Sounds We Cannot Hear

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Sounds

We Cannot Hear

B. KUDRYAVTSEV


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POPULAR SCIENTIFIC LIBR.O\RY

Prof. B. K UDRY A '7 T S E V

SOUNDSWE CANNOT HEAR

FOKEIf N LANGUAGES PUBLISmNG HOUSEM O i ~ O W 1958


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TRANSLATED FROM THE RUSSIAN BY D A V IDS 0 B 0 L F V

DESIGNED BY N. G R ISH I N


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CONTENTSPage

Introduction 7C h ap t e r 1. THE WORLD OF SOUNDS 9

First, About Audible Sounds 11Competing Sounds 13Limits of Audibility 17Supersonic Sounds 18Ch a p t e r 2. THE FIRST APPLICATIONSOF

SUPERSONICS -21Many Years Ago 21When France Was in Danger 23Sound and Light 24Wonderful Crystals 27To Nature's Aid 30How to Make a Supersonic Generator 33How to "Hear" Inaudible Sounds 37Why Do Transformers Hum? 39A Reliable Scout 41A Mechanical Watchman 47The Supersonic Echo Sounder 49Supersonic Waves Become" Visible 52

C h ap t e r 3. SUPERSONICS AND LIFECREATURES

The Riddle of Bats5959


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Along the Path Pointed Out by NatureAction Qf Supersonics on Simplest Living

CreaturesWhat Happened to the Fish?

C hap t e r 4. SOVIND AND CHEMISTRYThe ,First stepsElectrical Charges and BubblesGiant MoleculesDuality of the Properties of Inaudible SoundsA Mysterious GlowSupersonics Substitutes Time

C hap t e r 5. ONE OF MAN'S HELPERSSounds that CrushSupersonics Saves Human LivesScience and PracticeWashing Woollens with SupersonicsThe Supersonic Soldering IronPurifying the Air with SupersonicsA New Supersonic SourceSupersonics Helps to Intensify ProductionFluidization

C hap t e r 6. SUPERSONIC C O ~ T R O L

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Sound as InspectorThrough SoundingReflecting DefectoscopeA Mechanical MemoryOvercoming ObstaclesSupersonic DiagnosisSaving Human LivesHow Supersonics Helps to Investigate

Structure of the Earth . . . 4

the

109110112119120123125

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PageSupersonics Controls Chemical Change 130Ho w to Determine the E la st ic P ro pe rt ie s of

Substances 133Supersonic Flow Meter 136A Thickness Gauge 138

C h ap t e r 7. TH E SUPERSONlt MICROSCOPE 141Supersonic Optics 141Design of the Supersonic Microscope 144A New Design 147Practical Application of the Supersonic

"'\icroscope . . . . . . . .


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INTRODIJCTIONThis booklet deals with the achievements of a new, rapidly developing branch of science.Discovered very early in the 20th century, inaudi-ble sounds immediately drew the attention of researchworkers in various branches of science and engineerlng.Several thousand scientific papers have already been published on the properties of inaudible sounds and theirpractical application.Supersonic vibrations were 'first used for practical purposes in France. Especially noteworthy in this connectionare the works of Academician Langevin.Soviet scientists have also played a prominent part indeveloping the science of inaudible sounds and the meth 4ods of their practical application.Watching the development of science is like reading athrilling novel. We can all recall reading a book we couldnot tear ourselves away from. How breathlessly we fol-lowed the fate of the hero, how 'gla,d we were when he wassuccessful, how sorry when fate was cruel to him! Re-member how we yearned to know what awaited him in thefuture, how we tried to guess which of his plans would be

successful and which of them would remain unrealized!It is the same when you watch the development ofscience: you are always trying to catch a glimpse olf itstomorrow.

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In this book we shall tiel! of various discoveries in thefield of inaudible sounds. Some of the applicat ions of in-audible sounds may 'not justify our expectations in the fu-ture; it is quite possible that we shall make mistakes inour explanations of the action of supersonics, and 5 hanhave to begin our work anew. . . Then we shall recall thewords of Karl Marx that \"there'is no royal road to science,and only those who do not dread the fatiguing climb ofits steep paths have a. chance of gaining its luminoussummits."

If any of our readers should become interested enoughin the applications of inaudible sounds to try his hand inthis field, he will find the doors wide open before him intothe boundless and fascinating world of scientific discov-ery.The study of inaudible sounds is at present an unlimit-

ed field of activity f.or investigators of .nature, and it 'openup immense opportunities for the application of the creative p ower of man.


