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Chapter 4: The Synthesis of Sound by Computer

Section 4.8: Granular Synthesis


When we discussed additive synthesis, we learned that complex sounds can be created by adding together a number of simpler ones, usually sets of sine waves. Granular synthesis uses a similar idea, except that instead of a set of sine waves whose frequencies and amplitudes change over time, we use many thousands of very short (usually less than 100 milliseconds) overlapping sound bursts or grains. The waveforms of these grains are often sinusoidal, although any waveform can be used. (One alternative to sinusoidal waveforms is to use grains of sampled sounds, either pre-recorded or captured live.) By manipulating the temporal placement of large numbers of grains and their frequencies, amplitude envelopes, and waveshapes, very complex and time-variant sounds can be created.

Figure 4.20  A grain is created by taking a waveform, in this case a sine wave, and multiplying it by an amplitude envelope.

How would a different amplitude envelope, say a square one, affect the shape of the grain? What would it do to the sound of the grain?

Clouds of Sound

Granular synthesis is often used to create what can be thought of as "sound clouds"—shifting regions of sound energy that seem to move around a sonic space. A number of composers, like Iannis Xenakis and Barry Truax, thought of granular synthesis as a way of shaping large masses of sound by using granulation techniques. These two composers are both considered pioneers of this technique (Truax wrote some of the first special-purpose software for granular synthesis). Sometimes, cloud terminology is even used to talk about ways of arranging grains into different sorts of configurations.

Figure 4.21  Visualization of a granular synthesis "score." Each dot represents a grain at a particular frequency and moment in time. An image such as this one can give us a good idea of how this score might sound, even though there is some important information left out (such as the grain amplitudes, waveforms, amplitude envelopes, and so on).

What sorts of sounds does this image imply? If you had three vocal performers, one for each "cloud," how would you go about performing this piece? Try it!

Soundfile 4.29
"Implements of Actuation"

This is an excerpt of a composition by computer music composer and researcher Mara Helmuth titled "Implements of Actuation." The sounds of an electric mbira and bicycle wheels are transformed via granular synthesis.

There are a great many commercial and public domain applications to do granular synthesis because it is relatively easy to implement and the sounds can be very interesting and attractive.

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