When two TONEs are sounded simultaneously, other tones can sometimes be heard, the FREQUENCY of one of which is the difference between the frequencies of the two tones being sounded.
For example, if the two tones are 1500 Hz and 2000 Hz, the difference tone will be 500 Hz, if the two original tones have sufficient intensity - over about 50 dB. A police whistle, for instance, blown loudly enough will produce a difference tone which is heard as a low buzzing sound.
See COMBINATION TONES for a more detailed account. Compare: AURAL HARMONICS, BEATS, DISTORTION, HETERODYNE, SIDEBAND, SUMMATION TONE.
Sound Example: Two sine tones at 1500 Hz and 2000 Hz, followed by a 500 Hz tone as reference (the frequency of the difference tone).