Illustration showing the operation of "silent sound" with the human 
hearing system, using near-ultrasound, FREQUENCY MODULATED voice


One method for projecting either audible voice or voice-FM over
long distances, virtually undectable if line of sight, is the "acoustic
heterodyne" or "HyperSonic Sound" system, patented by American Technologies
Corporation, San Diego CA, http://www.atcsd.com




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