(French: “musical waves”), electronic musical instrument demonstrated in 1928 in France by the inventor Maurice Martenot. Oscillating radio tubes produce electric pulses at two supersonic sound-wave frequencies. They in turn produce a lower frequency within audible range that is equal to the difference in their rates of vibration and that is amplified and converted into sound by a loudspeaker. Many timbres, or tone colours, can be created by filtering out upper harmonics, or component tones, of the audible notes.

In the earliest version, the player's hand approaching or moving away from a wire varied one of the high frequencies, thus changing the lower frequency and altering the pitch. Later, a wire was stretched across a model keyboard; the player touched the wire to vary the frequency. In another version the frequency changes are controlled from a functioning keyboard. Works for the ondes martenot include those by the French-born Swiss composer Arthur Honegger, the French composer Darius Milhaud, and the American composer Samuel Barber.