    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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