The
microwave auditory effect, also known as the
microwave hearing effect or the
Frey effect, consists of audible clicks induced by pulsed/modulated
microwave frequencies that 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. These induced sounds are not audible to other nearby people. The microwave auditory effect was later discovered to be inducible with shorter-wavelength portions of the
electromagnetic spectrum. During the
Cold War era, the
American neuroscientist Allan H. Frey studied this phenomenon and was the first to publish (
1961) information on the nature of the microwave auditory effect; this effect is therefore also known as the Frey effect.
Research by
NASA in the
1970s showed that this effect occurs as a result of thermal expansion of parts of the human ear around the
cochlea, even at low power density.
Later, signal modulation was found to produce sounds or words that appeared to originate intracranially. It was studied for its possible use in communications but has not been developed due to the possible hazardous biological effects of microwave radiation.
Similar research conducted in the USSR studied its use in non-lethal weaponry.