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CHAP T ER 1TIlE 'VORLD OF SOlJNDS

The world we live in is full of sounds. Devoid of sounds,the world would be incomparably poorer. Our conceptionof a forest is inseparably connected with the singing ofthe birds, with the rustling of the trees; that of a meadow-with the chirring of the grasshoppers; of the sea-withthe roar of the waves and the 'Crash of the breakers; of acity-with the characteristic diversity of sounds known as"city noise," a sort of symphony combining the distanthooting of locomotives, the ding-a-ling of trams, scrapsof conversation or music, and the muffled rumble of numerous factories and plants.Very long ago man learned to find pleasant cornbina-

tions of sounds, to make up musical melodies. Musicis by right considered one of the earliest arts. The magiccharm olf musical melodies has given rise to many poetic

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legends. Our ancestors even used to attribute supernaturalproperties to music, They held that music could tame wildbeasts, move mountains from their seats, hold back floodsof water, 'calm down the raglng elements.,Man learned to make mirsical 'instruments very early. OnEgyptian pictographs we find images of musicians playingflutes and harps. The science of sound, or, as we call it today, acoustics, was founded by the ancients, The first ex-periments in acoustics of which any records have comedown to us are those of the Greek philosopher and scientist Pythagoras, who lived twenty-five hun dried years ago.Since that time man has put great efforts into the studyof the nature and properties of sound. And gradually, bythe end of the 19th century scientists came to believethat they knew practically everything there wa.s to knowabo-ut sound. It seemed that all that was left in acousticswas to describe already known phenomena, using improvedinstruments, to refine earlier determined values, butthat nothing new would ever be discovered in this fieldBut that was a mistake.Our knowledge of the surrounding world keeps contin

uously expanding and deepening, "and while yesterday"Lenin teaches us, "the profundity of this knowledge didnot go beyond the atom, and today does not go beyondthe electron and ether, dialectical materialism insists onthe temporary, relative, approximate character of all thesemilestones in the knowledge of nature gained by theprogressing science of man. The electron is as inexhausti-ble as the atom, nature is infinite. . . ."It turned out that the world of sounds also held secrets,which man had not yet guessed to exist, At the v,ery timescientists were about to yield to the idea that everythingin acoustics was already (known, a 'new Iascinating chapter was opened in. science, the door was thrown wide opel

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into a hitherto unknown kingdom of nature, the kingdomof inaudible sounds.This discovery was one of great importance. Having es-tablished the properties and peculiarities of inaudiblesounds, main began to use them successfully as a means ofdelving further into the secflets ()If nature, They became thehelpers of man.

FIRST, ABOUT AUDIBLE SOUNDSIt is impossible to understand the properties of super-sonic sounds without an acquaintance with ordinary au-dible sounds. Therefore, we shall tell the reader very brief-ly what is known about the nature and properties of theordinary sounds that we can hear.Let us fix our attention on the sounds that come to us

the moment w'e wake up. There, for instance, goes a fac-tory whistle.What happened when the whistle blew?The operator opened a valve, and. compressed air rushedforcibly out into the atmosphere and expanded, takingup a much greater volume. Under th.e impact the 'mole-cules of the air also shifted. But molecules cannot travelfar. Moving forward abruptly, they mix with the moleculesin the air layers in front of them and exert pressure onthem. Thus, for a very short 'moment the number of mole-cules in the adjacent layers becomes much greaterthan before. This means that for an instant the pressurein those air layers increases, and the air becomesdenser.The whistle sends forth an intermittent stream ofcompressed air, and these molecular impacts occur manytimes each second.

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At the instants the air stream is interrupted the shifting of th1e molecules leads to their momentary deficiency-in the" layer next to the compressed one. Therefore, nextIo the high pressure layer there arises a rarefied layer,where the pressure is low. As long as the whistle keepsblowing the compressed and rarefied layers keep runningin all directions,When they Iall on the human ear these alternate com

pressions and rarelactions give rise to the sensation ofsound.Thus, what we 'Call sound is a sequence of rapidly alter

nating compressions and rarefactions of the air.The individual particles of air, however, do not move

together with the propagated sound. They only oscillateunder the impacts of the compressed air, moving to and froover very sm.all distances,

A similar motion may be observed when a wave runsover the water and the surface becomes uneven: someparts of it ri.se, forming crests, while others sink, formingtroughs, 'as shown in Fig. 1.

Pig. l. Waves on the- surfaceof water


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A motion like this is called an undulatory or wave motion.If we obs.erve a float thrown on the surface of water,we shall. find that. it only oscillates, bobbing up and down,but does not move along the surface together w-ith the run-ning wav,e.This shows that the water molecules do not move to

gether with the wave, that they only oscillate on each sidleof certain mean positions. The undulations, on the. otherhand, are passed on land on by the molecules, in much thesame way as .the baton is passed from runner to ruoner ina relay race.On the surface of the water each wave crest is followed

by a trough; in air through which sound is propagatedcompressed layers alternate with rarefied ones, and in bothcases the separate -particles of substance oscillate.Due to the similarity in the motion of the adr particlesand water particles, the alternating compressions and rare-factions in the air are called sonic waves.

