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. The clicks 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 people nearby. 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 (Journal of Applied Physiology, Vol. 17, pages 689-692, 1962) 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.
The existence of non-lethal weaponry that exploits the microwave auditory effect appears to have been classified "Secret NOFORN" in the USA from (at the latest) 1998, until the declassification on 6 December 2006 of Bioeffects of Selected Non-Lethal Weaponry in response to a FOIA request.
The technology gained further public attention when a company announced in early 2008 that they were close to fielding a device called MEDUSA (Mob Excess Deterrent Using Silent Audio) based on the principle.
For centuries, humans have reported hearing unexplained noises in conjunction with meteors including "thunder-like sounds" at the scene of the Tunguska event on June 30, 1908. Astronomer Edmund Halley collected several such accounts after a widely-observed meteor burned up in the sky over England . The Leonid meteor shower in November 2001 also led to many reports of observers hearing crackling or fizzing noises. Similar observations have been reported by soldiers near the site of nuclear explosions.
Colin Keay, a physicist at the University of Newcastle in Australia, has advanced a hypothesis that purports to explain these phenomena. According to Keay's theory, meteor trails give off very low frequency (VLF) radio signals that the human ear cannot sense directly but are heard because a transducer on the ground must be converting the radio waves into sound waves. He has produced experiments that demonstrate that materials as commonplace as aluminum foil, thin wires, pine needles, and wire-framed glasses can act as suitable transducers.
Powerful VLF waves can induce physical vibrations in these objects, which are transmitted to the air as sound waves. Keay defines the field of geophysical electrophonics link to Colin Keay's geophysical electronphonics websiteas "the production of audible noises of various kinds through direct conversion by transduction of very low frequency electromagnetic energy generated by a number of geophysical phenomena."  Some scientists state that electrophonic effects may also be caused by lightning strikes, very bright auroras, and earthquakes.
Electroreception has also been studied in the animal world. Ritz et al., in "A Model for Photoreceptor-Based Magnetoreception in Birds", hypothesize that transduction of the Earth's geomagnetic field is responsible for the magnetoreception systems of birds. Specifically, they propose that this transduction may take place in a class of photoreceptors known as cryptochromes.
In general, there is scientific evidence that waves at different levels of the electromagnetic frequencies have physiological effects on people exposed to them, including auditory effects.
The first American to publish on the microwave hearing effect was Allan H. Frey, in 1961. In his experiments, the subjects were discovered to be able to hear appropriately pulsed microwave radiation, from a distance of 100 meters from the transmitter. This was accompanied by side effects such as dizziness, headaches, and a pins and needles sensation.
Devices exist for scaring birds away from aircraft near airfields by microwave hearing and induction of vertigo.
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