Monroe's patented audio method that plays slightly different tones into each ear so the brain manufactures a third, synchronizing one.
Documented
The binaural patent
Hemispheric Synchronization: feed 100 Hz to one ear and 104 Hz to the other and the brain generates a 4 Hz difference signal, nudging both hemispheres toward the same brainwave state. Monroe patented the method in 1975 and built every Gateway tape on it. Delta-theta states, where OBE reports cluster, are the target.
Assessment: Documented: the patent, the tapes, and the underlying psychoacoustics are real.
Stories
The elevator to the other side
Practitioner lore treats the tones as a literal transport mechanism: the signal is the vehicle, the headphones are the door. Thousands of Gateway Voyage graduates describe the same corridor imagery, which believers count as consensus evidence.
Assessment: Stories: shared imagery is shared instruction; the tapes tell you what the corridor looks like before you arrive.
Interesting claims
Entrainment as argument
The more interesting claim: if an external rhythm can march the brain into coherence, then every steady rhythm in the environment, screens, engines, chants, machines, is doing a quieter version of the same thing, and nobody consented to any of it.
Assessment: Interesting: entrainment is measurable; the leap to ambient mind-steering is the philosophy, not the patent.
Sources and further reading
U.S. Patent 3,884,218, 'Method of inducing and maintaining various stages of sleep in the human being,' Robert Monroe (1975)
CIA-RDP96-00788R001700210016-5, Analysis and Assessment of Gateway Process (1983)
The Gateway Experience audio series, The Monroe Institute
Worth asking
If two tones can steer consciousness, what else has been steering it?