Page 170 - Master Handbook of Acoustics

FIGURE 8-6 Schematic showing how oceanic refraction can affect sound propagation underwater.
(A) Sound speed decreases with depth in the upper reaches of the ocean (temperature effect) and
increases at greater depths (pressure effect), creating a sound channel at the inversion depth (about
700 fathoms). (B) A ray of sound is kept in this sound channel by refraction. Sound travels great

distances in this channel because of the lower losses.

A sound channel is created by this V-shaped sound-speed profile as shown in Fig. 8-6B. A sound
emitted in this channel tends to spread out in all directions. Any ray traveling upward will be
refracted downward, while any ray traveling downward will be refracted upward. Thus, sound
energy in this channel is propagated great distances with modest losses.
Refraction in the vertical plane is prominent because of the vertical temperature/pressure gradient.
There is relatively little horizontal sound-speed gradient and therefore very little horizontal

refraction. Sound tends to spread out in a thin sheet at this 700-fathom depth. Spherical divergence in
three dimensions is changed to two-dimensional propagation at this unique depth.
These long-distance sound channel experiments have suggested that such measurements can be
used to monitor global climate change by detecting changes in the average temperature of the oceans.

The speed of sound is a function of the temperature of the ocean. Accurate measurements of time of
transit over a given course yield information on the temperature of that ocean.