Subtractive synthesis

Subtractive synthesis is a method of sound synthesis in which partials of an audio signal (often one rich in harmonics) are attenuated by a filter to alter the timbre of the sound. While subtractive synthesis can be applied to any source audio signal, the sound most commonly associated with the technique is that of analog synthesizers of the 1960s and 1970s, in which the harmonics of simple waveforms such as sawtooth, pulse or square waves are attenuated with a voltage-controlled resonant low-pass filter. Many digital, virtual analog and software synthesizers utilise subtractive synthesis, sometimes in conjunction with other methods of sound synthesis.

The basis of subtractive synthesis can be understood by considering the human voice; when a human speaks, sings or makes other vocal noises, the vocal folds act as an oscillator and the mouth and throat as a filter. Consider the difference between singing "oooh" [uː] and "aaah" [ɑː], at the same pitch. The sound generated by the vocal folds is much the same in either case — a sound that is rich in harmonics. The difference between the two comes from the filtering applied with the mouth and throat. By changing the shape of the mouth, the frequency response of the filter is changed, removing (subtracting) some of the harmonics. The "aaah" sound has most of the original harmonics still present; the "oooh" sound has most of them removed (or, to be more precise, reduced in amplitude). By gradually changing from "oooh" to "aaah" and back again, a spectral glide is created, emulating the 'sweeping filter' effect that is the basis of the 'wah-wah' guitar effect.

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