253
11
Reverberation
Reverberation (reverb) is one of the most often used effects in audio produc-
tion. In this chapter, we will look at the causes, the main characteristics,
and the measures of reverb. We then focus on how to simulate reverb. First
we describe algorithmic approaches to generating reverb, focusing on two
classic designs. Then we look at a popular technique for generating a room
impulse response, the image source method. Next, we describe convolu-
tional reverb, which adds reverb to a signal by convolving that signal with a
room impulse response, either recorded or simulated (such as from using the
image source method). The chapter concludes by looking at implementation
details and applications.
Theory
In a room, or any acoustic environment, there is a direct path from any sound
source to a listener, but sound waves also take longer paths by reecting off
the walls, ceiling, or objects, before they arrive at the listener, as shown in
Figure11.1. These reected sound waves travel a longer distance than the
direct sound and are partly absorbed by the surfaces, so they take longer
to arrive and are weaker than the direct sound. These sound waves can
also reect off of multiple surfaces before they arrive at the listener. These
delayed and attenuated copies of the original sound are what we call rever-
beration, and it is essential to the perception of spaciousness in the sounds.
Reverberation is more than just a series of echoes. An echo is the result of a
distinct, delayed version of a sound, as could be heard with a delay of at least
40 ms. With reverberation from a typical room, there are many, many reec-
tions, and the early reections arrive on a much shorter time scale. So these
reections are not perceived as distinct from the sound source. Instead, we
perceive the effect of the combination of all the reections.
Reverberation is also more than a simple delay device with feedback.
With reverb the rate at which the reections arrive will change over time, as
opposed to just simulating reections that have a xed time interval between
them. In reverberation, there is a set of reections that occur shortly after the
direct sound. These early reections are related to the position of the source
and listener in the room, as well as the rooms shape, size, and material
254 Audio Effects: Theory, Implementation and Application
composition. The later reections arrive much more frequently, appear more
randomly, usually decay exponentially, and are difcult to directly relate to
the physical characteristics of the room. These late reections give rise to dif-
fuse reverberation. An example impulse response for a room is depicted in
Figure11.2. Each vertical line marks when the original sound is repeated,
and the height of each of these lines is the amplitude of the sound at that time.
A measure that is often used to characterize the reverb in an acoustic space
is the reverberation time, often denoted RT
60
. This is the time that it takes for
sound pressure level or intensity to decay by 60 dB, i.e., 1/1,000,000th of its
original intensity, or 1/1000th of its original amplitude (see Chapter1). A long
reverberation time implies that the reections remain strong for a long time
before their energy is absorbed. The reverberation time is associated with
room size. Small rooms tend to have reverb times on the order of hundreds
of milliseconds, though this can vary greatly depending on the acoustic
treatment and other factors. Larger rooms will usually have longer reverber-
ation times since, on average, the sound waves will travel a larger distance
between reections. Concert halls typically have reverberation times around
Early
Reections
Late
Reections
Time
FIGURE 11.2
Impulse response of a room.
Source
Microphone
FIGURE 11.1
Reverb is the result of sound waves traveling many different paths from a source to a listener.

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