When two TONEs are sounded simultaneously, a third tone has been claimed on occasion to be heard, the FREQUENCY of which is the sum of the two original tones. If the two tones are 1500 Hz and 2000 Hz, the summation tone will be 3500 Hz. Summation tones are usually harder to hear than DIFFERENCE TONEs, and there is doubt that they occur at all.
See: COMBINATION TONES, DISTORTION. Compare: AURAL HARMONICS, HETERODYNE, SIDEBAND.