MICROWAVE‐INDUCED ACOUSTIC EFFECTS IN MAMMALIAN AUDITORY SYSTEMS AND PHYSICAL MATERIALS *

One of the most widely observed and accepted biologic effects of low average power electromagnetic (em) energy is the auditory sensation evoked in man exposed to pulsed microwaves.l-e The effect appears as an audible clicking or buzzing sensation that originates from within and near the back of the head and that corresponds in frequency to the recurrence rate of the microwave pulses. The effect is of great interest, because it can be elicited by average incident power levels far below those believed to be of thermal significance. The mechanism of the effect, however, has remained obscure. Sommer and Von Gierke' have suggested that radiation pressure may be sufficiently high to couple acoustic energy to the inner ear by bone conduction. Freya has discounted this hypothesis, however, and does not believe the interaction is due to transduction of em to acoustic energy. His belief is based on his failure to observe cochlear microphonic potentials associated with the pulsed microwave stimulation of the auditory systems of cats and guinea pigs and on the low levels of incident power at the threshold for perception by the human subject. Frey and Messenger' have observed that the loudness of the sensation was proportional to the peak power, whereas Guy and colleagues5 have observed that the threshold of the sensation is proportional to energy per pulse. It is important that the origin and threshold of the microwave interaction be identified and understood to assess potential hazards of pulsed microwave power. The present ANSI safety standarde does not restrict the peak power density as long as the power density, as averaged over any 6-min period, does not exceed I mW hr/cmz, or 3.6 J/cm2. This value is five orders of magnitude greater than the threshold level for producing an audible sensation by a single short pulse. The work reported here was formulated to apply a bioengineering approach for establishing the threshold of the effect in man and animals as a function of pulse power or energy, pulse shape, and carrier frequency; the locus of action of the effect, that is, whether it is initiated at a central or a t a peripheral site; and whether the stimulation is due to direct action of the em fields on the nervous system or to transduced acoustic energy acting on the auditory system. The studies involved the following procedures: establishment of incident field and modulation characteristics a t the threshold for auditory sensation in humans; com-