the output from the mixing process can be shown to contain sine waves having frequencies that are the sum and difference of the two input frequencies. thus, as the variable, known frequency is adjusted close to the unknown frequency, the frequency of the output difference frequency term approaches zero. this zero-beat phenomenon can be detected by actually listening to the audio beat frequency resulting from mixing the standard frequency source with the unknown. the beat frequency method has an apparent dead zone where the mixed difference frequency lies below the human range of hearing.