Jet Silencer

Airplane designers are confident that many jet airliners will be flying some day, but they dread the reception they will get from people living near airports. The noise of four or six big jet engines has to be heard to be believed. If not reduced in some way, the noise will drive strong men to desperation for miles around the runways.

United Aircraft Corp., producer of the biggest, most powerful and presumably loudest jet engine, the Pratt & Whitney J-57, is working on ways to reduce its roar to a tolerable level. One method worked out by Engineers John M. Tyler and George B. Towle utilizes the fact that the frequency (pitch) of the noise generated by a stream of gas varies with the stream's diameter. The big stream that shoots from the tailpipe of a jet engine stirs up a lot of low-frequency sound that carries for miles as a thunderous roar. Small gas streams, e.g., air escaping from a compressor hose, give high-frequency sound. Much of it is too high-pitched to be heard at all, and much of the rest is absorbed by the air before it has traveled far.

Tyler and Towle first tried shooting the engine's stream of hot gas through a sheet-metal plate perforated with small holes close together. This did not work very well. The wakes of the little jets of gas acted upon each other and caused violent turbulence that made too much noise of its own. Next they added to the tail pipe a metal cylinder with holes all around and closed at the rear end with a metal cone. It worked well in reducing noise level, but since the gas jets pointed every which way, the engine lost nearly all of its thrust.

Final trick was to stud the tail pipe cylinder with holes made in such a way that the gas streams escaping through them pointed almost directly backward. This device preserved most of the engine's thrust, and also eliminated nearly all of its low-frequency noise.

Pratt & Whitney's silencer has not yet been tested in flight. For actual installation in airliners, it will be made of several telescoping cylinders fitting around the tail pipe, and a segmented cone that can be closed or opened. When the airplane takes off within earshot of neighbors, the cylinders will be extended and the cone closed. The mighty stream of hot gases will be broken into small and comparatively quiet jets. After the aircraft is high in the air, the cylinders will be drawn back into the engine's nacelle and the cone will be opened. Then the engine will have full thrust for economical cruising, and its noise, muffled by distance and altitude, will not matter.

Like other silencing devices, this one will cost a good deal in weight, complication and loss of thrust on takeoff. But the cost may be justified by necessity. The big jet airliners, unsilenced, will make too many enemies.

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