Sound is a form of vibrational energy that affects all living things. We can hear and feel sound waves, our brain and nervous system respond to sound, and sound has been used as a tool for healing for thousands of years. Traditional cultures have used prayers, chants, drumming, bells and singing bowls, chimes, gongs, didgeridoos, flutes, rattles, and other instruments to access the healing power of sound vibration and to guide the mind into meditative states of consciousness.
Modern scientists and healing practitioners have been able to use technology to better understand and refine the use of sound in the healing arts and personal development field. One pioneer in this area was author Robert Monroe, founder of the The Monroe Institute in Virginia. Monroe wrote books about his out of body experiences and developed the use of binaural sound to influence the brain in ways that could help facilitate various states of consciousness. Monroe referred to the binaural sound recordings with the term "hemi-sync" due to the fact that hearing a slightly different tone in each ear leads the brain hemispheres to synchronize and create a brainwave frequency to bridge the difference.
Besides leading the left and right hemisphere to coordinate with one another, this method would be useful for guiding the brain to generate specific frequencies. The brain naturally produces wave patterns that fall into a spectrum of frequency ranges. The lowest frequencies are known as "delta" and fall in the range between 0.5 and 4 cycles per second (hertz). From about 4 to 7 hertz the frequencies are called "theta." The 7 to 14 hertz range is known as "alpha" and 14 to 30 hertz range is referred to as "beta." Above that there are "gamma" waves. When we shift states of consciousness the level of brainwave activity in these ranges changes. When we are sleeping most of the waves are in the delta range, while when we are awake and very focused the majority of waves would be in the beta range. During meditation we tend to have a lot of theta and alpha brainwaves.
Since our ability to hear sounds is limited and we don't hear sounds in the range that matches the brainwave frequencies, binaural tones of higher frequencies are played, such as 250 hz in one ear and 257 hz in the other ear. In that case, the difference between the tones is 7 hz, so our brain will create that frequency in response to the tones played through headphones. We can use this method to train the brain to produce more of any type of brainwave frequency. This could help people improve concentration, foster relaxation and sleep, induce meditative and out of body states, and enhance creativity and learning. It can also foster physical healing, since brain hemisphere synchronization, a relaxed nervous system, and reduction of stress are all aids to helping the body heal or recover from illnesses and injuries.
Audio recordings using binaural tones have become popular and abundant in recent years, with several companies selling these products and many people creating their own using computer software. A quick search on Google or Youtube will bring up many binaural sound recordings. One can experience a similar effect when listening to Tibetan singing bowls or chimes (tingshas), where it sounds like the sound is wavering between different frequencies. Simultaneously playing two crystal singing bowls that are within one tone or note from each other (such as C and D or C and C#) will also generate this effect. You may find your consciousness rapidly shifting due to this sound entrainment process.
If you are looking for an aid for sleep, meditation, concentration, or relaxation, binaural sound and brainwave entrainment may be helpful. There are some precautions related to these audio recordings. Those with epilepsy and other seizure disorders would need to consult with their physician to see if this would be safe for them or not, since stimulating a synchronization of the brain hemispheres could lead to seizures in people with some disorders. Also, since these affect your mental state you would not use something that might cause you to enter a state that interferes with your ability to drive or operate equipment or machinery.