COMPETING SOUNDS

Sound waves rise and are propagated through the airas a result of the vibration of any body, whether it be astring, a gramophone membrane, the diffuser of a loudspeaker, etc.Air is not the only conductor of sound waves.Just before the Battle of Kulikovo dn 1380 the Russian

Prince Dmitry Donskoi rode out to reconnoitre and,putting his ear to the ground, heard the thud of horses'hoofs: the enemy cavalry was approaching. In thiscase the sound waves were propagated through theground.


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The rate at which sound waves are propagated throughvarious substances is 'not the sam,e.The velocity of sound iriair is comparatively small andequals only 1,088 feet per second under ordinary conditions. If we could shout loudly enough for the soundto travel from Moscow to Leningrad (a distance of373 miles) we should be heard there only half an hourlater. ,11Sound is propagated t.hrough water much more quickly:in one s'econ,d it travels about 4,900 .feet. Thus it would

take sound about seven minutes to get to Leningrad fromMoscow "by water."The rate of propagation of sound through solids is still

greater. For instance, -in a steel rod sound travels about16,400 feet per second and through a steel rail it wouldcover the distance between Moscow and Leningrad inabout two minutes.

lin everyday life we distinguish sounds by tone and intensity.The tone of a sound depends on the frequency with

which the body issuing the sound vibrates, The greater thefrequency, the greater the number of compressions andrarefactions arising each second in the sound wave, and thehigher the tone of the sound,

Frequency is measured In units called cycles per second,A cycle is one complete oscillation or vibration. One thousand cycles make a kilocycle,It is noteworthy that t.he rate of propagation of soundsof different tones is usually the same. Therefore, withsounds of higher frequency the adjacent densified and rare-fied zones are closer to each other than with sounds oflower frequency.

The distance between two adjacent compressed air zones14


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Fig. 2. Distribution of air molecules in two wavesof different frequenciesor between two rarefied zones is called the wavelengthof the sound. The higher the frequency of the sound, theshorter is 'its wavelength (Fig. 2).The human ear is very sensitive to the tone of sound. l\

person gifted with an ear for music can distinguish between two sounds, one of which has a frequency of 1,000,and the other 1,003 vibrations per second.However, two sounds of the same tone may have a dif

ferent effect on our sense of hearing: we say that one ofthem is more intense or lo-uder than the other. With thesame frequency, the intensity of sound depends on theswing or amplitude of the vibrations of the soundingbody.A sounding body vibrating with a wider swing will cause

greater changes in the air, and the sound will be moreintense; hence, the greater the change in pressure, thehigher the intensity of the sound (Fig. 3).

In recent years scientists have engineered sound sourcesgiving tremendous intensities.

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F i ~ . 3. Dependence of the intensity of soundon'.the swing- of the vibrating' body

"If we try to transform the energy of sound into heat, weshall see how little energy is radiated by ordinary sourcesof sound in comparison with modern powerful sound generators. Indeed, in order to heat a glass of water to theboiling point by transforming into heat the energy we ex ...pend in talking, we should have to speak continuously foranything between 75 and 2,000 years, depending on theloudness of our voice. If, however, w,e used the sonic ener-gy radiated by a modern sound source, it would take onlyabout seven minutes,

We usually appraise the intensity of sound by ear, butthe intensity of sound cannot be measured in this way,because the sensitivity of the human ear has certain peculiarities. These peculiarities kept us from knowingabout the existence of supersonic sounds for a long time,which accounts for the fact that in such an old branch ofknowledge as acoustics whole large fields remained unexplored like the "blank spots" on the map.

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LIMITS OF AUDIBILITYThe human ear senses sounds of different frequenciesdifferently. It is especially sensitive to sounds within thefrequency range between 1,000 and 3,000 vibrations persecond. Within this range we can ,even sense sound waves

in which the change of pressure is 1,000 times less thanthe pressure felt by a human hand when a mosquito settles on it. If the ear were just a little more sensitive itwould hear as sound the random increases in the densityof the air caused by the chaotic movement of its molecules.And since s-uch increases occur 'continuously, the worldaround us would in that case be full of an incessantnoise.The sensitivity of the ear is characterized by the leastintensity needed to make a sound heard-this is the threshold of hearing. Naturally, the higher the sensitivity, thelow-er the threshold of hearing.As the frequency of the sound decreases, our ability tohear it also decreases and the threshold of hearing 'correspondingly rises.In order to be heard, a sound of very low tone, with afrequency of 100 vibrations per second, must be more intense than, for instance, a sound with a frequency of 3,000vibrations pier second.Sound waves with very slow vibrations, say, from 16 to20 per second, cannot 'be sensed at all by our ear.Such waves are inaudible infrasonic sou-nds.The insensibility of the human ear to low-frequencyvibrations is very important to man, since it keeps himfrom hearing the beating of his own heart, which otherwise would sound like a'n incessant rumble.


