Isochronic Tones are an effective method of brainwave entrainment.

They are short sounds which are repeated in quick succession. They are equally spaced and of equal length and volume. The speed is dependent upon the frequency of the targeted brainwaves.

To induce a state of relaxation, for example, both brain hemispheres must be synchronised to the Alpha frequency, which is around 10 Herz. This means that the impulse signal must be repeated ten times per second

Further reading: Healthline