There is an interesting effect, named the “binaural beats” that arises when several musical instruments or the chorus of people sing in the same key is. Most strongly this effect is manifested in ethnic music.
While listening to the sounds whose frequencies are close to each other, one can experience these binaural beats. The greatest effect is achieved when using stereo headphones.
If you send to a stereo headphone two signals with difference of frequency (e.g. 250Hz in one ear, and 255 Hz in the other one), the hemispheres of human brain begin work together. The man hears a 5Hz phantom sound that does not exist in reality. The most interesting is that these oscillations are produced exclusively in the brain, because there is no any summation of real sound waves in the physical space (remember, you are listening in headphones).
Binaural sounds are a consequence of a harmonious work of the cerebral hemispheres. Researchers have had the idea to effects on the brain through the waves to produce certain psychophysical states. Stimulators may be very different, for example: sounds, flashes of light or super-weak pulses of electric current. Important aspect is to make frequencies interconnected in a specific ratio.
Corresponding to a certain frequency range waves can lead people to different state namely: waking state, relax passively or sleep.
For example waves of 14-42 Hz occur to increasing attention to surrounding events. Deficiency of naturally generated beta waves in the brain causes depression, poor selective attention and problems with remembering information. Excessive amounts of beta waves on the contrary leads to an increased level of tension, and even ordinary events can plunge human stress.
The waves of 8-13 Hz lead to a state of tranquility and pleasant relaxation, similar to abstract background music. At the same time, such waves help us to better assimilate the information. Ancient martial arts masters have noticed this feature and widely used it in their teaching methods. It is noteworthy that when brain stays in alpha state the muscle reaction speed increases many times. Lack of alpha waves also leads to stress, inability to proper rest and active learning, as well as to a variety of mental disorders.
The slowest waves (4-8 Hz) appear when the quiet wakefulness becomes drowsy. This state is a border between sleeping and waking. Theta state provides access to the contents of the unconscious part of the mind, sudden insights and creative ideas.
It is worth noting that the people in the theta state extremely susceptible to external shocks, as the impact of waves reduces efficiency of the mental protecting mechanisms. Information perceived in this state, penetrates deep into the subconscious. It is noteworthy being in this state, our brain secretes the largest amounts of growth hormone and the body recovers itself most intensely.
There are a lot of audio programs on the market today with binaural beats to enhance the intellectual capacities of man. The most famous are audio materials produced by the Monroe Institute.