What a Difference Tone Is

When two pure tones are played together, it sometimes sounds as though a third tone is also present. For example, a person simultaneously hearing two loud tones of 800 and 1000 hertz may also report hearing another, quieter tone at about the pitch one would expect from a 200 hz stimulus. More generally, if two generating tones (or primary tones) of frequency U (for the upper tone) and L (for the lower tone) are played together, people will under some conditions report hearing a difference tone (or first difference tone) that sounds like a tone presented at frequency U - L, despite the fact that waveform analysis may show no sound wave at that frequency.

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