Why can a slightly mistuned unison sound noticeably out of tune?

A slightly mistuned unison sounds noticeably out of tune because of the phenomenon known as beats.12

When two sound sources produce waves of slightly different frequencies, they create interference as they arrive at the ear. At certain intervals, these waves align in phase to produce a louder sound, while at other times they cancel each other out, resulting in a softer sound. This periodic fluctuation in sound intensity—the pulsation or throb—is perceived as a beat.31

In music, absolute consonance occurs only when the ratio between two frequencies is numerically simple, such as the 1:1 ratio of a perfect unison. When a unison is slightly mistuned, the resulting beats create dissonance. The human ear perceives this interference as a lack of clarity or a "beating" effect, which is judged as being out of tune. While piano tuners may intentionally set unison strings slightly out of tune to achieve specific tonal characteristics, this practice relies on the ability to hear and control these beat rates.243