Adding Reverberation

Reverberation is the collection of reflected sounds from the surfaces in an enclosure like an auditorium. There are a number of circumstances where there has been a desire to add reverberation. Currently, this is typically done with electronic reverberation units which can simulate a number of acoustic environments and give control over the amount of reverberation added.

Historically, reverberation has been added in a number of ways. One notable example is that of the Capitol Records facility in Hollywood where there are controllable reverberation chambers in the basement. The signal to be recorded can be sent to a speaker in one of the chambers and then picked up by a microphone at another point in the chamber. This is adding real reverberation formed by the sound actually bouncing off the walls of the chamber.

Most means of adding reverberation are less ambitious, and less realistic. One popular method used in the past was plate reverb where a speaker transducer was welded offcenter on a metal plate and a microphone transducer was placed at another spot on the metal plate. Some of these were quite elaborate, with various perforation schemes for the plates to discriminate against certain resonances, and damper systems to control the decay time. One example was a plate 8 feet by 4.5 feet, with a damper system which could vary the reverberation time from 0.5 to 5 seconds.

A cheaper and more portable system of reverberation was the spring reverb used in many sound mixing boards and guitar amplifiers. Coiled springs were stretched between a transmitting device and a receiving device. Although having the advantage of small size, the reverberation was mechanical-sounding and often harsh. And as many sound-board operators found out, if you bumped into the sound board during a performance, everybody knew it!

Tape reverb was also utilized by constructing a tape-loop device with a record head and multiple playback heads at intervals along the tape. Known as tape delay or tape echo, the signals from the successive playback heads were progressively diminished to attempt to mimic real reverberant decay, but it was always at the regular intervals dictated by the head positions.

Why Add Reverberation?

Why add Reverberation? One simple answer is to make a studio sound like an auditorium. A studio is usually constructed to be acousticall "dead", i.e., with a very short reverberation time. The reason is that it gives a greater degree of control over the quality of the recording. You eliminate extraneous noises, and can get clearer recordings of different sections of a musical group, providing more control in the mixing process. If you can then add back a controlled amount of realistic reverberation, you can add richness and fullness to the sound after the mix.

Adding reverberation is often desirable in live performances to make an amplified performance sound more natural. For example, if a soloist is close to the microphone, then the amplified sound to the auditorium is deficient in room reverberance. The microphone has intercepted the sound signal before it has significant reverberant content. The direct sound into the microphone is too dominant over the sounds coming from the surfaces of the room. The natural voice sound in the room has a natural complement of reverberant sound from the surfaces by the time it reaches the ear of a typical listener. You would not want to put your ear at the position of the microphone to listen to the performance. Adding back a judicious amount of reverberation, typically from an electronic reverberation unit, can help "color" this close-field sound so that it is more like that which would be heard by a listener to an unamplified voice in a good auditorium.