When two tones, or pitches, are sounded simultaneously, other tones can sometimes be heard, at a frequency that is the difference between the frequencies of the two tones being sounded. For example, if the two tones are 1500Hz and 2000Hz, the difference tone will be 500Hz (if the two original tones have sufficient intensity – over about 50dB).
The difference tone appears when the two tones are not part of a harmonic series (i.e., octaves, fifths, etc.) and have a frequency difference of 20Hz or greater. A difference of less than 20Hz will cause the difference of the two notes to be perceived as a pulse, called beating.
In acoustic music this effect is often noticeable when two clarinets are playing in thirds.