When Gerard Oster, a biophysicist from NYC, developed a process of reproducing the entrainment effect using audio technology in the early 1970's the process of creating audio-entraining binaural beat frequencies was born. If you have a different tone in each ear your brain takes the difference between the two tones and resonates at this differential frequency. With a beat frequency of 97 Hz in the left ear and 103 Hz in the right ear, the brain will perceive it as a 6 Hz differential, and it will resonate at this frequency in the Theta range.
The Monroe Institute is one of the most well-known producers of entrainment-related products. They sell Hemi-Sync Tapes designed and developed using their Hemi-Sync Technology. Tapes are embedded with multi-plexed binaural beat frequencies. Each binaural beat frequency causes the brain to resonate at a particular frequency. Orchestrating these frequenices have created products that alter the user's consciousness. Robert Monroe, known for his journeys out of the body, studied altered states of consciousness and with his team of researchers and scientists have expanded knowledge about it.
Several other companies use forms of brainwave entrainment technology. Dr. Jeffrey Thompson, a well-known neuroacoustic pioneer, has information about entrainment and products he developed that utilize this technology. I purchased his Brainwave Suite at Border's Books a few years ago when I experienced insomnia associated with fibromyalgia. Using the Delta CD, I experienced great entrancing entrainment and experienced the deep Delta wave of healing sleep, where the body could repair itself. I'm not affiliated with Dr. Thompson.