Reverberation

This causes numerous reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air.

[2] This is most noticeable when the sound source stops but the reflections continue, their amplitude decreasing, until zero is reached.

Reverberation occurs naturally when a person sings, talks, or plays an instrument acoustically in a hall or performance space with sound-reflective surfaces.

[5] Although reverberation can add naturalness to recorded sound by adding a sense of space, it can also reduce speech intelligibility, especially when noise is also present.

It is defined as the time it takes for the sound pressure level to reduce by 60 dB, measured after the generated test signal is abruptly ended.

Reverberation time is frequently stated as a single value if measured as a wideband signal (20  Hz to 20 kHz).

In the late 19th century, Wallace Clement Sabine started experiments at Harvard University to investigate the impact of absorption on the reverberation time.

A loud noise is produced, and as the sound dies away the trace on the level recorder will show a distinct slope.

This two port measurement system utilizes a Fourier transform to mathematically derive the impulse response of the room.

It is often difficult to inject enough sound into the room to measure a decay of 60 dB, particularly at lower frequencies.

In 1965, Manfred R. Schroeder published "A new method of Measuring Reverberation Time" in the Journal of the Acoustical Society of America.

This made it possible to show the variation in the rate of decay and to free acousticians from the necessity of averaging many measurements.

The total absorption in sabins (and hence reverberation time) generally changes depending on frequency (which is defined by the acoustic properties of the space).

The equation does not take into account room shape or losses from the sound traveling through the air (important in larger spaces).

Sabine concluded that the reverberation time depends upon the reflectivity of sound from various surfaces available inside the hall.

As a result, the Eyring equation is often implemented to estimate the reverberation time in recording studio control rooms or other critical listening environments with high quantities of sound absorption.

The Sabine equation tends to over-predict reverberation time for small rooms with high amounts of absorption.

Conversely, a thick, smooth painted concrete ceiling would be the acoustic equivalent of a mirror and have an absorption coefficient very close to 0.

Gregorian chant may have developed in response to the long reverberation time of cathedrals, limiting the number of notes that could be sung before blending chaotically.

These simulate reverb through means including echo chambers, vibrations sent through metal, and digital processing.