Ocean depth creates a wave-guiding sound speed minimum around 800 meters

Evidence

Going deeper into the ocean, temperature steadily declines, but the pressure is rising. High pressure increases water density and causes an increase in sound speed. At first, the temperature decline wins, and sound speed decreases with depth. Eventually, at a depth of about 800 meters, the rising pressure overcomes the decline in temperature and causes the sound speed to go up again (see figure 2.21). Thus, as discovered in the United States toward the end of World War II and independently in 1946 in Russia (and kept top secret during the Cold War by both the American and Russian navies), there is a band of minimum sound speed, called the Sound Fixing and Ranging (SOFAR) channel, just under 1 km deep. Since waves always refract toward regions of lower speed, the SOFAR channel is a waveguide for sound. Communication over long distances is possible, since the sound energy is held to the channel rather than spreading in three dimensions. — Eric J. Heller, Why You Hear What You Hear- An…, 219

Categorization

• XREF: Connects to the reader’s soundscape/noise work on wave behavior and sound propagation — and to whale communication in One Square Inch of Silence. — connects to: Ocean acidification physically amplifies how far sound travels in seawater — Almo Farina, Soundscape Ecology: Principles,…, 1332
• DEFINE: Defines the SOFAR channel and the mechanism (temperature vs. pressure balance) that creates the low-speed waveguide.
• SHARE: The SOFAR channel, kept secret by both US and Russian navies during the Cold War yet discovered independently, is a perfect nerdy tidbit for anyone into naval or acoustic history.
• CANDY: The detail that whales dive nearly a kilometer to use the natural sound conduit of the deep is a light, shareable fun fact.