The microwave auditory effect, also known as the Frey effect, consists of the human perception of audible clicks, or even speech, induced by pulsed or modulated radio frequencies. The communications are generated directly inside the human head without the need of any receiving electronic device. The effect was first reported by persons working in the vicinity of radar transponders during World War II. In 1961, the American neuroscientist Allan H. Frey studied this phenomenon and was the first to publish information its nature.
Research Origin: Human auditory system response to Modulated electromagnetic energy - Allan.H.Frey (1961) (unpaywalled)
In his experiments, the subjects were discovered to be able to hear appropriately pulsed microwave radiation, from a distance of a few inches to hundreds of feet from the transmitter. In Frey's tests, a repetition rate of 50 Hz was used, with pulse width between 10–70 microseconds. According to Frey, the induced sounds were described as "a buzz, clicking, hiss, or knocking, depending on several transmitter parameters, i.e., pulse width and pulse-repetition rate.
Research Evolution: From the book "Military Neuroscience and the Coming Age of Neurowarfare" - by Armin Krishnan (2017):
"In 1975, an article by neuropsychologist Don Justesen discussing radiation effects on human perceptions referred to an experiment by Joseph C. Sharp and Mark Grove at the Walter Reed Army Institute of Research during which Sharp and Grove reportedly were able to recognize nine out of ten words transmitted by "voice modulated microwaves". Since the radiation levels approached the (then current) 10 mW/cm² limit of safe exposure, critics have observed that under such conditions brain damage from thermal effects of high power microwave radiation would occur, and there was 'no conclusive evidence for MAE at lower energy densities'".
Research Application: Microwave Weapons Are Prime Suspect in Ills of U.S. Embassy Workers - NYtimes (2018)
"Doctors and scientists say microwave strikes may have caused sonic delusions and very real brain damage among embassy staff and family members."
Research Foundation: Microwave Auditory Effects And Applications - Book by James C. Lin, PhD (1978) (PDF)
"The purpose of the book is to bring a body of research literature, scattered in a large number of journals and reports, into some compact form for the convenience of students and researchers. It will deal with selected experimental and theoretical topics in an interdisciplinary field which is 'undergoing explosive growth." - James C.Lin (1978)
Research Today: Mostly classified. The following from an independent researcher: Microwave Auditory Effect And Its Aplication - Research Project by Makoto Koike (2019)
Project Goal: "Microwave auditory effect refers to the phenomenon that pulse-modulated microwave induces auditory perception. The head acts as an acoustic transducer to convert the microwave into a theremoelastic wave, thereby invoking bone conduction. I am exploring the application of the microwave auditory effect onto a novel telecommunication system as well as a conspiracy theory that the novel telecommunication induces psychosis with symptoms of hallucination and delusion."
Review of microwave auditory effect: rediscovery of radiofrequency hearing phenomenon. - by Makoto Koike (2016, JP only) (PDF)
The Myth concerning Not Hearing Microwave - Presentation by Makoto Koike (2019, The 99th CSJ Annual Meeting, The Chemical Society of Japan)
These patents by the U.S. Air Force, suggest that wireless, receiver-less communication (based on the Microwave Auditory Effect) could be used in military communication today: