the microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of audible clicks (or, with modulation, whole words) induced by pulsed/modulated microwave frequencies. The clicks are generated directly inside the human head without the need of any receiving electronic device. These induced sounds are not audible to other people nearby. signal modulation was found to produce sounds or words that appeared to originate intracranially. It was studied for its possible use in communications.
The existence of non-lethal weaponry that exploits the microwave auditory effect appears to have been classified "Secret NOFORN" in the USA from the mid 90's, until the declassification on December 6th 2006 ,in response to a FOIA request. Application of the microwave hearing technology could facilitate a private message transmission. Quoting from the above source, "Microwave hearing may be useful to provide a disruptive condition to a person not aware of the technology. Not only might it be disruptive to the sense of hearing, it could be psychologically devastating if one suddenly heard "voices within one's head".