差频（英文：beat note）一词来源于声学上两个频率相近但不同的声波的干涉，所得到的干涉信号的频率是原先两个声波的频率之差，因此叫做差频。这个概念也用到了光学和电子学中，指两个频率不同的信号进行合波后得到频率为两者之差的新信号。

In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies.

When tuning instruments that can produce sustained tones, beats can readily be recognized. Tuning two tones to a unison will present a peculiar effect: when the two tones are close in pitch but not identical, the difference in frequency generates the beating. The volume varies like in a tremolo as the sounds alternately interfere constructively and destructively. As the two tones gradually approach unison, the beating slows down and may become so slow as to be impe****tible.