Binaural beats
Two slightly different frequencies, one in each ear: the brain perceives a phantom beat. But can it actually 'tune' your brainwaves?
Play 200 Hz in the left ear and 210 Hz in the right: you will perceive a 10 Hz beat that exists in neither sound. That is the binaural beat, a very real auditory phenomenon, described as early as 1839.
The promise of brainwave entrainment
The popular idea: this phantom beat would entrain your brainwaves toward its frequency — 10 Hz for alpha relaxation, 4 Hz for deep meditation, and so on. That is the claim behind countless apps, and historically behind the Monroe Institute’s Hemi-Sync, meant to facilitate OBE-like states.
What the literature says
Studies are mixed: the EEG entrainment effect is weak or inconsistent across protocols, but subjective effects (relaxation, focus) come up regularly — hard to untangle acoustics from placebo. Good news: the experiment costs nothing but stereo headphones and twenty minutes.