Inaudible binaural beats in the alpha or theta frequency range. Mini Review and insights.

Binaural beats (BB), the perceptual auditory phenomenon of an apparent oscillating sound of a specific frequency, which coincides with the frequency difference between two distinct frequency inputs presented to each ear, have been extensively employed in clinical studies as a prompt to psychological state and/or cerebral function. In parallel with a growing interest in music therapy, due to measurable effects of sound on brain activity as recorded by electroencephalography (EEG), there are several studies on the effect of BB in the frequency range of 1-40 Hz, which overlaps with the main EEG bands in humans. While the input sounds used for the BB creation in existing literature are mainly within the audible frequency range, it is possible to create practically inaudible BB, if employing ultrasound frequencies as a background, i.e., higher than about 16.000 Hz. To the best of our knowledge, currently there is not a review covering the specific field. This mini review summarizes the so far existing clinical data on the effect of experiencing inaudible binaural beats, on healthy young adults. The existing effect of the inaudible BB input has been established as independent from an audible counterpart for both alpha and theta frequencies, while its main positive impact to cognitive function is found to be relevant to visuospatial memory enhancement in case of alpha frequencies. Further insights into future applications of the technique are also discussed.