What is the Frey effect?

The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of the human perception of audible clicks, or even speech, induced by pulsed or modulated radio frequencies.

The electromagnetic field of the heart

Allan H. Frey was the first American to publish on the microwave auditory effect (MAE). Frey’s “Human auditory system response to modulated electromagnetic energy” appeared in the Journal of Applied Physiology in 1961. 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. The perceived loudness was found to be linked to the peak power density, instead of average power density. At 1.245 GHz, the peak power density for perception was below 80 mW/cm². 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”. By changing transmitter parameters, Frey was able to induce the “perception of severe buffeting of the head, without such apparent vestibular symptoms as dizziness or nausea”. Other transmitter parameters induced a pins and needles sensation. Frey experimented with nerve-deaf subjects, and speculated that the human detecting mechanism was in the cochlea, but at the time of the experiment the results were inconclusive due to factors such as tinnitus.

Auditory sensations of clicking or buzzing have been reported by some workers at modern-day microwave transmitting sites that emit pulsed microwave radiation. Auditory responses to transmitted frequencies from approximately 200 MHz to at least 3 GHz have been reported. The cause is thought to be thermoelastic expansion of portions of auditory apparatus, and the generally accepted mechanism is rapid (but minuscule, in the range of 10⁻⁵ °C) heating of brain by each pulse, and the resulting pressure wave traveling through the skull to the cochlea.

In 1975, an article by neuropsychologist Don Justesen discussing radiation effects on human perception 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”.