Sounds You Can’t Hear
Silent noise can be used to age whisky, pasteurize milk, kill rats, disperse fog, mix paints — maybe even detect and cure cancer
GEORGE H. WALTZ
A GOOD MANY scientists, at the moment, are busily finding jobs for sounds that no one has ever heard. Called “silent” sounds —because they are too high-pitched for the human ear to hearthese high-frequency vibrations are being harnessed by science to do such amazing things as homogenize milk, mix medicines and paints, sterilize foods, repel pests, kill germs, dispel fog and wash clothes. They even show some promise, according to recent reports from the famed Mayo Institute, of providing a medical weapon that not only will detect cancer but treat it! & At best, the most keen-eared of us hears only a fraction of the many noises that fill the air. There
is a whole weird world of sounds beyond our hearing. The human hearing system is arranged that way—it responds only to the relatively lowpitched sounds and perhaps it is just as well. If it were otherwise, we never would know the meaning of that golden word “silence.” What to our ears is soothing quiet actually is a nervewracking din. What we revel in as the relaxing calm of a summer countryside in reality is shattered with the incessant babble of millions of insects. All these sounds are around us but we fail to hear them. To us humans, they are truly “silent” sounds and it is man-made inaudible sounds of this general type that the scientists are now applying to practical everyday uses.
In the technical language of the physicist, a sound is the result of vibrations—a series of waves—in
the air. These waves can be caused by a vibrating member like a reed or a wire string, by a pulsating column of air, or by some sort of oscillating diaphragm such as the head of a drum.
A sound’s pitch—or its position in the scale of sounds—is determined by its frequency or the number of waves a second required to produce it. The fewer the number of vibrations a second, the lower the sound’s pitch; the greater the number of vibrations, the higher the pitch.
Theoretically, the scale of possible sounds is unlimited, ranging from the sound caused by the smallest possible fraction of a wave a second to sound with a frequency of many millions of waves. The human ear, however, responds only to a relatively narrow segment of this broad sound spectrum. Normally, we hear only those sounds that have frequencies somewhere in the range between about 16 waves a second (the lowest bass note on a large organ) and 18,000 waves a second (a sound pitched about five times higher than the highest note on your piano). Beyond this upper audible limit is the world of “silent” sounds—the sounds we never hear. Scientists have dubbed it the realm of “ultrasonics.”
Although silent sounds are a comparatively new scientific tool they are rapidly being put to use in industry and medicine. For one thing, supersounds are fairly easy to produce. They can be generated in a number of ways—either mechanically by a high-speed Continued on page 24
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Sounds You Can't Hear
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supersiren similar in basic construction to the siren on your local fire engine, or by an electrically vibrated source, which may be a powerful electromagnet not unlike the working innards of your telephone receiver or a quartz crystal that contracts and expands rapidly when subjected to high - frequency electrical impulses. Actually, anything that can be made to vibrate very rapidly can be used to produce silent sounds.
All sounds represent energy, but silent sounds, because their vibrations or waves come in rapid-fire order, are concentrated pellets of power that can be utilized in a number of ways. They can be focused to produce heat or violent vibrations, or they can be beamed radarlike to seek out the inner secrets hidden inside metals, plastics and even human tissue.
Among the top ultrasonic pioneers in the United States is a group of experimenters at the Pennsylvania State College in central Pennsylvania. Their supersound generator, of the siren type, has been responsible for many of the basic explorations into the world of ultrasonics. When visitors are around, its builders enjoy demonstrating some of the powers of silent sounds. With the siren whirling at high speed and literally chopping out a silent sound, one of the experimenters will hold a piece of steel wool in the sound’s path. Almost instantly it bursts into sparkling flame.
There’s no magic about this action. The supersonic vibrations simply set the particles of steel wool in motion and the resulting friction produces a rise in temperature. Water placed in the sound beam boils. Insects exposed to the intense vibrations are shattered. Marbles placed over the siren’s throat actually float in the sound beam like the familiar balls that bob around on top of streams of water in amusementpark shooting galleries. These are only spectacular laboratory demonstrations to be sure, but they demonstrate the usable power of high-frequency sounds.
