If external stimulus is applied to the brain, it becomes possible to entrain the brain frequency from one stage to another. For example, if a person is in beta stage (highly alert) and a stimulus of 10Hz is applied to his/her brain for some time, the brain frequency is likely to change towards the applied stimulus. The effect will be relaxing to the person. This phenomenon is also called the frequency following response.

When the brain's state is close to the applied stimulus, entrainment works more efficiently. Thus, when doing a sweep from one frequency to another, the starting frequency should be as close to your current brain state as possible. The sweep speed should be such that your brain's state changes steadily with it, so that the difference never gets very large. In practice, it is difficult to determine your brain state without extra equipment (like EEG devices). However, you can quite safely assume that during the day your brain is in the beta stage (about 20Hz) and you can start the sweep from there. If you are already somewhat relaxed, you can use a start frequency of 15Hz or a few Hz lower.

Stimulating the brain with binaural beat frequencies

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 the brain this way, is called a binaural beat.