* The easiest way of applying stimulus to the brain is via ears. Other senses could be used as well, and vision is actually used quite often (often in addition to hearing). However, humans cannot hear sounds low enough to be useful for brain stimulation, so special techniques must be used. One such special technique used is called binaural beats. If the left ear is presented with a steady tone of 500Hz and the right ear a steady tone of 510Hz, these two tones combine in the brain. The difference, 10Hz, is perceived by the brain and is a very effective stimulus for brainwave entrainment. This 10Hz is formed entirely by the brain. When using stereo headphones, the left and right sounds do not mix together until in your brain. The frequency difference, when perceived by brain this way, is called a binaural beat.
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* Delta 0.5Hz - 4Hz high (up to 200uV) Deep sleep
* Theta 4Hz - 8Hz low (5uV - 20uV) Drowsiness (also first stage of sleep)
* Alpha 8Hz - 14Hz high (up to 200uV) Relaxed but alert
* Beta 14Hz - 30Hz low (less than 10uV) Highly alert and focused

http://www.bwgen.com/