A short discussion on binaural beats:

My career as an audio Mixer (sound designer) has been fascinating both in terms of the people I've met and in terms of the stuff that I've learned about the technical aspects of creating and manipulating sounds. Perhaps one of the more interesting of the technical aspects has been the binaural beat technology (BBT). In nature, two sounds that are similar but slightly different in frequency will beat to produce two new frequencies which are the average and the difference of the original two sounds. For example, a 300 Hz tone and a 310 Hz tone will form a ~305 Hz tone which will beat 10 times per second.(Don't let this technical stuff get you down - thats about as hard as it gets) The brain produces a similar phenomenon internally, resulting in low-frequency pulsations in the loudness of a perceived sound when two tones at slightly different frequencies are presented separately, one to each of a subject's ears, using stereo headphones. A beating tone will be perceived, as if the two tones mixed naturally, out of the brain. Heinrich Wilhelm Dove discovered this more that 100 years ago. This idea was picked up by Gerald Oster's article "Auditory Beats in the Brain" Scientific American, 1973). Since then, much research has been done documenting the effects of biaural beats on the brain and nervous system. As you probably know, your brain is composed of billions of neurons (nerve cells) which exhibit a characteristic signature, which can be measured with an EEG. The dominant frequency determines your current state. For example, if in someone's brain alpha waves are dominating, they are in the alpha state (this happens when one is relaxed but awake). However, also other frequencies will be present, but with smaller amplitudes.
Frequency range Name Usually associated with:
> 40 Hz Gamma waves  Higher mental activity:(perception, problem solving, fear, and consciousness)
13–40 Hz Beta waves Active, busy or anxious thinking and active concentration, arousal, cognition
7–13 Hz Alpha waves Relaxation (while awake), pre-sleep and pre-wake drowsiness
4–7 Hz Theta waves Dreams, deep meditation, REM sleep
< 4 Hz Delta waves Deep dreamless sleep<, loss of body awareness