Combination tone

Also called a Tartini tone, a combination tone is a usually lower pitch produced inside the inner ear by the presence of two external pitches. The frequency of the most commonly heard combination tone is the difference between the frequencies of the two pitches, f2 - f1, if f2 is the higher frequency. Though much less common the following frequencies may also be heard:

2f1 - f2,3f1 - 2f2,...,f1 - k(f2 - f1)