Sound microwave-induced

Human subjects, whose heads were irradiated with rectangular pulse modulated microwave energy with peak incident power density on the order of 300 mW/cm, perceived an audible sound. The frequencies of these microwaves ranged from 200 to 3000 MHz, while the pulsewidths varied from 1 to 150 js (1,6,9,10,11). The sensation appeared as a barely audible click, buzz or chirp depending on such factors as pulsewidth and repetition frequency of the incident radiation, and usually was perceived as originating from within or near the head. When earplugs were used to attenuate ambient noise, the subject would indicate an apparent increase in the level of microwave-induced sound. The sensation occurred instantaneously and was independent of the subject s orientation in the microwave field. 

Thus, microwave-induced sound is a function of sound propagation speed (v), and the radius (a) or circumference (27Ta) of the head. Figure 5 illustrates measured cochlear microphonic frequency in cats and guinea pigs (8,1.5) and calculated fundamental sound... 

Figure 5. Microwave-induced sound frequency as a function of head radii or circumference (([J) constrained surface (O) stress-free)...

Figure 6. Variation of microwave-induced sound pressure and brainstem potential amplitudes as a function of pulse width (21)...

There are two alternate routes that might be taken by the microwave-induced thermoelastic pressure wave to reach the cochlea the well-known bone conduction route and a yet unspecified but perhaps a direct route from brain matter to the cochlea. While current information precludes elaboration of the latter, compressional bone-conduction appears to be the most likely candidate for the former, since the frequency of microwave-induced sound is very high and is inversely proportional to the radius or circumference of the head (15,26,27). 

It should be mentioned that a recent publication (H ) showed that the pitch (frequency) of sound induced by microwave pulses of widths less than 50 ps persisted as the subject s head was lowered into saline water, while the loudness diminished roughly in proportion to the depth of immersion. Upon complete immersion, auditory sensation disappeared. For pulse widths longer than 50 ps, even partial immersion resulted in loss of perception. This was interpreted as being at odds with the thermoelastic theory. There is, however, an explanation that does seem to fit the data. 

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