Only some combination tones are readily perceived, and none by all listeners.

Evidence

These tones are always below the frequency of the lower pure tone and occur at integer multiples of the difference tone frequency below the lower tone. No listeners hear all and some hear none of these combination tones. The difference tone and the combination tones for n = 1 and n = 2, known as the “second-order difference tone” and the “third-order difference tone,” are those that are perceived most readily (e.g., Rasch and Plomp, 1982). — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1578

Categorization

• QUESTION: Why do some listeners hear combination tones and others none at all, and does this vary with training?