Is it farther or further?
in musical acoustics, faint tone produced in the inner ear by two simultaneously sounded musical tones. Because such tones are caused by the ear rather than by the external source of the sound, they are sometimes called subjective, or resultant, tones. There are two varieties: difference tones (D) and summation tones (S), generated respectively by the frequency differential of the two pitches or the sum of their frequencies. The most commonly heard are difference tones lying below the original pitches; these were discovered by the celebrated violinist-composer Giuseppe Tartini (1692-1770), who regarded the "third tone" as an excellent means of correcting faulty intonations of double stops on the violin.