Additive synthesis
Additive synthesis is a sound synthesis technique that creates timbre by adding sine waves together.
The timbre of musical instruments can be considered in the light of Fourier theory to consist of multiple harmonic or inharmonic partials or overtones. Each partial is a sine wave of different frequency and amplitude that swells and decays over time due to modulation from an ADSR envelope or low frequency oscillator.
Additive synthesis most directly generates sound by adding the output of multiple sine wave generators. Alternative implementations may use pre-computed wavetables or the inverse fast Fourier transform.
Additive synthesis is useful for xenharmonic music because it allows timbres to be created to match a given tuning (eg for a 17edo piece, one could use timbres with a weak 5th harmonic and a strong 11th harmonic).