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Since sound is reflected at the interface of differing elasticities, and since the elasticity of an inhomogeneous medium varies with pressure, it is in principle possible to map a standing wave of low frequency in, say, a biological medium, with ultrasound. The conditions for doing so are, alas, beyond the capability of the CIA: if they want to know whether you are listening to communist music or plotting the downfall of the Bilderberg-military-industrial complex, they will have to use a microphone, just like everyone else. Or they could just ask you, but where's the fun in that? Aliens use an anal probe.