Industrially, one of their most widespread uses, at the moment, is as a means of detecting flaws and internal fatigue points in large metal parts. When directed toward a piece of metal, silent sounds bounce off, or are absorbed, according to a definite pattern that is related to the metal’s hardness and density. Any flaws that are present within the metal reveal themselves by altering the reflected path of the high-frequency sounds. An operator, watching a dial or an electronic screen similar to that in a television set, can tell at a glance whether the piece of metal he is testing is flawless or contains weakening hidden faults that make it unfit for use. Such ultrasonic metal testers, now being manufactured by a number of wellknown concerns including the General Electric Co., save invaluable inspection time in railroad locomotive repair shops and large shipyards. With silent sounds, large metal parts such as axles, drive shafts and crank pins can be checked easily and quickly for internal wear and tear without removing them from place.
By harnessing this same radarlike quality of supersounds and directing them against the human body, Drs. J. P, Herrick and E. J. Blades of the Mayo Institute for Experimental Medicine recently reported the possibility of detecting human flaws in the form of cancer. Cancerous tissue, they have
found, acts just like the flaw in a metal part, reflecting the silent sounds to produce a detectable “echo” and indicating the presence of a malignant growth!
By stepping up the power of their high-frequency sounds and focusing them to a knifelike point, these same doctors also have been repeatedly successful in shattering cancerous growths by blasting them apart cell by cell. So far, these experiments have been limited to laboratory animals, but the results are extremely encouraging. They show that knifeless surgery with silent sounds is a distinct possibility.
This same idea also is being explored by two staff members of Columbia University’s Neurological Institute in New York City as an aid in performing delicate brain operations. Although their experiments too have, up to the present, only been performed on laboratory animals, the technique is such that it could be applied to humans. By directing a powerful, focused sound beam at an animal’s unopened skull they have been able to destroy any part of the brain tissue they desired—a fact that would seem to indicate that supersounds might eventually replace the surgeon’s scalpel and trepanning saw in performing a prefrontal leueotomy (the removal of the forward lobes of the brain) on a human to relieve certain types of insanity.
Naturally, these amazing medical uses of silent sounds are still very much in the experimental laboratory stage. Nevertheless, the same principles of employing their power already have many actual and practical applications in science and industry.
Have you ever tried to mix oil and water so they will stay mixed? It’s an almost impossible feat, even with the best mechanical mixers, yet silent sounds can do it! Their high-frequency vibrations easily break down the globules of any pair of hard-to-mix liquids and blend them into a smooth mixture. As a result, paint manufacturers see in supersounds a means of producing permanently mixed paints that will require no further stirring no matter how long they remain on a paint dealer’s shelf, food processors recognize the possibilities of using them as supermixers for turning out supersmooth salad dressings that won’t settle out, and drug companies are experimenting with the mixing of medicines that you no longer will have to “shake well before using.”
Silent sounds already are being used to homogenize milk in a number of large dairies. Milk made to flow over metal plates or diaphragms vibrating at ultrasonic frequencies is jolted so hard and fast that the individual droplets of fat are disintegrated to form a homogenous easily digested fluid. So intense can the action of the supersounds be made, as a matter of fact, that they can be used to pasteurize milk. Ordinary high-intensity heat — the conventional Pasteur process— requires hours to lower the bacterial count of milk to a safe level. Silent sounds do a better job in a matter of seconds.
A Good Whisky Ager
The bacteria-killing qualities of silent sounds have endless possibilities in the broad field of food sterilization. Pioneering sound experts see them as a means of killing the harmful spoiling bacteria in solid as well as liquid foods. Once man succeeds in killing all the bacteria in a food—and silent sounds come closer to doing it than any other preserving method’—that food will last almost indefinitely without spoiling-
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Milk, for instance, could be kept without. refrigeration.
Other practical uses for high-frequency sounds are being tried in the food and drink industries. Silent sounds, agricultural experts have found, rid processed flour of harmful weevils and germs. Brewers and distillers testing the possibilities of ultrasonics have found that a short exposure to sound waves speeds up the ageing of whisky—a normal four-year ageing period being telescoped into a matter of a few minutes.
