A Remote Sound Source by Microwave-Induced Acoustic Effects at Medium Interface

A thorough formulation of acoustic coupling by high power microwaves (HPM) is presented. It is found that the coupling may arise from thermoacoustic effect, electrostrictive effect, and electronagnetic energy of dielectric polarization. The formulation takes into account dielectric discontinuities at material interfaces and tansient charcter of microwave pulses. The result shows that, through themacoustic effect at an air-water interface, an absorption of microwave enery by water at the rate of 4.5 kW/gram produces an acoustic pressure as high as 10% of the atmospheric pressure, and that a rise time of 1 nanosecond is the marginal microwave rate, beyond which the electrostrictive effect contributes more to the pessure waves than the thermoacoustic effect It is also shown that, through electostrictve effect electromagnetic field modifies the effective compressibility tensor of a dielectric, thereby changes its acoustic velocity. This effect may be utiliz for direct measurement of transient electric field of RPM pulses.