Binaural beats occur when each ear receives a pure tone of slightly different frequency, for example 400 Hz on the left and 410 Hz on the right, usually through headphones. The brain combines them and perceives a third, pulsing beat at the difference, here 10 Hz. The phenomenon was described in the nineteenth century and drew renewed interest after a 1973 Scientific American article by Gerald Oster. It works only with fairly low carrier tones and a difference of no more than about 30 Hz.
Apps and videos often claim that beats in the alpha, theta or delta range tune the brain for focus, relaxation or sleep. A 2023 systematic review of EEG studies found the evidence for such brainwave entrainment inconsistent: five of fourteen studies supported it, eight contradicted it and methods varied widely. Meta-analyses on memory, attention or pain report small to moderate effects, but from heterogeneous and mostly small trials.
They can be an interesting listening experience at a sensible volume, but they are not a medical treatment; persistent sleep, anxiety or attention problems call for a doctor.