Binaural Beats:

A binaural beat is an illusion generated by listening at the same time to two tones with slightly different frequencies. The brain interprets the two tones, which align with brain waves, as its own beat with a different frequency. What’s called the frequency-following effect is when brain activity matches the frequency set by the beat, allowing you to listen to binaural beats to entrain (synchronize or be in rhythm with) your mind to reach a certain mental state. Also see Entrainment