The Microwave Auditory Effect: Allan Frey’s Discovery That Changed Nothing

In 1962, neuroscientist Allan H. Frey published a paper in the Journal of Applied Physiology that should have transformed the field of electromagnetic bioeffects research. The paper documented a phenomenon that violated the prevailing assumption that non-ionizing radiation could not produce neurological effects at sub-thermal power levels: pulsed microwave radiation, at specific frequencies and pulse characteristics, produced audible sounds inside the human head. The subjects heard clicks, buzzing, or hissing — sounds that were not audible to others nearby and that did not require functioning cochlear hair cells. Deaf subjects with nerve damage rather than cochlear damage could hear them. The mechanism was eventually identified as thermoelastic expansion — rapid, minute thermal pulses creating pressure waves in cranial tissue. The physics was understood. The military implications were immediately classified. Public research into the phenomenon slowed to a crawl. Sixty years later, the most basic questions about the microwave auditory effect’s implications for public health remain unanswered.

The Discovery

Frey’s initial observation was accidental and personal. Working at General Electric’s Advanced Electronics Center at Cornell University in the late 1950s, he perceived clicking sounds while standing near operating radar equipment. The sounds were not audible to everyone present — Frey could hear them; some colleagues could not. This variability itself was significant: it suggested that the perception was not a simple acoustic phenomenon (which would be equally audible to all) but something involving individual physiological variation in the interaction between microwave radiation and the auditory system.

Frey systematically investigated the phenomenon over several years, publishing his landmark paper — “Human Auditory System Response to Modulated Electromagnetic Energy” — in the Journal of Applied Physiology in 1962. The paper documented the following:

Pulsed microwave radiation in the 200 MHz to 3 GHz range produced auditory perception in human subjects. The effect required pulsed radiation — continuous-wave (CW) exposure at the same average power did not produce audible perception. The threshold power level was well below the level that would produce measurable tissue heating. Subjects with conductive hearing loss (damaged cochlear hair cells) could not perceive the effect. Subjects with sensorineural hearing loss (intact cochlea but damaged auditory nerve) could not perceive it either. But subjects with cochlear damage who retained functional auditory nerves could perceive it — demonstrating that the effect bypassed the normal air-conduction hearing pathway and directly stimulated the auditory nerve through a mechanism that did not require the cochlea’s normal transduction process.

The Mechanism

The physical mechanism was not immediately understood but was eventually identified through the work of multiple researchers, including Kenneth Foster, Edward Finch, and James Lin. The thermoelastic model, now widely accepted, works as follows:

Each microwave pulse, upon absorption by cranial tissue, produces a rapid, extremely small temperature increase — on the order of millionths of a degree Celsius. This temperature increase causes correspondingly rapid thermal expansion of the tissue. The expansion generates a pressure wave — an acoustic pulse — that propagates through the skull to the cochlea, where it is transduced as sound by the normal mechanisms of bone conduction. The effect requires pulsed rather than continuous radiation because it is the rapid onset of heating, not the sustained temperature increase, that generates the pressure wave.

The thermoelastic mechanism is well-understood, experimentally verified, and not controversial in the electromagnetic engineering literature. It is taught in graduate-level bioelectromagnetics courses. It is the physical basis for one of the applications investigated under military research programs: the possibility of using pulsed microwaves to transmit audible messages directly into a target’s head without any external sound source.

The Military Interest

The Defense Intelligence Agency, the Advanced Research Projects Agency (now DARPA), and the U.S. Air Force funded follow-up research into the microwave auditory effect through the 1960s and 1970s. Declassified documents confirm that the military investigated the phenomenon as both a communications technology (transmitting voice messages into a soldier’s head without radio equipment) and a potential weapon (using high-power pulsed microwaves to produce disorienting or disabling auditory effects in enemy personnel).

The classified research trajectory is, by definition, incompletely known. What is documented is that public academic research into the microwave auditory effect slowed dramatically after the initial period of military interest. Frey himself — in interviews given decades after his original publication — described institutional resistance to continued research, including difficulty obtaining funding and what he characterized as pressure from military-affiliated researchers to minimize the significance of his findings.

The most recent comprehensive review of the phenomenon’s implications for public health was published by James Lin and Zhangwei Wang in Health Physics in 2007 — forty-five years after Frey’s original paper. A forty-five-year gap between a fundamental discovery and its public health review is not typical of phenomena that institutions take seriously.

The Havana Connection

The National Academy of Sciences’ 2020 assessment of Havana Syndrome — the mysterious neurological symptoms experienced by U.S. diplomatic personnel beginning in 2016 — identified “directed, pulsed radiofrequency energy” as the mechanism most consistent with the reported symptoms. The microwave auditory effect was specifically cited as a documented mechanism by which pulsed RF energy could produce neurological effects including headache, hearing disturbance, vertigo, and cognitive symptoms.

Frey’s 1962 discovery — ignored for public health purposes for six decades — is now the leading hypothesis for a phenomenon that has affected hundreds of government personnel and generated multiple intelligence investigations. The research that was not funded for civilian public health was apparently funded, in classified form, for military and intelligence applications. TFRi observes this asymmetry as part of the broader pattern documented in our centennial synthesis.