According to sound engineers, the possible uses of silent sounds are just about endless. Not long ago, a Texas manufacturer of carbon black, an important ingredient in many industrial processes, wondered if supersounds might not help him to speed up his production. He took his problem to the Ultrasonics Corporation, manufacturers of high-frequency sound generators located in Boston. Today, in a pilot plant, a large sound generator mounted at the base of a large chimney at the Texas factory sets up sound waves that literally shake the carbon particles loose from the flue gases. They fall like large flakes of black snow to be collected and sold as carbon black.
In much the same way, silent sounds can wring the soot out of smoke and dust and pollen out of the air. They even can change fog into rain by shaking the tiny fog droplets so hard that they join together to form larger drops that fall as rain. Out near Areata, Calif., one of the foggiest regions in the United States, sound scientists are experimenting with this system as a means of clearing airports of fog.
When civilian sound experts and U. S. Navy technicians first tried their fog-dispelling noisemaker its tone was too close to the audible level of the average person’s hearing. The giant siren succeeded in getting rid of the fog over the test airport but neighbors with sensitive ears complained. At the moment, a larger more powerful siren that will produce sounds well within the silent range is being constructed. If its tests are successful and completely silent, a supersoundmaker may well become standard equipment at civilian and military airports for use during foggy weather.
A silent-sound household gadget that will keep your home free of insects and mice also is a definite future possibility. Experimenters with supersounds have found that ultrasonic waves act as a sure-fire repellent against household pests — insects, like flies and roaches, just don’t like the sound of silent sounds, because unlike humans they can hear them. The same appears to be the case with rats and mice. It is not stretching things too far, according to many of the experts, to predict that it probably won’t be too long before we will be able to buy a portable silent - sound generator housed in a little black box about the size of a table radio, plug it into a wall socket and keep our homes completely pest-free. As Dr. Hubert Frings, who has been experimenting along these lines at Pennsylvania State College, puts it, “Confidentially, the Pied Piper of Hamelin may have been on the beam with his ultrasonic flute.”
But if these silent sounds can kill germs and insects, what about their effects on humans? Might they not, after prolonged exposure, cause some gradual breaking down of human tissue or perhaps internal injuries?
All the answers are not yet known, but scientists are seeking them. They are not particularly worried about any ill effects that might come from the industrial or medical uses of silent
sounds. In those applications the sounds can be controlled and the operating personnel can be protected if necessary. What they are concerned about is the possibility that the silent sounds hidden in the blast of a jet engine may have some effect on the ground crews and pilots working with supersonic airplanes.
Many cases of dizziness—aviation medical men have dubbed it “supersonic sickness”—have been reported by workers in laboratories and test pits where jet engines are run for long periods. From aircraft factories in England have come reports of ear troubles and fatigue among jet-engine test engineers. Similar reports have been made in Canada and the United States. Laboratory men working around ultrasonic generators also have complained of odd sensations while the equipment is in operation.
Whether these symptoms of “sound sickness” are real or perhaps imagined no one knows. So far, however, there is no definite evidence to show that there are any lasting had effects that can be traced to the silent sounds from a jet engine or to those that come from a silent-sound generator. Experimenters in Canada, the United Kingdom and the United States are working hard to come up with a definite answer. The RCAF Institute of Aviation Medicine in Toronto is investigating alleged “illness” arising out of ultrasounds in jet engines.
Meanwhile, experts of the National Research Council of Canada and of other similar research groups throughout the world are pushing deeper and deeper into the broad frontiers of sound, audible as well as inaudible. Inventors are busily suggesting all manner of new applications for the “sounds that can’t be heard” ranging from silent-sound “vacuum” cleaners that will literally shake the dust out of carpets and draperies to ultrasonic dental drills that work so fast that they cause little pain. Opposite the inventors are the industrialists who hope that silent sounds will solve many of their production problems. Their requests range from equipment for use in metallurgy to a supersiren that might successfully combat the bacteria that attacks paper stock in Canadian paper mills. One owner of a large duck farm recently contacted a sound engineer in the hope that silent sounds might provide a good way of defeathering ducks for market.
As outlandish as some of these suggestions seem, silent-sound experts are hesitant to laugh them off. Ultrasonic research is a relatively new field in which just about anything is possible. The uses of supersounds are growing day by day. *
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