Super-Sound Detector Picks Up Silence, Makes It Roar
CAMBRIDGE, Mass., Jan. 15., 1934 — The fact that a suit of clothes creaks with every movement of the body is revealed by a super-sound detector at the Harvard university physics laboratory.
When this supersonic listener is turned on in a room perfectly quiet to the ear, the air is filled with noises. They are supersounds, air waves vibrating at 20,000 or more a second, too high to be audible to the human ear.
The noises arise apparently from every kind of motion, however slight. Among these supersounds the creaking of a good suit of clothes is among the noisiest. Stand in front of the mechanical listener and slowly bend one elbow. “Grrump,” goes the racket of the flaxlug fabric and keeps up this sound as long as the arm is in motion. Light a match, and above the soft flare which is audible to the ear the supersonic instrument picks up another noise resembling the distant rumble of a street car. Rubbing the palms of the hands together emits a stream of these sounds. Tearing a piece of paper sounds like far-off machine gun fire.
A roomful of men trying to remain entirely quiet registers on the supersonic device like the noises of a herd of elephants. This detector, designed by Dr. G. W. Pierce, Rumford professor of physics, is part of an extensive investigation of supersounds. In the air these inaudible waves travel farther than ordinary sounds. They also are transmitted by the other substances which carry sound, suggesting possible normal uses for signalling. In water they can be heard nearly 10 miles.
Music can be transmitted over a specially arranged supersound beam. For this Dr. Pierce uses the inaudible sound of a Gallon whistle, which sends out vibrations at the rate of about 25,000 a second. When the listening device is turned to the whistle it makes a high, clear note. But when a phonograph record is connected electrically with the whistle, the clear note modulates to carry faithfully every variation in sound of a full orchestra. These modulations are all in supersound frequencies, so that the music cannot be heard except when the listening device is cut in to receive the whistle and reduce its high frequencies to the audible range.
—The Bee, Danville, VA, 1934