Audio Frequency Modulation (FM) is a synthesis technique used in music production to generate complex waveforms by modulating the frequency of one waveform with another waveform. In FM synthesis, a carrier waveform is modulated by a modulator waveform, resulting in a new waveform that contains a rich spectrum of harmonics.

The modulator waveform is typically an audio frequency oscillator that is used to modulate the frequency of the carrier waveform, which can be a sine wave, sawtooth wave, or another waveform. By adjusting the frequency and amplitude of the modulator waveform, producers can create a wide range of complex and evolving timbres.

FM synthesis was popularized in the 1980s by the Yamaha DX7 synthesizer, which used 6-operator FM synthesis to create a wide range of sounds. Since then, FM synthesis has become a popular technique for creating digital sounds in a wide range of music genres, including electronic, pop, and hip-hop.

FM synthesis can be implemented using hardware or software synthesizers, with many digital audio workstations (DAWs) featuring FM synthesis plugins. It can also be used in combination with other synthesis techniques, such as subtractive synthesis, to create even more complex and unique sounds.