    Most of the transmitters used to date in the experimentation have
    been pulse modulated with no information placed on the signal.  The
    rf sound has been described as being a buzz, clicking, hiss, or
    knocking, depending on several transmitter parameters, i.e., pulse
    width and pulse-repetition rate (PRF).  The apparent source of these
    sounds is localized by the subjects as being within, or immediately
    behind the head.  The sound always seems to come from within or
    immediately behind the head no matter how the subjects twists or
    rotates in the rf field.

    Our early experimentation, preformed using transmitters with very
    short square pulses and high pulse-repetition rates, seemed to
    indicate that we were dealing with harmonics of the PRF.  However,
    our later work has indicated that this is not the case; rather, the
    rf sound appears to be incidental modulation envelope on each pulse,
    as shown in Fig 1.

    Some difficulty was experienced when the subjects tried to match the
    rf sound to ordinary audio.  They reported that it was not possible
    to satisfactorily match the rf sound to a sine wave or to white
    noise.  An audio amplifier was connected to a variable bypass filter
    and pulsed by the transmitter pulsing mechanism.  The subjects, when
    allowed to control the filter, reported a fairly satisfactory match.
    The subjects were fairly well satisfied with all frequencies below
    5-kc audio were eliminated and the high- frequency audio was extended
    as much as possible.  There was, however, always a demand for more
    high-frequency components.  Since our tweeter has a rather good
    high-frequency response, it is possible that we have shown an
    analogue of visual phenomenon in which people see farther into the
    ultraviolet range when the lenses is eliminated from the eye.  In
    other words, this may be a demonstration that the mechanical
    transmission system of the ossicles cannot respond to as high a
    frequency as the rest of the auditory system.  Since the rf bypasses
    the ossicle system and the audio given the subject for matching does
    not, this may explain the dissatisfaction of our subjects in the
    matching.

                                    -34-