Additive synthesis
Additive synthesis could be considered the reverse approach to subtractive synthesis. See Sound basics overview, Tones, overtones, harmonics, and partials, and How subtractive synthesizers work.
To obtain an insight into the additive synthesis method, consider the fact that all sounds are a sum of various sine tones and harmonics.
In additive synthesis, you start out with nothing and build a sound by combining multiple sine waves of differing levels and frequencies. As more sine waves are combined, they begin to generate additional harmonics. In most additive synthesizers, each set of sine waves is viewed and used much like an oscillator.
Depending on the sophistication of the additive synthesizer you’re using, you will either have individual envelope control over the level and pitch of each sine wave, or you will be limited to envelope control over groups of sine waves—one envelope per sound and its harmonics, or all odd or all even harmonics, for example. In practical terms, working with groups of related harmonics is the best approach due to the mathematical relationships between them and the impact this has on the overall tone when adjusting them en masse, rather than individually.
Logic Pro for Mac Alchemy can be used as a true additive synthesizer, where you create sounds from scratch with sine waves, with full control of the level, pitch, and pan position of each harmonic. Logic Pro for Mac Alchemy and Sample Alchemy also allows you to resynthesize imported samples with additive (and spectral) synthesis techniques. See Resynthesis.
Some aspects of the additive synthesis approach are also used in Vintage B3 and other drawbar organs. In Vintage B3 you start with a basic tone and add harmonics to it, to build up a richer sound. The level relationships between the fundamental tone and each harmonic are determined by how far you pull each drawbar out. Because Vintage B3 doesn’t provide envelope control over each harmonic, it is limited to organ emulations.
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