130 HANDBOOK OF ACOUSTICS. 





It is evident from the above, that when the two generators are at a greater interval apart than an octave, the differential tone lies between them; when they are at the exact interval of an octave, it coincides with the lower generating tone; but when at a less interval than an octave it lies below this latter, and the smaller the interval, the lower relatively will be the differential.
Any of the partials of compound tones may act as generators, if sufficiently powerful. Thus, if j * be two compound generators, we see from the above table that the fundamentals d^{1} and n may give rise to the differential tone SJ but the 2nd partials d^{2} and n^{1}, if sufficiently powerful, may also generate the differential s ; or the partial n^{1} and the fundamental d^{1} may produce differential dj and so on. It is only in rare cases, however, that the overtones will be strong enough to produce audible differentials. 
