Production of Difference Tones

Abstract

IN providing a course of instruction in preliminary radio physics for war purposes, there were in our laboratory several audio-frequency generators which were capable of producing tonal frequencies up to and above the limits of hearing. It occurred to me that with two such instruments difference tones might easily be generated. The instruments were connected, preferably in parallel, with a small loudspeaker, and the pitch of each was gradually raised by itself to a frequency just above the limit of audibility. When both instruments were simultaneously adjusted by the dial to the same high frequency, no tone was heard. But when one of them was altered to a small extent, a deep low difference tone of a frequency near the lower limit of audibility was loudly heard by itself. By gradually changing the frequency of one generator, the pitch of the difference tone rises continuously to any height desired. It is sufficiently loud to be heard over a large lecture room, and is now so demonstrated.