When each ear hears a slightly different pure tone through headphones, the brainstem's superior olivary complex compares the two signals to help locate sound in space. When the two frequencies are close together, the neurons instead lock onto the tiny difference between them, and the brain perceives a rhythmic pulsing loudness — a binaural beat — that does not exist in the physical air pressure at all. It is generated entirely inside the head.
f_L, right ear hears f_R.|f_R − f_L|, shown here as the pulsing glow between the two wave ribbons.The term "brainwave entrainment" claims listening to a given beat frequency nudges EEG activity toward that same band (e.g. 10Hz for relaxed alpha waves). Evidence for a strong causal effect on mood or cognition is mixed and much weaker than popular claims suggest — but the auditory illusion itself is real and well documented since Heinrich Wilhelm Dove described it in 1839.
Two pure tones, one per ear, ribbon across the scene as scrolling sine waves; where they overlap, a pulsing marker traces the beat envelope your brainstem perceives as a rhythmic throb equal to the difference between the two frequencies.
The perceived beat frequency equals |f_right − f_left|. The simulation shows both carrier waves plus their interference sum, and pulses the ear markers and lighting in sync with that difference frequency.
Set the left ear's carrier frequency and the beat (right-minus-left) frequency, or jump to a brainwave-band preset. Press Play tones to actually hear the effect through stereo headphones while watching it pulse in 3D.
Binaural beats were first documented by physicist Heinrich Wilhelm Dove in 1839. The beat itself is a genuine neural phenomenon, but it only exists inside the listener's head — there is no physical 10Hz sound wave in the room.