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SUPERSONIC SOUNDSThe human ear does not hear sounds of vJery high Irequency either. Depending 011 his age and individuality,

man ceases to hear sounds with frequencies above 16,00020,000 vibrations per second.These high-frequency sounds, inaudible to the human

ear, are called supersonic or ultrasonic sounds.The physical nature of all sounds, however, is the same,

and as we shall see, the division of sound waves into audible and inaudible ones is conventional and is connectedmerely with the peculiarities of our 'ear.Among the waves olf frequencies corresponding to au

dible sounds our ear is incapable of hearing either veryweak or very powerful sounds,When the intensity of the sound becomes high enough

man stops hearing the sound and senses the elastic vibrations as pressure or pain. This Intensity of sound is calledthe threshold of feeling.Experiment shows that sounds of different frequencies

begin to 'cause pain at different intensit ies; hence, it maybe concluded that the threshold of feeling varies with thefrequency of the sound. In the region ()If maxirnurn sensitivity the ear can stand very powerful sounds without feeling any pain.

If the intensity of the weakest sound discerned by theear is conventionally accepted as unity, the intensity ofthe most powerful sound of the same frequency which doesnot cause pain will have to be expressed as a figure con-sisting of a "1" followed by twelve naughts.This is explained graphically in Fig. 4. The sound fre

quencies are plotted along the horizontal axis, and the intensities-along the vert ical axis. The lower curve is the

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s.10I - o - - --+ -+ - - -+ - -+ - - -+ - -+ - - -+ - - -+ - - -+

-1010 . - . - - - + - + - - + - - ~ . , . . . . . . t : - - 2 I f I J I ~ - - - t20 -.50 100 200 500 1000 1000 5000100002QOOO

---__._- Souna frequencyFig. 4. The audible sound zone

threshold of hearing, and the upper-the threshold of.feeling.The figure shows that the upper and lower curves approach each other both in the high-frequency and low-frequency regions. Thus, a definite zone of frequencies markedoff in the figure corresponds to the waves sensed by thehuman ear as sound. The shaded parts of the figure embracethe waves used in human speech and in music. As wesee, these comprise but a very small pa.rt of the waves th'ehuman ear can sense.

Many of our readers will doubtlessly wonder whetherthere is any l,imit beyond which the frequency of soundvibrations cannot be increased.At the close of last century the remarkable Russian, phys

icist P N. Lebedev, who was the first to make use of supersonics in research work, noticed that the damping ofhigh-frequency sounds limits their propagation throughair. Lebedev estimated that sounds with frequencies ofabout five million vibrations pier second could not practi-2* 19


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cally be p ro pa ga te d t hr ou gh air: they would be dampedright at their source.A lt ho ug h so un d i.s damped incomparably more slowly inliquid and solid bodies, the frequency of sound cannotbe increased indefinitely even in these bodies. Sooneror later we are bound to reach frequencies correspondingto the thermal vibrations of molecules. Those frequenciesare the 'upper boundary of supersonic vibrations. But toreach the upper limit it will be necessary to increase thesupersonic frequencies we have attained so far severalthousandfold,

Some of the remarkable properties ,af supersonic SOUI1,d,such as its ability to accelerate c he mi ca l r ea ct io ns or itscapacity 'for dispersing substances, are due more to .itspower than to any peouliarities of this kind of s ound. Whenscientists succeeded in creating powerful enough audiblesounds, it turned out that the latter have similar effects.Hence, in our times when the practical applications of su-personics are discussed, mention is often made of the pos-sible applications of powerful audible sounds.


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C I IA PTER 2l'HE FIRSrr APPlJCA1'IONS OF SUPERSONICS

MANY YEARS AGOThe first practical application of supersonics dates back

to the times when our knowledge of sound in general wasvery scanty Even the nature of sound was still scarcelyknown to rnan, and he had no idea of supersonics,Observing the world around him, man noticed that dogsreacted to certain sounds which he himself could not hear.This observation led to the first application of supersonics.From olden times, poachers were cruelly prosecut.ed bylaw, They would use a special kind of short whistle, calleda "poacher's whistle." 'This whistle emitted a sound of

such high frequency that man could not hear it, but hisdog could.Concealed in the bushes, the poacher could unperturba-bly call his dog to his side without fear of the warden,21


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though he might be close by. This is due to the factthat dogs have a different zone of audibility than humanbeings.However, poachers pondered no marie over the nature ofsupersonics than primeval man did over the transforma ..tion of energy as he drew fire by striking one stoneagainst another.Superson-ics became an object of study a comparativelyshort time ago.There was an immense jump in the progress of sciencein the end of last and the beginning of this century, Thoseyears saw the complexity of atomic structure established,the spontaneous transmutation of some elements intoothers discovered, various "invisible" rays brought to light,radio invented. All these achievements paved the way forthe penetration into one more hitherto unknown sphere ofnature-s-into the world of supersonics,

