Intensity 
The intensity of the sound in all parts of the direct 
beam is by no means the same. In the absence of refraction it 
would diminish in proportion to the inverse square of the dis- 
tance from the source. But because of refraction the energy in 
some parts of the direct beam has all come from a very small 
angle at the source. Compare, for example, the spacing of the 
sound rays in Figures 1 and 4. In Figure 1 at ranges between 
2000 and 3000 yards and at depths below 120 feet all the energy 
hiaoe Cone micon an vaneike or jo ab the source. in echo: rangine? 
unless a very carefully tuned receiver is used and the back- 
ground noise level is low, an echo from a target in this zone 
might not be detected. Certainly the bottom reflected energy 
from the outer part of this interval would be too weak to re- 
turn an echo. However, in listening this weak part of the beam 
might be used. 
In a general way, the spreading of the sound rays in 
the diagrams provides a rough measure of intensity. It will be 
noticed that under conditions of strong downward refraction (as 
in Figure 8) most of the energy from the source is concentrated 
over a relatively short distance on the bottom. It is believed 
that under such circumstances the effectiveness of bottom re- 
flections in increasing the range is usually considerably in- 
creased; in such a case even a MUD AND SAND bottom may be likely 
to produce at least one "bounce." 
Low Frequency Sound 
Sound in the audible frequencies is refracted and re- 
flected in the same way aS supersonic sound, although the re- 
flecting power of different sediments at low frequencies has not 
been accurately determined. With low frequency sound, however, 
the boundaries of the shadow zones will be much less sharp owing 
to diffraction. It is not known how effective diffracted sound 
is in increasing listening ranges. 
Although for audible frequencies, the data areeven 
less complete than for supersonic frequencies, it may be assumed 
that over a SAND bottom the listening conditions may possibly be 
good even when several surface and bottom reflections are re- 
quired. 
The Bathythermograph 
This is an instrument for measuring the vertical tem- 
perature gradients from a vessel proceeding at speeds up to 15 
or 20 knots. From such an observation the refraction pattern 
and the "Assured Range” of echo-ranging equipment can be deter- 
mined. Obviously the usefulness of sedimentary charts in sub- 
