Reverberation

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Reverberation is the persistence of sound in a particular space after the original sound is removed. When sound is produced in a space, a large number of echos build up and then slowly decay as the sound is absorbed by the walls and air, creating reverberation, or reverb. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they can no longer be heard. Large chambers, especially such as cathedrals, gymnasiums, indoor swimming pools, large caves, etc., are examples of spaces where the reverberation time is long and can clearly be heard. Different types of music tend to sound best with reverberation times appropriate to their characteristics.

(Compare with echo: "If so many reflections arrive at a listener that he is unable to distinguish between them, the proper term is reverberation.")

Short sample of reverberation effect (file info) — play in browser (beta)
- Clean signal, followed by different versions of reverberation (with longer and longer decay times).
- Problems listening to the file? See media help.

1 Reverberation Time
- 1.1 The Sabine Equation
2 The sound decay rate
3 Creating Reverberation Effects
4 External links

[edit] Reverberation Time

Reverberation time is the time required for a sound in a room to decay by 60 dB (called RT₆₀). A century ago, Wallace Clement Sabine started experiments at Harvard University to investigate the impact of absorption on the reverberation time. Using a portable wind chest and organ pipes as a sound source, a stopwatch and a clean pair of ears he measured the time from interruption of the source to inaudibility (roughly 60dB).

The best reverberation time for a space in which music is played depends on the size of the room and the type of music. Rooms for speech require a shorter reverberation time than for music. A longer reverberation time can make it difficult to understand speech. If the reverberation time from one syllable over laps the next syllable, it may make it difficult to identify the word [1]. "Cat", "Cab", and "Cap" may all sound very similar. If on the other hand the reverberation time is too short, tonal balance and loudness may suffer. Reverberation effects are often used in studios to "smooth" sounds; the effect is commonly used on vocals to help remove inconsistencies in pitch.

The reverberation time RT₆₀ and the volume V of the room have great influence on the critical distance d_c (conditional equation):

<math>

d_c = 0{.}057 \cdot \sqrt \frac{V}{RT_{60}} </math>

Where:

Critical distance, r_H in m.
Volume, V in m³.
Reverberation time, RT₆₀ in s.

[edit] The Sabine Equation

Sabine's reverberation equation was developed in the late 1890's 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:

<math>

RT_{60} = \frac{c \cdot V}{Sa} </math>.

Where:

RT₆₀ = Reverberation time in seconds
c = 0.161 m³ or 0.049 cu ft
V = volume of room
S = total surface area of room in sq m or sq ft (depending on c)
a = average adsorption coefficient of room surfaces
Sa = total adsorption in sabins

It is worth noting that the total adsorption in sabins (and hence reverberation time) generally changes depending on frequency (dependent on the which is defined by the acoustic properties of the space), and that the equation does not take into account room shape or dimensions, nor losses from the sound travelling through the air (important in larger spaces). In general most rooms adsorb less in the lower frequencies, causing a longer decay time.

[edit] The sound decay rate

There is some debate as to the value of using RT₆₀ measurements with rooms that have a reverberation time under around 1 second as they are not statistically reverberant. Measuring the RT₆₀ in such rooms may also be difficult due to ambient noise e.t.c. In rooms with a shorter reverberation time it is common to use the sound decay rate.

The is the time rate at which sound pressure level decreases in a room at a stated frequency. The unit is decibel per second (dB/s). The decay rate D_R in a reverberant room is related to reverberation time by RT₆₀ = 60 dB / D_R. eg a RT₆₀ of 0.4 of lol a second is equivalent to a decay rate of 60 dB/0.4 sec = 150 dB per second

Sometimes the reverb time is mistakenly called the sound decay rate, this is incorrect; reverb time in seconds and sound decay in decibels per second are related, but not identical.

[edit] Creating Reverberation Effects

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.

[edit] Chamber reverberators

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.

[edit] Plate reverberators

A plate reverb system uses an electromechanical transducer, similar to the driver in a loudspeaker, to create vibration in a plate of sheet metal. A pickup captures the vibrations as they bounce across the plate, and the result is outputted as an audio signal.

[edit] Spring 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. 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.

[edit] Digital reverberators

Digital reverberators use various signal processing algorithms in order to create the reverb effect. Since reverberation is essentially caused by a very large number of echoes, simple DSPs use multiple feedback delay circuits to create a large, decaying series of echoes that die out over time. More advanced digital reverb generators can simulate the time and frequency domain responses of real rooms (based upon room dimensions, adsorption and other properties). In real music halls, the direct sound always arrives at the listeners ear first because it follows the shortest path. Shortly after the direct sound, the reverberant sound arrives. The time between the two is called the 'arrival time gap'. This gap is important in recorded music because it is the cue that gives the ear information on the size of the hall, better digital reverbs can incorporate this arrival time gap and hence sound more realistic. Digital reverb systems are commonly implemented as software plugins.