Supersonic waves were produced in physical laborato]ries at the enid of last century by means of very small tun:ing forks, no more than several millimetres in length. Thefrequency of the sound produced was up to 90,000 cyclesper second. Special kinds of whistles, "Galton's whistles,"were also used for this purpose (Fig. 5). But inau ..dible sounds did not find practical application at that

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Fig. 5. A modern whistle for producing supersound22


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time, and that was why this branch of science developedso slowly at first.The situation changed abruptly, however, when supersonics began to find practical application.

WHEN FRANCE WAS IN DANGERDuring the world war of 1914-18 the French Navy suf

fered great losses from German submarines. The spaciousocean became a literal trap for ships. For a long time scientists strove in vain to find a way of fighting submarines.

One of the scientists who devoted their efforts andknowledge to the cause of the defence of their country wasthe famous physicist Paul Langevin.In 1916 he suggested inaudible sounds as a means oide-tecting submarines. His idea was very simple: a specialprojector was to send a short supersonic signal in a predetermined direction under the water. If the path wasclear, the signal would go on and on and be lost in theocean, If, however, anything of a density other than thatof water happened to be in its w,ay, the sound would bereflected from It and would return to the projector as anecho, showing that there was a foreign object in the sea.At the same time the distance to the obstruction thus revealed could also bedetermined.

Suppose the reflected signal returned three seconds afterit had been sent. As is known, sound t ravels through waterabout 4,900 feet per second; therefore, in three secondsit will have travelled about 14,700 fleet. Considering thatthe sound has to travel forward first and then return,this figure should be divided by two. Hence, in our example the object detected is 7,350 feet, i.e., somewhat under1 1/2 miles, away.

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Our readers, naturally, may ask: why was supersonicsound needed for this device? Could not ordinary audiblesounds have served th'e same purpose?In 1912, near the coast of North America, the hugeBritish steamer Titanic ran into an iceberg and went rap-idly to the bottom with most of its crew and passengers.News ol the tragic fate of the Titanic spread swiftly allover the world. People eet to thinking how to avoid suchcatastrophes in the future.Could not a sonic echo warn the ship's crew of the im-pending danger?However, all attempts to make an instrument of this kind\V1ere in vain, owing to one of the fundamental propertiescf sound,

SOUND AN"I) LIGHTImagine yourself in a garden on a summer night byan open window,The melodious sounds of a piano 'c-ome pouring out ofthe room and are slowly lost in the stillness of the night.Note how sharply the bright rectangle of the windowdefined on the sand ()If the garden walk.If you wish to read anything by the light falling fromthe window, all you have to do is get into the way of thelight rays; and it is enough to take but a step or two asideto find yourself in complete darkness. Light waves travelin straight Iines.Sound behaves otherwise.Step aside from the window, and that will n-ot keep you

from hearing the music. You may even stand next to thewindow right up 'close to the wall of the house, and stillthe sound waves will reach you. Do not think that thesounds you hear come through the wall. If you close the24


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window you will see that the sounds had been comingthrough the window.Why does the light wave travel in a clearly definedbeam, while the sonic wave spreads in all directions likethe waves formed on the surface of water when a stoneis thrown into it?This difference is due to the difference in wavelengths.Whether the waves will ,be propagated in definite directions like light or .in all directions atonce like sound,depends on the ratio between the dimensions of the sourceof undulations (projector or aperture in an obstruction)and the wavelength,

Ilf the size of the aperture is smaller than the wav,e-length or nearly equal to .it, the wave will be propazatedin all directions at once, as in a, Plate I (p. 35).This is exactly the case with the sounds coming out of theopen window. The highest notes of the piano have a wavelength of about three feet, which is close to the dimensionsof the window through which the sound emerges into thegarden-that is why the sound spreads in all directions.If the source of wave motion is much larger than thewavelength, the emission 'will be directional: the wavewill be propagated in the form of .a ,beam with more orless defined ledges, as in b, Plate I.Light rays have wavelengths of ten-thousandths of amillimetre. Compared with the dimensions of the light wavethose of the window are enormous, and that is preciselywhy the light beam is so sharply defined. The propagationof a wave sent out by a projector .is similar to its propagation from an aperture in an obstruction 'in the path ol thewave. Hence, the pecul iar ities of sonic waves which manifest themselves when the sound comes from the window will be observed also if the window is substituted bya sound projector,


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If desired, sound can be directed as well as light; to dothis it is necessary either to increase the dimensions ofthe sound projector or decrease the wavelength of thesound, Le., increase its frequency. In practice, k get acomparatively non-divergent sound beam supersonic wavesmust be used.It was noticed in the very first experiments with super-

sonic sound that it actually does travel in narrow beams,At present the reason for this is quite clear. Indeed, inwater sound with a frequency of 20,000 cycles per secondhas a wavelength of about three inches; thus, the vibrator,which has a diameter o.f 20 inches, is about 6.6 times aslarge as the wavelength, The radiations of such a vibrator lare directiona 1, like light rays.

