, given a transducer and pickup. Spring reverberators were often integrated into instrument amplifiers, and are generally considered to have the least "real" sound. An exception to this are the high-quality AKG BX-series spring reverbs.
- DSP Reverberators use electronics and signal-processing algorithms to create the effect of reverberation, through the use of large numbers of long delays with quasi-random lengths, combined with equalization, envelope-shaping, and other processes. Digital reverberators are by far the most ubiquitous.
- Chamber Reverberators. The most simple implementation of "artificial reverb." A large room with solid walls, such as the empty, excavated concrete foundation of a builiding is equipped with a loudspeaker at one end and microphones at the other. Chambers can produce long reverbs with a natural sound, but because of the space required, they are not in common use anymore. (Legend has it that the entire foundation of the Capitol Records building in Los Angeles was configured into one large echo chamber, but had to be renovated into underground parking for economic reasons.) Also, it is difficult to control the shape and characer of a chamber reverb, for obvious reasons.