Page 344 - NS-2 Textbook

PHYSICAL SCIENCE 339

deeper a sOlmd ·wave is, the faster it travels. This pres-
sure effect is smaller than temperature effect, but it can-
not be neglected, since it increases about 2 feet per sec-
ond for each 100 feet of depth.
Seawater has high mineral content or salillity. 11,e
density of seawater is about 64 pOlmds per cubic foot;
that of fresh water is only about 62.4 pounds per cubic
foot. This variation is the result of the salt content in the
seawater. The saltier the 'vatel~ the greater its density,
and hence the faster the speed of sound in it. The speed
of sOlmd increases about 4 feet per second for each part-
per-thousand increase in salinity-a lesser effect than
that of temperature, but greater than that of pressure.

SONAR
A refracted sound wave in the ocean.
The principal means of detecting and tracking sub-
by more than 30 degrees from the surface to a depth of marines at sea is called sonar (short for sOillld navigation
450 feet. So temperature changes in the sea have a great and ranging). The earliest sonar device, used in World
effect on the speed of sound in the seawater. War I, was a simple hydrophone, which could be lowered
Sound also travels faster in water lUlder pressure, into the water to listen for noises generated by submerged
since the density increases somewhat as pressure in- submarines. Three ships equipped with this equipment
creases. Pressure increases as depth increases, so the could pinpoint the location of U-boats by triangulation-

A frigate fitted with a bow-mounted sonar dome rests in drydock at the Boston Naval Shipyard.