In order to make ordinary audible sounds just as di-rectional it would be necessary to engineer a sound sourceabout 30 feet large. It would be practically impossibleto make use Q1f such an .apparatus,That is why the attempts to use audible sounds for the

detection of obstacles in the ship's path failed. With ordinary sound sources the echo will return not only .from ob-jects in front ol the ship, but from objects at its sides andbehind it as well.

Now it is clear to us why Langevin employed supersonics for the detection of submarines, this type of soundbeing 'easy to send in the form of a narrow beam in thedesired direction.

It would seem that the problem of detecting submarineswas solved. But this impression was misleading. Therewere still a great many difficulties to be overcome beforethis essential ly simple idea could be put into practice.Tuning forks and Galton's whistles gave very feeblesounds, which could not be used to detect submarines.And for lack of suitable sources supersonics could not be

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Fig. 6. Quartz anda piezoelectric plate

used to detect icebergs, although such suggestions weremade after the wreck of the Titanic.Practice had set a new problem before science, that ofcreating .a powerful source of supersonic sound.

WONDERFUL CRYSTALSMany of our readers have seen' beautiful specimens of

rock crystal or quartz, as it is called in chemistry (Fig. 6).A small slice cut from such a crystal possesses wonder

ful properties: if it is compressed, unlike electrical chargesappear on the opposite faces of the plate. This phenomenonis called piezoelectric effect, which means the formation oielectricity under the action of pressure.

Electrical charges can also be produced by stretchingthe plate, but they will be opposite to those which appearedupon compression.IJ we keep alternately compressing and stretching theplate, unlike charges will keep appearing at its oppositefaces. The signs of thecharges will keep chang-ing in accordance with thechanges in the shape of theplate.This is not the only won-derful property of the sliceof quartz. It was found thatif unlike electrical chargesare applied to its oppositefaces the plate changesshape each time thecharges are reversed,growing alternately thickerand thinner.

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Now let us place the plate in a gas or liquid. As the plategrows thicker its faces move like pistons in the cylindersof a steam engine and compress the substance in whichthe plate is submerged. On the other hand, when the platecontracts, the zone of substance adj acent to its surIace .is rarefied. Thus, the repeated changes in the shapeof the plate give ri.se tQ alternate compressions and rarefactions in the substance surrounding it. Spreading intospace these compressions and rarefactions form a wave;the plate becomes a source of waves, i.e., a projector(Fig. 7).

The shape of the plate can be changed 'at any frequencyby changing the signs of the electrical charges on its facesat the corresponding rate.

At present it is possible to make the plate vibrate thousands of millions of times per second, nor is this th-e ulti-mate figure.

It must be rem-embered that the change in the dimensions of the quartz plate is very small. If a voltage of, say,about 1,000 volts is applied to the plate its thickness willincrease or decrease by only two ten-millionths of a centimetre, i.e. by a length that will hold only about 10 to 15atoms,

But the swing of the plate's vibrations can be .increased.Perform the following experiment: tie a small weight

to a string and set it swinging like a pendulum. Note themoment the weight passes through its position of equilibrium by the second hand of a watch, then count off 20oscillations and find how much time they took. Now, increase the throw by giving the w-eight a harder push. Youwill find that exactly the same time is required for 20 O'S>cillations of greater swing. In this experiment the weight

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was oscil lating freely and wehave shown that the frequen-cy-free, or as we say, natu-ral oscil la tions of a body areindependent of the swing, or,

~ ~ ~ c h o : \ ~ ~ e o : ~ ~ t ~ t ~ ~ ~ s ~ m p l i - .But what does the frequen-

cy of natural oscillations depend on?The frequency of natural

oscillations of the weight canbe changed by merely short-ening or lengthening thestring it is fastened to. The Fig-. 7. Piezoelectric projectorshorter the ' st ring the greaterthe frequency of oscillations.

Each osc illat ing body has a char.acterrstic frequency ofnatural oscillations. Thus, for instance, if a swing is pushedit begins to move to and fro with quite a definite frequency, The throw of its oscillations 'Can be increased bygiving it a push. To increase the throw considerably, itis necessary, as we know, to push the swing "in time'with its oscillations, i.e., at a frequency equal to that ofits oscillations, if it is leit to swing by itself.

