The ondes Martenot ( /ˈoʊnd mɑrtɨˈnoʊ/ or OHND mar-tə-NOH;, "Martenot waves"), also known as the ondium Martenot, Martenot and ondes musicales, is an early electronic musical instrument invented in 1928 by Maurice Martenot. The original design was similar in sound to the theremin. The sonic capabilities of the instrument were later expanded by the addition of timbral controls and switchable loudspeakers.
The instrument's eerie wavering notes are produced by varying the frequency of oscillation in vacuum tubes. The production of the instrument stopped in 1988, but several conservatories in France still teach it.
In 1997, the Ondéa project began designing an instrument based on the ondes Martenot. Since the Martenot name is still protected, the new instrument is called Ondéa, but has the playing and operational characteristics of the original ondes Martenot. In 2001, a completed prototype was first used in concerts. These instruments have been in regular use since 2005.
Since 2008, Jean-Loup Dierstein, with the support of Maurice Martenot's son, has been developing a new, officially named ondes Martenot instrument based on the model used when production stopped in 1988.