At one time in our experimentation with deaf subjects there seemed to
be a clear relationship between the ability to hear audio above 5 kc
and the ability to hear rf sounds.  If a subject could hear above 5
kc, either by bone or air conduction, then he could hear the rf
sounds.  For example, the threshold of the subject whose audio gram
appears in Fig. 2 was the same average power density as our normal
subjects.  Recently, however, we have found people with a notch
around 5 kc who do not perceive the rf sounds generated by at least
one of our transmitters.

THRESHOLDS 

As shown in Table 1, we have used a fairly wide range of transmitter
parameters.  We are currently experimenting with transmitters that
radiate energy at frequencies below 425 mc, and are using different
types of modulation, e.g., pulse-repetition rates as low as 3 and
4/sec.

In the experimentation reported in this section, the ordinary noise
level was 70-90 db (measured with a General Radio Co. model 1551-B
sound level meter.)  In order to minimize the rf energy used in the
experimentation, subjects wore Flent antinoise ear stoppers whenever
measurements were made.  The ordinary noise attenuation of the Flents
is indicated in Fig. 3.  Although the rf sounds can be heard without
the use of Flents, eventhough they have an ambient noise evel of 90
db, it appears that the ambient noise to some extent "masked" the rf
sound.

Table 2 gives the thresholds for the perception of the sounds.  It
shows fairly clearly that the critical factor in the perception of
the rf sound is the peak power density, rather than the average power
density.  The relatively high value for transmitter B was expected
and will be discussed below.  Transmitter G has been omitted from the
table since the 20-mw/cm2 reading for it can be considered only
approximate.  The field-strength-measuring instruments used in that
experiment did not read high enough to give an accurate reading.  The
energy from transmitter H was not perceived, even when the peak power
density was as high as 25 w/cm2.

When the threshold energy is plotted as a function of the rf energy
(Fig. 4), a curve is obtained which is suggestive of the curve of
penetration of rf energy into the head.  Figure 5 shows the
calculated penetration, by frequency of rf energy, into the head. Our
data indicate that the calculated penetration curve may well be
accurate at the higher frequencies but the penetration at the lower
frequencies may be greater than that calculated on this model.

                                -39-