Percussion synthesis


Audio: percussion synthesis (0:06)

Percussion synthesis
Figure: percussion synthesis

PARTIAL #1234567891011
Relative frequency0.560.560.920.921.191.702.002.743.003.764.07
Offset frequency (Hz)0101.70000000
Relative amplitude1.000.671.001.802.671.671.461.331.331.001.33
Relative duration1.000.900.650.550.3250.350.
Table: percussion synthesis

percussion synthesis plays three synthetic percussion instruments in succession: a bell, a bass drum and a snare drum. The percussion synthesis figure shows the composite waveform. The percussion synthesis table shows the specification for the bell.

Percussion synthesis is the process of creating drum sounds using a computer.

You can use percussion synthesis to build your own drum machine or electronic drum. It is an alternative to using a drum sample and is much less hassle than recording drums with a microphone.

The shape of the percussion envelope is a major consideration in percussion synthesis. This is invariably an envelope with two segments, an attack and a decay. Both segments are exponential (curved) not linear. The attack segment is very fast, usually 1-2 milliseconds (all the instruments in percussion synthesis have a 2ms attack). The decay segment is the critical factor and its shape has a major influence on the sound. In general, the lower the frequency, the shorter the decay.

The frequencies present in the percussion determine its timbre. The sound can be as simple or as complex as you like and there is plenty of scope for experimentation. Any of the synthesis techniques covered in the guide can be used and all sorts of sound effects added.

The bell in percussion synthesis is created by additive synthesis. This is a good technique for creating percussion instruments that contain relatively few partials such as a bell, gong or chime. The specification for the bell is shown in the percussion synthesis table. This is a Risset bell, named after its creator, the composer Jean-Claude Risset, who designed synthetic instruments in the 1960s. The partials are sine waves and inharmonic. There is an implied fundamental, which is 440Hz in percussion synthesis, but which is not sounded. As an example of how the table works here is the detail for the second partial: frequency 247.4 (440*0.56+1); amplitude 0.044 (0.67/15.26); and duration 4.02 seconds (6*0.67).

The bass drum in percussion synthesis is created by Frequency Modulation (FM). FM is an excellent technique for generating percussive sound although it does require a bit of experimentation. In percussion synthesis the carrier is 75Hz and the modulator is 10Hz. The modulator amplitude is a percussion envelope with a 2ms exponential attack rising from 0.001 to unity, peak amplitude, and a 148ms exponential decay back to 0.001. The same envelope is scaled up by a factor of ten to form a variable modulation index.

In percussion synthesis the snare drum was created by mixing a 200Hz sine wave with white noise in the proportion 90:10. Both sounds are enclosed in a 250ms exponential percussion envelope.