Interaction of an Electromagnetic Wave with the Acoustic Field of a Traveling Point Source of Sound. Part II: Application in a Standard Atmosphere

ABSTRACT

In Part I (Asvestas, 1994) we developed the theory for the interaction of a plane electromagnetic (EM) wave with the acoustic field of a traveling point source of sound. Here we look at this phenomenon in the context of a standard atmosphere. We first examine the variation of permittivity and acoustic absorption coefficient with atmospheric conditions, and we use our findings to analyze the backscattering cross section of the first two interaction waves in the resonance region and under typical atmospheric conditions. We find that the resonance peak of the cross section can be substantial but, also, that both the EM and acoustic 3dB bandwidths of the resonance lobe are very narrow. This indicates that, if the acoustic frequency is unknown, it may be very hard to locate the EM resonance frequency; moreover, even when the EM resonance frequency can be located, that a slight drift in the acoustic frequency may ruin the resonance effect. The sharpness of the acoustic resonance lobe indicates that an acoustic source with bandwidth will appear to the EM wave as being monochromatic, thus doing away with the need for studying the broadband case separately. We conclude by showing that, if the acoustic power is spread through any 1/3-octave band, then it requires an EM system with a 23.16% bandwidth to capture it.