- In synthesizers, capable of sounding two voices, or notes, at a time. Compare: monophonic, polyphonic.
- Duophonic is also a term used to refer to a sound process by which a monaural recording is turned into a kind of "fake stereo" by splitting the signal into two channels, delaying the left and the right channels by means of delay lines and other circuits (desynchronizing them by fractions of a second), and cutting the bass frequencies in one channel with a high-pass filter, then cutting the treble frequencies in the other channel with a low-pass filter. The end result was a synthesized stereo effect, without giving the listener the true directional sound characteristics of real stereo.
"Duophonic" was used as a trade name for the process by Capitol Records for re-releases of mono recordings in the mid-to-late 1960s through the 1970s. For nearly ten years, Columbia used the banner "Stereo 360 degree sound" to differentiate their true stereo LPs from the Duophonic LPs.
The process was used for some of their biggest releases, including a variety of albums by The Beach Boys and Frank Sinatra. Over the years however, some Duophonic tapes were confused with true stereo recordings in Capitol Records' vaults, and wound up getting accidentally reissued on CD throughout the 1980s and 1990s.