Exploring with Sound

Weather balloons, used by scientists to study the upper atmosphere, can reach up more than 21 miles. Rockets fly much higher, but they are too expensive and uncertain to send up on daily errands. Last week a group of upper-air explorers from the University of Denver started using cheaper messengers: sound waves, which soar up 30 miles or more and curve back to earth with valuable information about the air they have passed through.

The project, backed by the Air Force through its Cambridge, Mass, research laboratories, has been farmed out to the University of Denver Industrial Research Institute. Working under Colonel Victor C. Huffsmith, the project's scientists have laid out a rough cross with 100-mile arms on the flat plains of eastern Colorado. Spaced 20 miles apart on the arms are detonation points where the explosion of 200-lb. charges of TNT will send powerful sound waves into the sky. At points on a circle 160 miles from the center of the cross, complicated sets of microphones mounted on two trucks will listen for the waves when they return to earth.

The waves that serve best as messengers are of very low frequency, normally inaudible to the human ear. Only under exceptional circumstances can a human hear the TNT explosions 160 miles away. The microphones, developed in wartime for locating enemy artillery, can pick up the waves accurately and record their direction and time of arrival on paper tapes.

A study of these records gives clues about the high atmosphere. The waves that make the 160-mile jump experience various vicissitudes. In the warm air near the earth they move fast. Then they slow down gradually as the air grows colder. Passing through the stratosphere (temp. -- 70DEG F.) they hit a warmer layer of air 30 to 35 miles above the earth which turns them back down to the microphones.

The time the waves take to make the trip (up to 14 minutes) and the direction in which they are moving on arrival tell scientists the temperature and wind velocity in the zone above 20 miles. Such information is important to meteorology because the winds of the upper atmosphere affect the weather below. It is also important to the designers of long-range military rockets, which spend much of their flying time above 20 miles.

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