An audio amplifier takes a signal from a radio tuner, CD player or other source and boosts the electrical current to drive a set of loudspeakers. A guitar amplifier, though similar, is designed to work with a guitar signal. In the amplifier, a preamplifier circuit gives the signal an initial boost and routes it to a reverberation circuit, which gives the sound a reverb effect. An electronic mixer combines the original signal and reverb signals. A power amplifier circuit takes the combined signal and sends it to a loudspeaker.
In the 1930s and 40s, a sound engineer created artificial reverb by placing a speaker at the base of a stairwell and a microphone on a landing. The microphone picked up the sound reverberated by the walls of the stairwell. Another original reverb method from this era, and still popular, uses a set of metal springs about a foot long. A mechanism called a transducer turns an amplified signal into mechanical waves that travel back and forth through the spring. A pick-up at the other end of the spring turns the waves back into electronic signals. An amplifier sends electronic sound signals to the transducer, and the springs add reverberation to the sound. Spring, also called tank, reverberation systems have a rich sound but are affected by shock and vibration. The springs pick up vibrations from foot stomping, dancing and other movement, and annoying "twanging" sounds enter the music. Though some musicians deliberately slap the reverb unit to get this effect, it is usually unwanted.
Sound engineers have developed specialized computer circuits that simulate the reverberation of many different musical venues, such as small clubs, rooms or concert halls. This digital reverb works by converting music signals into a rapid stream of data. A microprocessor and software introduce complex delays in the data stream, which other circuits convert back into musical signals. With digital reverb, you can select from a variety of different reverb effects. As the effect does not involve mechanical parts, vibration can't distort it.
The illusion of an auditorium's space in the sound of reverb is related to the time it takes for the reverberation to die down. Larger venues, and those with flat, parallel walls have more prolonged reverb. Generally, reverb adds richness to sound. Choral and orchestral music benefit from long reverb times, but more intimate kinds of music, such as jazz and folk, do better with the short reverb times you get in a club-like setting. A spring reverb has a fixed reverb time, usually between one and two seconds. You can choose different amounts of reverb time on a digital reverb unit, dialing it up or down.
A dial on the reverberation amplifier lets you control the amount of reverberation in the music for different situations. With the control set to zero, or no effect, you get a "dry" mix without any reverb. A "wet" mix has the control fully on, giving an extreme amount of the effect. With simply-arranged music, such as string quartets, too much reverb causes a "muddy" sound that drowns out subtle details. You may not want to hear any reverb on announcers' voices, as it makes them sound like they are speaking in a cave. The soundtrack to a space movie may have lots of reverb, as it imparts an ethereal, spacious feeling to the sound. Classic rock has moderate to generous amounts of reverb. Blues and ballads have modest reverb, or none at all.