'Beats' is a phenomenon that occurs when freencies of two harmonic waves are

Beats are a physical phenomenon that occurs when two harmonic waves of nearly the same frequency interfere with each other. This interaction results in a periodic variation in the amplitude of the resultant wave, which is perceived as a pulsing sound that fluctuates in loudness [1]. The frequency of these pulses, known as the beat frequency, is mathematically equal to the absolute difference between the frequencies of the two individual waves [2]. If the frequencies are exactly equal, the interference is constant rather than periodic; if they are too far apart, the human ear cannot distinguish the rapid fluctuations as distinct beats [1]. Therefore, the phenomenon is specifically characterized by waves with frequencies that are very close or nearly the same [1].

Sources

1. [1] https://www.britannica.com/science/sound-physics/Beats
2. [2] https://www.khanacademy.org/science/physics/mechanical-waves-and-sound/standing-waves/v/beat-frequency