Many musical instrument amplifiers, such as those used for electric guitars, feature a reverb effect. One of the more popular methods sends audio signals through a set of special metal springs; sound vibrations bounce back and forth through the springs, which impart reverberations in the sound. Another method processes sound signals with digital circuitry, generating the effect through complex time delays. Although the digital method gives you a wider variety of effects, many musicians prefer the traditional sound of spring reverb.
Amplifiers generally have one control for reverb that adjusts the amount of the effect you hear in the amp's output. Set at zero, you hear the instrument amplified but without reverb. At maximum, the effect may be excessive, sounding like you're playing in a giant underground cavern. In-between, you get varying amounts of reverb to match the acoustics of a venue or the style of music you're playing.
With natural reverberation, sounds decay slowly and gradually rather than immediately; cathedrals and large concert halls have the longest decay times; smaller rooms have shorter times. A spring-type reverb has a fixed amount of decay time, amounting to a few seconds, so amplifiers with spring units do not have a decay control. Digital reverbs, on the other hand, can simulate anything from small rooms to cathedrals, and typically have a knob you adjust for decay time. This lets you set the reverb for particular kinds of music; a choir, for example, is well-suited to long decay times, and club jazz works with shorter, more intimate-sounding decay.
The versatility of digital reverb units allows them to replicate many different types of reverb. A good digital reverb will, for example, have settings that sound like spring, plate and tape delay as well as simulating different kinds of rooms. Plate reverb refers to vintage effects units that used a large metal plate instead of a set of springs. Although more bulky and expensive than spring units, plate reverb has a more natural, even sound, lacking the "twangy" character of springs. Digital technology gives you these and other effects in an inexpensive, portable and rugged device. A control knob on the device lets you easily select from different types of reverb.