    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-