Combination tones at frequencies greater than the primary tones.

The existence of audible combination tones at frequencies greater than the primary tones that generate them has long been problematic. With primary tones at frequencies f1 and f2, combination tones at f1 + f2 - f1, and other frequencies can be demonstrated and measured by using a contralateral probe tone to establish a binaural interaction with a given combination tone. The estimated amplitudes of these higher-frequency combination tones are generally 20 to 40 decibels below the amplitude of the primary tones.