beat, in physics, the pulsation caused by the combination of two waves of slightly different frequencies. The principle of beats for sound waves can be demonstrated on a piano by striking a white key and an adjacent black key at the bass end of the keyboard. The resulting sound is alternately soft and loud—that is, having characteristic pulsations, or throbs, called beats. Toward the treble end of the keyboard, the beat frequency is greater because the difference in frequency of adjacent keys is more than at the lower end. The Figure depicts two waves n₁ and n₂ with respective frequencies of 24 and 30 vibrations per second (hertz); the beat frequency N is their difference, 6 beats per second.

The phenomenon of beats is employed in various ways. For example, in the tuning of instruments, if a tuning fork and piano key of the same note are struck simultaneously and no beat is heard, then they are of identical pitch. Ultrasonic vibrations (having a frequency higher than is audible), such as the vocal sounds made by bats and dolphins, may be detected by superimposing a sound of different frequency to produce audible beats. The principle is also used in the superheterodyne reception of radio waves, in which a low-frequency signal from an oscillator is beat against an incoming high-frequency radio signal to produce an intermediate (beat) frequency that can be amplified to produce audible signals.