Jet Sound Effects

A touchy subject in aviation circles is the effect of jet engine noise on human nerves and bodies. Some of the airmen hate to mention it; they fear that people who live near jet bases will blame all sorts of ailments on the screaming jets. The truth about jets seems to be that their noise, when heard in the open and at a reasonable distance, is not at all harmful. But the intense "sound fields,"* which extend a short way behind the tail pipe, can have alarming and possibly harmful effects on people who enter them. When afterburners come into general use, adding their basso profundo to the scream of the jets, the fields of sound may become serious menaces.

The Navy is specially worried about the sound hazard. On the narrow space of a carrier's deck, many men must work in powerful sound fields. Last week the Navy was carefully investigating the effects of jet noises on its crews.

Hot Fingers. One method was to borrow an F94 (with afterburner) from the Air Force and lash it to the deck of the carrier Coral Sea. Under the watchful eyes of flight physicians,volunteers walked into its sound field. Few emerged without respect for what sound waves can do. When they get strong enough, the sound waves not only hurt the ears but make other parts of the body vibrate. A man standing in a sound field of 120 decibels (common near the tail pipe of a jet) feels the waves in surprising ways. If he holds out his hand, his fingers get painfully hot whenever they touch one another. If he partially opens his mouth, his nasal cavities may resonate like organ pipes. Sometimes his lower jaw vibrates so strongly that he has to grit his teeth to quiet it down. His ears get hot as they ride the waves; his nostrils get hot too. He may see only vague blurs as his eyeballs dance, and individual muscles resonate like plucked guitar strings.

Sometimes in a sound field a healthy man will crumple without warning and fall. Once he is out of the sound field, he is all right again, and when he has gained experience, he learns how to stand up in spite of wobbling knees. Apparently the sound waves interfere with the functioning of his nervous system, which normally maintains his equilibrium without conscious signals from his brain. With training, the brain can be made to tell the muscles how to keep the man standing.

Sound-Proof Helmet. All these effects can be felt with ordinary jet engines. Afterburner noise makes them more intense and adds some effects of its own. Men exposed to an afterburner's sound field for the first time are overwhelmed by terror and panic. They want desperately to escape, or as one victim put it, "to roll yourself into as small a ball as possible."

The Navy does not know yet how loud afterburners will get or what will have to be done to shield men from the effects of their shattering sound. It hopes that the temporary deafness that jets often cause will not turn into permanent afterburner deafness. Meanwhile, as it studies the problem theoretically, it is working on a protective helmet, proof not against bullets but sound.

*Not to be confused with the explosive bang made by a jet pulling out of a supersonic dive. This is caused by a shock wave which is intensified during the turn and then detaches itself from the wing and slams down to the ground.

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