The frequencies of naturally occurring electromagnetic signals, circulating in the electrically resonant cavity bounded by the Earth and the ionosphere, have governed or determined the ‘evolution’ or development of the frequencies of operation of the principal human brain-wave signals.
We have also discussed the benefits for human well-being and relaxation by entraining with natural rhythms (The Diamond Body, 1981). It is the same technique used for decades by light and sound machines designed for neural signaling. Binaural beats technologies (hybrid sound devices) use out of beat-sync auditory output however, the effective frequency derived by them are within this same range as well.
When one discusses Schumann at 7.83 Hz, one must be cognizant that Schumann encompasses an entire range of frequencies from .01 to well over 100 Hz; The peaks below 15 Hz keep us tuned low, as soon as a higher amplitude is introduced, the biological organism will entrain (tune) itself to it. Notice the amplitude of the 60 Hz signal below.
Hear Dr. Robert O. Becker and Andrew Marino, PhD on CBS 60 Minutes circa 1977 warning of the effect of 60 Hz fields.