Thomas Robinson, age 15, of Butte, Mont., for his question:
HOW DOES SONAR WORK?
Sonar is a detecting device that uses sound. Sonars generally use sound frequencies of 5,000 to 25,000 hertz. One type is used in navigation to find the depth of water.
Sonar signals are generated in a transmitter within the ship. They are sent to a transducer, an antenna like device in the water under the ship. The transducer changes the electrical signals to sound waves and sends them through the water. The waves travel outward or downward until they strike an object and reflect back to the transducer.
As the signal is sent out, part of it is routed within the ship to a receiver and indicator, which resembles an electric stop watch. It marks zero time as it receives a signal that sound waves are being sent out. It then begins to keep time.
As the waves return from an object, they are changed by the transducer to electric signals, which again are sent to the indicator. When it receives these signals, it indicates the range.
Since sound travels in water at about 1,600 yards each second, the indicator can give the distance in yards. Depth finding equipment gives the distance in fathoms.
In addition to the type of sonar used in navigation to find the depth of water, a second type is used by warships and military aircraft to find submarines and then attack them.
Both types of sonars operate on the same principle. They simply send out sound waves into the water. When the waves meet an object, it reflects back. The distance to the object is found by measuring the time taken by the sound to return. This is called "echo ranging."
The term "sonar", comes from these four words: sound navigation and ranging. The word is made up of the first two letters of the first word and the first letters of the final three words.
Some animals find direction by means of echo sounding. As an example, bats make ultrasonic, high frequency squeaks that bounce back from obstacles.
At first, all ranging devices were built on a "searchlight" principle. The transducer was moved back and forth like a searchlight to catch the sound waves. As the "searchlight" crossed the target, the bearing could be measured and range recorded.
But it takes a long time to search through a circle with such a device. Then, during World War II, it was learned that a cylinder could be built of staves or sections, each of which was a transducer.
Sound waves could be sent in all directions. The electronic circuits were devised to scan two or more sections of the transducer, forming their signals into a beam. The result was a' scanning sonar with electronic beam rotation which can be used for rapid search.
Sonar actually provides ships with "ears" to hear underwater.