Maurice Ewing was a physicist who predicted, and then verified, the existence of the SOFAR sound channel in the oceans. (SOFAR stands for SOund Fixing And Ranging. To learn more about Ewing -- not required for this course -- you can read a short biography.)
The SOFAR sound channel is a layer in the oceans, about 1 km deep, that is somewhat isolated from outside. Sound produced at this depth tends to travel long distances without significant amounts getting to the surface. Likewise, sound created at the surface (from ships and waves) does not easily get into the sound channel.
The origin of the sound channel is the way the velocity of sound varies in water as a function of depth. Contrary to what I said in class, sound speed decreases with a decrease in water temperature, about 5 meters per second per degree C. If this effect dominated, the deeper it got, the slower sound would go. However there is a counter effect. Sound speed increases with pressure. (The pressure effect is more important than the salinity effect that I mentioned in class. The salinity effect is present only in relatively shallow water.) At a depth of about 1 km, the pressure effect overcomes the temperature effect, and there is a minimum in the sound speed.
Sound tends to bend towards this depth. Sound produced at this depth at an upward or downward angle, is bent back to this depth. The sound gets trapped. Since sound has no good path to leave this layer, it turns out that sound has no good way to enter the layer. It is thus very quiet. There is very little sound in this layer except the music of whales, who use it to communicate thousands of miles across the oceans. Wonderful recordings of these whale songs have been made; see, for example, The Song of the Humpback Whale.
In World War II, Ewing proposed that the channel could be used for emergency
communications. A small metal sphere dropped into the ocean, designed to crush
at 1 km depth, would emit a ping (sharp sound) that travelled in the sound channel
for thousands of kilometers. Microphones, placed at a similar depth, could hear
this ping. With three such microphones recording the time of arrival, the location
of the sphere could be accurately measured. This was useful for locating downed
pilots during World War II. The same principle is used in GPS (Global Positioning
System) satellite systems. (I used a GPS receiver -- it cost $119 at REI
-- for backpacking.) In GPS, the receiver measures the distance to several satellites.
From this distance, and knowledge of the locations of the satellites (which
are broadcast), the GPS receiver can tell you where you are to within a few
feet, anywhere in the world.
After World War II, the SOFAR system was developed into the LOFAR system. LOFAR stands for LO Frequency Analysis and Ranging. It listened for low frequency sounds, such as those from submarine engines. The US ultimately built a complex multibillion dollar SOSUS (Sound SUrveillance System) network. Eventually sensitive microphones were put on submarines, or on long arrays trailing behind them. It was such a system that undoubtedly gave the US detailed information on the recent Russian submarine disaster.
The use of the sound channel in submarine detection is described in great detail in the novel "The Hunt for Red October" by Tom Clancy. In this novel, submarines go to great effort to avoid the sound channel. This novel is highly recommended for anyone interested in the subject of submarine detection and intrigue. It is full of information that was (and still is) highly classified. But I won't tell you which facts are classified, and which facts Clancy actually got wrong. (He did make some mistakes.)