In order to create the Frey effect of hearing and sensation of pressure within the head, there are four distinct steps involving the energy conversion from radio frequency (RF) to acoustic modalities. First, the RF energy penetrates the skull and couples to the neural tissue as a function of impedance matching and absorption in the tissue, with penetrations of 2-4 cm for frequencies of 915 MHz to 2.45 GHz (Brace, 2010). This coupling, in turn, creates a rapid oscillation of temperature changes that leads to a rapid, volumetric thermal expansion and contraction of local tissues (i.e., the increase in thermal energy causes an increase in kinetic energy of atoms, pushing against neighboring atoms to create an expansion or swelling in all directions). The oscillating tissue expansion and contraction launches a thermoelastic pressure wave (Lin and Wang, 2007; Yitzhak et al., 2009). If operated at the right pulse repetition frequency, the thermoelastic pressure wave can propagate to and excite the cochlea and vestibular organs at the resonance frequency of the cranium (Lenhardt, 2003; Yitzhak et al., 2014). Intracranial focusing is possible depending on the incident angle of the incoming RF radiation. Localization and intensity effects within a room can be achieved through nonlinear beat wave effects with careful design of the RF source and antenna. The absence, however, of electromagnetic disruption of other electronics within the immediate home/office environment suggests an upper bound to the RF energy, with implications for a potential RF system design. The average power densities associated with some of these effects (e.g., Frey effect hearing) are so low that they would not disrupt nearby electronics in a fashion similar to high-power microwaves (HPM) (Hoad, 2007; Jinshi et al., 2008). The lack of perceptual heating would also rule out other non-lethal HPM systems that have been developed for crowd control (e.g., Department of Defense’s 95GHz Active Denial System that only penetrates the skin to 1/64 an inch but heats the skin to uncomfortable levels within seconds) (D’Andrea et al., 2008; DoD, 2020; Nelson et al., 2000).
It is well-known that the vestibular end organs and regions of the brain involved in processing of space and motion information may be excited by energy sources other than rotational or linear accelerations. External sonic, galvanic, and magnetic stimuli are used for diagnostic, experimental, and therapeutic purposes in neuro-otology and vestibular research such as generating vestibular evoked myogenic potentials (sonic), investigating vestibular response thresholds (galvanic), and as emerging therapies for chronic dizziness (transcranial magnetic and electrical stimulation) (Cha et al., 2013). Clinical observations also suggest that certain patients with vestibular disorders (e.g., Ménière’s disease) may be susceptible to exacerbations of their symptoms in response to rapid changes in atmospheric pressure as occur with quickly moving weather fronts or changes in elevation during air or land travel (Gürkov et al., 2016). However, the potential for RF sources to stimulate the vestibular end organs via thermoelastic pressure waves or to excite central nervous system pathways via transduction akin to the Frey effect are not known. If these effects exist, then a few observations may be made about their potential manifestations. A thermoelastic pressure wave would be omnidirectional thereby stimulating the vestibular end organs in a non-physiological manner. This unusual form of vestibular stimulation could lead to very confusing percepts as central vestibular pathways do their best to resolve the non-physiological pattern of end organ stimulation resulting in sensations of physically impossible motions, unexpected reflexive postural responses to them, and faulty inferences about external
forces causing them. Affected individuals could report different sensations in response to the same external stimulus; thus, immediate reports of affected individuals may not be veridical and sensations may vary from one individual to another. If a Frey-like effect can be induced on central nervous system tissue responsible for space and motion information processing, it likely would induce similarly idiosyncratic responses.
Brace, C. L. 2010. Microwave tissue ablation: Biophysics, technology, and applications. Critical Reviews in Biomedical Engineering 38(1):65-78.
Cha, Y.-H., Y. Cui, and R. W. Baloh. 2013. Repetitive transcranial magnetic stimulation for mal de debarquement syndrome. Otology & Neurotology 34(1):175-179.
D’Andrea, J., D. Cox, D., P. Henry, J. Ziriax, D. Hatcher, and W. Hurt. 2008. Rhesus monkey aversion to 94-GHz facial exposure. Naval Health Research Center Detachment Directed Energy Bioeffects Laboratory, Technical Report–NHRC DEBL TR-2006-07.
DoD (Department of Defense). 2020. Active denial systems. https://jnlwp.defense.gov/Future-Intermediate-Force-Capabilities/Active-Denial-Technology (accessed June 27, 2020).
Gürkov, R., R. Strobl, N. Heinlin, E. Krause, B. Olzowy, C. Koppe, and E. Grill. 2016. Atmospheric pressure and onset of epidsodes of Ménière’s disease. PLoS One 11(4):e0152714.
Hoad, R. 2007. The utility of electromagnetic attack detection to information security. Ph.D. dissertation. University of Glamorgan, United Kingdom.
Jinshi, X., L. Wenhua, Z. Shiying, Z. Jinjua, and X. Changfeng. 2008. Study of damage mechanism of high power microwave on electronic equipments. Paper presented at the 2008 China-Japan Joint Microwave Conference, Shanghai.
Lenhardt, M. L. 2003. Ultrasonic hearing in humans: Applications for tinnitus treatment. The International Tinnitus Journal 9(2):69-75.
Lin, J. C., and Z. Wang. 2007. Hearing of microwave pulses by humans and animals: Effects, mechanism, and thresholds. Health Physics 92(6):621-628.
Nelson, D. A., M. T. Nelson, T. J. Walters, and P. A. Mason. 2000. Skin heating effects of millimeter wave irradiation: Thermal modeling results. IEEE Transactions on Microwave Theory and Techniques 48:2111-2120.
Yitzhak, N. M., R. Ruppin, and R. Hareuveny. 2009. Generalized model of the microwave auditory effect. Physics in Medicine and Biology 54(13):621-628.
Yitzhak, N. M., R. Ruppin, and R. Hareuveny. 2014. Numerical simulation of pressure waves in the cochlea induced by a microwave pulse. Bioelectromagnetics 35(7):491-496.