Audio Effects

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 room’s 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.