Sabine's reverberation equation was developed in the late 1890s in an empirical fashion. He established a relationship between the RT60 of a room, its volume, and its total adsorption (in sabins). This is given by the equation:
.where c is a mathematical constant measuring 0.161, V is the volume of the room in m, and reverberation time RT60 is measured in seconds.
The Sabine Equation
The absorption coefficient of a material is a number between 0 and 1 which indicates the proportion of sound which is absorbed by the surface compared to the proportion which is reflected back into the room. A large, fully open window would offer no reflection as any sound reaching it would pass straight out and no sound would be reflected. This would have an absorption coefficient of 1. Conversely, a thick, smooth painted concrete ceiling would be the acoustic equivalent of a mirror, and would have an absorption coefficient very close to 0.
Absorption
Historically reverberation time could only be measured using a level recorder (a plotting device which graphs the noise level against time on a ribbon of moving paper). A loud noise is produced, and as the sound dies away the trace on the level recorder will show a distinct slope. Analysis of this slope reveals the measured reverberation time. Modern digital sound level meters carry out this analysis automatically, on digital data.
Two basic methods exist for creating a sufficiently loud noise (which must have a defined cut off point). Impulsive noise sources such as a blank pistol shot, or balloon burst may be used to measure the impulse response of a room. Alternatively, a random noise signal such as pink noise or white noise may be generated through a loudspeaker, and then turned off. This is known as the interrupted method, and the measured result is known as the interrupted response.
Reverberation time is often given as a measurement of decay time. Decay time is the time it takes the signal to diminish 60 dB below the original sound.
Measurement of Reverberation Time
It is often desirable to create a reverberation effect for recorded or live music. A number of systems have been developed to facilitate or simulate reverberation.
Creating Reverberation Effects
The first reverb effects created for recordings used a real physical space as a natural echo chamber. A loudspeaker would play the sound, and then a microphone would pick it up again, including the effects of reverb. Whilst this is still a common technique, it requires a dedicated soundproofed room, and varying the reverb time is difficult.
Chamber reverberators
A plate reverb system uses an electromechanical transducer, similar to the driver in a loudspeaker, to create vibration in a large plate of sheet metal. A pickup captures the vibrations as they bounce across the plate, and the result is output as an audio signal.
Plate reverberators
A spring reverb system uses a transducer at one end of a spring and a pickup at the other, similar to those used in plate reverbs, to create and capture vibrations within a metal spring. Guitar amplifiers frequently incorporate spring reverbs due to their compact construction and low cost. Spring reverberators were once widely used in semi-professional recording due to their modest cost and small size. Due to quality problems and improved digital reverb units, spring reverberators are declining rapidly in use.
Many musicians have made use of spring reverb units by rocking them back and forth, creating a thundering, crashing sound caused by the springs colliding with each other.
Digital reverberators