Dr. Cheryl Fraser: How To Have Better Sex & Intimacy
The idea behind brainwave entrainment goes back further than most people assume. In 1839, a German physicist named Heinrich Wilhelm Dove noticed something odd about how the ear and brain handle sound. When two slightly different tones are played separately into each ear, the brain registers a third, pulsing tone that exists nowhere in the room. It only appears inside perception.
Dove was no fringe figure. He held a fellowship in the Royal Society, worked as a physicist, and ran the Prussian Meteorological Institute. His observation sat mostly as a curiosity for more than a century.
Things changed between the 1950s and the 1970s, when the effect he first described was turned into a working audio technique. Several people have claimed credit for building the practical technology, and the history there is tangled, but the underlying discovery traces back to Dove.
The appeal, for people interested in mindfulness and relaxation, is simple enough. A specific auditory stimulus seems able to nudge the listener toward a calmer mental state, and that has made it popular as a tool for winding down rather than as any kind of medical treatment.
If you want to check the claims yourself, the research exists. Searching PubMed for terms like mindfulness and the psychological effects of brainwave entrainment will turn up the published studies, which is a better starting point than taking anyone's word for what the sound does.