Combination tone
An additional audible tone produced when two tones sound together, such as a difference tone.
What it means
A combination tone is an extra tone heard when two sufficiently loud tones sound together, even though it comes from no separate sound source. It arises from non-linear distortion in the ear itself or in a vibrating system, and usually lies well below both original tones.
Musical use
The most important type is the difference tone, whose frequency equals the difference between the two source frequencies; there are also summation tones, whose frequency equals the sum, and higher-order tones. Combination tones were described by Tartini and are also known as "Tartini tones"; string players use them to check the intonation of double stops, since a combination tone stands out most clearly when the interval is perfectly in tune. Combination tones differ from overtones: overtones belong to the sound of a single tone, while combination tones only arise when several tones sound together.
Performance practice
Practise hearing combination tones on well-tuned double stops or loud, pure intervals; as the tuning drifts, the combination tone weakens or clouds over, giving you a built-in check.