The easiest way of applying stimulus to the brain is via ears and eyes. Since humans cannot hear sounds low enough to be useful for brain stimulation, special techniques must be used. One such special technique used is called binaural beats.
Perceived human hearing is limited to the range of frequencies from 20 Hz to 20,000 Hz, though Infrasound - sound below 20Hz - still have scientifically observable effects on humans, however, it is not readily audible, especially at low volume levels. While the frequencies of human brain waves are below about 40 Hz. To account for this lack of perception, binaural beat frequencies are used.
Binaural beats or binaural tones are auditory processing artifacts, or apparent sounds, which arise in the brain independent of physical stimuli. The brain produces these sounds [low-frequency pulsations] when two tones at slightly different frequencies are presented separately, one to each of ear, using stereo headphones.
A beating tone will be perceived, as if the two tones mixed naturally, out of the brain. The frequency of the tones must be below about 1,000 to 1,500 hertz for the beating to be heard. The difference between the two frequencies must be small - below about 30 Hz - for the effect to occur; otherwise the two tones will be heard separately and no beat will be perceived.
Binaural beats may influence functions of the brain besides those related to hearing. This phenomenon is called frequency following response. When one receives a stimulus with a frequency in the range of brain waves, the predominant brain wave frequency is likely to move towards the frequency of the stimulus - a process called entrainment.
In addition, binaural beats have been demonstrated to entrain brainwave rhythms, according to the frequency following response, at various sites in the brain.The stimulus need not be aural; it can also be visual - or a combination of aural and visual. CAUTION - using alpha frequencies with such stimuli can trigger photosensitive epilepsy.
When the perceived beat frequency corresponds to the delta, theta, alpha, beta, or gamma range of brainwave frequencies, the brainwaves entrain to or move towards the beat frequency.
For example, if a 315 Hz sine wave is played into the right ear and a 325 Hz one into the left ear, the brain is entrained towards the beat frequency [10 Hz, in the alpha range]. Since alpha range is associated with relaxation, this has a relaxing effect or if in the beta range, more alertness.
Beta, Delta, Theta Range
Binaural sound stimulation using beat frequencies in the Beta range and Delta/Theta range, can result in better vigilance performance and mood in those on the awake alert state of Beta range stimulation.
Binaural beat stimulation has been used fairly extensively to induce a variety of states of consciousness, there has been some work done in regards to the effects of these stimuli on relaxation, focus, attention, and states of consciousness. Studies have shown that with repeated training to close frequency sounds that a plastic reorganization of the brain occurs for the trained frequencies and is capable of asymmetric hemispheric balancing.
As you listen, your brainwave activity will become more coherent. You will feel more relaxed, but lucent.
The different wave patterns have different effects on the brain: