Hemi-Sync is a trademarked brand name for a patented process used to create audio patterns containing binaural beats, which are commercialized in the form of audio CDs. Interstate Industries Inc., created by Hemi-Sync founder Robert Monroe, is the owner of the Hemi-Sync technology.
Hemi-Sync is short for Hemispheric Synchronization, also known as brainwave synchronization. Monroe claimed that the technique synchronizes the two hemispheres of one's brain, thereby creating a 'frequency-following response' designed to evoke certain effects. Hemi-Sync has been used for many purposes, including relaxation and sleep induction, learning and memory aids, helping those with physical and mental difficulties, and reaching altered states of consciousness through the use of sound.
The technique involves using sound waves to entrain brain waves. Wearing headphones, Monroe claimed that brains respond by producing a third sound (called binaural beats) that encouraged various brainwave activity changes. In 2002, a University of Virginia presentation at the Society for Psychophysiological Research examined Monroe's claim. The presentation demonstrated that EEG changes did not occur when the standard electromagnetic headphones of Monroe's setup were replaced by air conduction headphones, which were connected to a remote transducer by rubber tubes. This suggests that the basis for the entrainment effects is electromagnetic rather than acoustical.
Replicated, double-blind, randomized trials on anesthetized patients have found Hemi-Sync effective as a partial replacement for fentanyl during surgery. A similar study found it ineffective at replacing propofol however.