The frequency at which the swing has to be pushed tomake its throw especially large is called the resonancefrequency. All oscillating bodies have a resonance frequency. In those cases where the body is set oscillatingat its resonance frequency the swing of i ts oscillations becernes especially large. There is a case known to historywhere a small detachment of soldiers marching across abridge and keeping good step accidentally fell into res-onance with the vibrations of the bridge. As a result of

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the resonance, the vibrations of the bridge became sogreat that it fell to pieces,If the signs of the electrical 'charges on the faces of thequartz plate are varied at its resonance frequency withoutchanging the voltage applied, the swing of its vibrationswill increase and the intensity of the sound emitted by itwill grow.Each plate has its own, resonance frequency.The thinner the plate the higher its resonance frequency.

For a plate one millimetre thick it is 2.88 million cycles persecond and for a thickness of 0.5 mm.-5.76 million cyclesper second. Plates can be made thinner than cigarette pa-per. The resonance frequency of such a plate is very high, butplates as thin as that are very weak and are not of much use.

And so we see that plates possessing piezoelectric properties are all-important for the production of supersonicsound, For this reason we shall say a few words moreabout the materials they lar1e made of.

TO N:\TURE'S AIDQuartz is one of the most common minerals. Ordinary

sand consists of tiny particles of quartz. This mineral of-ten occurs in cobblestones which are still used in someplaces to pave roads. If sand is heated to a very high tem-perature it fuses into transparent quartz glass which iswidely used in chemical laboratories.

It would seem there was no lack of materials for themanufacture ()If supersonic generators.

Actually, however, this is not so.Quartzglas.s does not possess piezoelectric propertiesand hence cannot be used for the production of super-sonic wave projectors.

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quartz crystals

Piezoelectric properties are peculiar only to crystals ofquartz, but large quartz crystals are found very rarelyand therefore plates with large surface areas are expen ...sive.One of the remarkable achievements of science was thereoenldevelopment of a method of growing large quartzcrystals artificially. It was found that quartz crystals canbe grown just like crystals of common salt, alum andother water-soluble substances.At first sight it may seem strange that beautiful rockcrystals can be grown from such a stable, insoluble sub ..stance as sand or cobblestone. Of course, under ordinaryconditions it is impossible.For this purpose a rod of quartz glass is suspended bymeans of a fibre in a special receptacle filled with anaqueous solution of certain chemical substances, and belowit a tiny quartz crystal is ;-'- - - - - - - - -placed (Fig. 8). The receptacle is closed and the temperature within it is raisedto 350 0 C., causing the pressure in the receptacle to risevery high.

Under these conditions thequartz glass rod dissolves inthe water, and the quartz molecules, having passed into solution, deposit again on thecrystal, increasing its dimen..sions. Part of the dissolvedquartz precipitates out on thewalls of the receptacle, cover- ,ing them with a layer of tiny FIg. 8. Receptacle for growingcrystals. After about eighteen

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Filg. 9. Bariumtitanate projector

hours the rod has dissolvedcompletely. Then the recepta-cle is opened and another rodis hung in. If this operationis repeated four or five timesa crystal several centimetreslarge can be grown; suchcrystals are large enough tobe made into piezoelectric pro-j ectors.

Further improvement ol this 'method will make it possibleto manufacture quartz plates of still greater dimensions,Supersonic generators can be made also of 'crystals of Ro-chelle salt, ammonium dihydrophosphate and several othersubstances. \ I

Besides, Soviet physicists have recently evolved! newsubstances, called titanates, which di.splay an immense piezoelectric effect. Especially prospective for the generationof supersonic sound are ceramlc wares of barium titanate.This compound does not naturally possess piezoelectricproperties, but the latter can be induced -in it, just as mag-netism 'Can be induced in steel, which is 'not naturally mag-netic, to make it an artificial magnet.

Since the piezoelectric properties of barium titanate arebuilt up artif icially, we can prepare projectors of any shapeand make them vibrate in any way desired.For instance, the barium titanate can be Iashioned into a

tube which will emit supersonic waves towards its inside.Iv1aking the walls of this tube vibrate, we cain subject anyliquid we pass through it to the action of powerful super-sonic waves.Fig. 9 shows a barium titanate projector in the shape ofa cylinder. The conical horn at one end of the cylinder concentrates the supersonic vibrations, making them very in-

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F i ~ . 10. Lang-evin's sonicvibrator

tensive. The displacement of the point of this vibrator inoperation is about 0.005 millimetre.At the time Langevin designed his apparatus the artificial production of quartz crystals was unknown. So werethe wonderful properties of barium titanate. Scientists andengineers had to content themselves with the small quartzcrystals found in nature.

