Note:  Do not rely on this information. It is very old.

Combinational Tones

Combinational Tones. When two notes are sounded together a third note will be faintly heard whose frequency is the sum of the frequencies of the two given notes; this is called the summational tone. Also a fourth note will be produced whose frequency is the difference of the frequencies of the two given tones; this is the differential tone. Thus c' (256 vibrations per second) and g' (384) give e" (=640 = 256 + 384). They also give c (= 128 = 384 - 256). These two extra notes are called combinational tones.