Put on headphones and turn your volume down. Set a carrier frequency and the beat offset in hertz, then press Play; the two tones fade in gently and stop whenever you press Stop.
When each ear receives a slightly different frequency, the auditory system perceives a pulsing at the difference between them — 200 Hz and 204 Hz produce a 4 Hz beat. The pulse exists only in perception, not in the air.
Claims about focus, relaxation and sleep are popular but the research is mixed and far from settled. This tool is offered for general interest and experimentation and is explicitly not a medical device, a treatment, or a diagnostic instrument.
The effect depends on each ear receiving a different tone. Speakers let both signals reach both ears, which produces an ordinary acoustic beat in the room rather than a binaural one, so the effect disappears.