Quicker Than the Ear

Working with some unusual accessories (including canary birds, guppies and nervous women), Inventor S. Young White is digging into a complex study called ultrasonics.* Last week, in Audio Engineering, Inventor White described one of his gadgets: a sound-maker no bigger than a milk bottle. The White siren can generate: 1) "silent" sounds powerful enough to set paper afire; 2) audible sounds so loud that they knock strong men (including Mr. White) silly for five minutes.

Mr. White's doomsday trumpet is a steel wheel 3 1/2 inches in diameter, with 80 square teeth around its edge. It spins against a thick steel disc drilled with 80 small holes arranged in a circle to match the 80 teeth. Compressed air rushes through these holes. When the wheel revolves, its teeth chop the air into pulses; each pulse becomes a sound wave.

Loud Holes. This is roughly the way many sirens work, but Inventor White has added something extra. The channels in his steel disc are designed to act as resonators, i.e., to intensify sound waves. When they are closed by the wheel's teeth, the air rushing through them stops suddenly. A compression (sound) wave builds up, reverberates back & forth as if the channel were a tiny organ pipe. When the wheel is revolving at proper speed, the wave snaps back just in time to find the end of the channel uncovered. It pops out into the open, carrying with it a monster pulse of sound energy.

With the wheel spinning at 18,000 r.p.m., the sound has a pitch of 24,000 cycles--too high for the normal human ear. But if two sheets of paper are placed in the beam, the nearer is cooled by the air blast, while the second bursts into flame. Once Mr. White held his hand in the path of the silent sound waves. He felt a "scintillating" sensation, as if his skin were covered with rapidly alternating hot and cold spots. The hand was not damaged. Ultrasonic sound is no comic-strip death ray; 99.98% of its energy is reflected harmlessly by flesh.

Paralyzing Racket. Though intended primarily for ultrasonics, Inventor White's siren can also produce ordinary, audible sounds. Low in pitch, their power is still enormous. To show what they can do, White adjusts the wheel's speed so that it will generate 800-cycle sound waves --just below the top of a soprano's range.

Then little by little he turns on the compressed air, increasing the power while the pitch remains the same. From the start, the sound is painfully loud. Above a certain intensity its apparent loudness does not increase; the ear contains safety devices which keep it from registering sounds too loud for it to handle.

But other effects take over. As the giant sound waves rocket around the room, White and his assistants feel their eyes go out of focus. Their muscles jerk and their jaws drop.

Once, as the frightful sound bellowed louder, White's fingers froze to the control valve. He forgot what he was doing, or why. So did all the others present. For five minutes they stood paralyzed until an outsider ran in and broke the clinch.

Nervous Guppies. Neighbors being what they are, the White siren is worked most of the time in the ultrasonic range. The average man cannot hear sounds above 14,000 cycles. Women are receptive to slightly higher pitches. White cites one exceptional woman, "a nervous, Park Avenue type," who can hear sounds in the neighborhood of 24,000 cycles. So can canaries. If annoyed with ultrasonic heckling, canaries turn their heads to an angle of least annoyance.

Guppies react much the same way. During the war, White was trying to trace the complicated track of the silent waves as they bounced around inside a tank of water. Baffled by their gyrations, he bought a school of guppies and dumped them into the tank. The little fish lined right up to outline the waves.

White believes that ultrasonics has a brilliant future. Some possible uses: killing bacteria; breaking up suspensions of solid particles; precipitating smoke and dust; speeding up chemical reactions. The sound waves can also pull large molecules apart, turning heavy oils into gasoline. Last week, from Britain, came a report that the little waves may soon be used in laundering, to knock dirt from soap-starved British clothes.

* Sounds that are too high-pitched for the human ear to hear. They were once called "supersonic," but airmen, trying to fly faster than sound, stole the name.

This file is automatically generated by a robot program, so reader's discretion is required.