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.
Additive synthesis generates sound by adding the output of multiple sine wave generators. It may also be implemented using pre-computed wavetables or inverse Fast Fourier transforms.