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-