In the attempt to obtain a sharply defined supersonic rayLangevin cemented a whole mosaic of small slices of quartzon a steel plate and covered them with a second steel plateto form the transducer shown in Fig. 10. Now all that was leftwas to apply unlike electrical charges to the plates and makethe signs of these charges keep continuously alternating.

HOW TO l\'1AKE A SUPERSONIC GENERATOR

In ourdays the signs of the charges on the surfaces ofthe quartz plates are kept automatically alternating byconnecting the plate to anelectronic oscillator of thesame kind as is used in radiotransmission.

A diagram of a simple piezoelectric generator is shownin Fig. 11.

If it is desired to producesupersonic waves of frequencies from 500 thousand to onemillion cycles per second the lroil AC must be about 3 inch- ~ .es in diameter and the copper .wire wound on it should be ~ " , : w . : ~ ; : ; ; ; : J ~ ~. i IoI l Ir";: :-about 80 to 120 rnrn. (5/64 to1/8 inch) in diameter. Three3-132 33


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Fig-. 11. Diag-ram of piezoelectric generator:I-tube; 2-resistance; 3 and 4-condensers; 5-battery; 6-quartz plate

turns are wound between points A and B, and six turns between Band C. The distance between turns is about 5/32"The quartz plate 6 is placed on a metallic base connected toterminal K ~ ; the plate is covered with a piece of thin aluminium foil which is held down tight against it by means of alight spring. The latter is connected 'to terminal K2 Caremust be taken to keep the spring from touching the base.

The high voltage applied to the faces of the plate sometimes causes electrical discharges in the form of sparksrunning around the edges of the plate. In order to avoidsparking the plate is usually immersed in a liquid of goodinsulating properties, such as transformer oil.

When the quartz plate vibrates powerfully, the free S,Ufface of the oil is raised into a mound, as shown in c, Plate I.

If it is desired to produce especially powerful supersonicwaves the quartz plate is shaped into a concave mirror.The concave projector converges the sound energy into apoint, that is, concentrates it, with the result that the magnitude of power produced in a small space is difficult evento imagine.

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Plate Ia-passage of a wave through an aperture smaller than the wavelength;b-passage of a wave through an aperture larger than the wavelength;c-oil mound forming above a flat vibrating plate; d-oil fountain form-ing above a concave vibra ting pla te


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If the intensity of a locomotive whistle is taken as unityfor the sake of comparison, the intensity of the sound con-centrated in the focus of the concave quartz plate will be afigure consisting of a "1" followed by nine naughts, Le., athousand million times as great as that of the locomotivewhistle.

The pressure variations at this point amount to 120 atm.But the 'construction of such projectors is very complicatedand they are very expensive,(In Langevin's apparatus a l it tle motor running at a con-

stant speed applied a high voltage to the vibrator at definitetime intervals, thus making it send short supersonic signalsinto the ocean.

It now remained only to learn to detect th,e weak super-sonic echoes returning after reflection from some obstacle.

HOW TO "HEAR" INAUDIBLE SOUNDSTo detect inaudible sounds P. ,N. Lebedev made use of

their property of exerting pressure on objects placed in theirpath. This pressure is very small and is measured with aspecial sensi tive instrument, the supersonic radiometer(Fig. 12).The design of the supersonic radiometer is as follows: ahorizontal rod with a light mica fan on one end is soldered

to a very thin wire stretched vertically, The incident super-sonic wave exerts pressure on the mica, deflecting it andtwisting the wire slightly. The stronger the sound, thegreater the pressure, and the more the mica will be de-flected.

To determine the deflection of the mica a small mirroris attached to the wire at the point the horizontal rod issoldered to it. If a ray of light is reflected by this mirror,the least deflection of the mica will cause a perceptible

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Fig. 13.Piezoelectricsupersonicreceiver

Fig. 12. Radiometer

movement of the reflection.Supersonic sounds can be detected by watching for themovements of this reflectionand their intensity can bemeasured by measuring itsdeviation.To protect the radiometer

from the influence of variouscurrents which always existin the air it is placed in a special chamber. The supersonicvibrations pass through awindow covered with verythin tissue paper.The radiometer is still usedin laboratory investigations.

But for practical purposes-s-Ior the detection of submarines-the radiometer was of nouse, and Langevin employed for this purpose the same transducerwhich sent the supersonic beam out on itsreconnoitering mission.Having sent the signal the transducer

automatically switched over to receptionand "listened in" for the returning echo.Falling on the quartz vibrator the supersonic wave gave rise to electrical chargesin it which could be detected after ampli-fication. Piezoelectric receivers of this kindare very sensitive (Fig. 13).

As a result of strenuous efforts an apparatus for the detection of submarines, theso-called supersonic hydrophone, was finally created.

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Modern supersonic hydrophones are very different fromthe apparatus designed by Langevin. The sources of supersonic waves in modern hydrophones are usually magnetostrictive instead of piezoelectric oscillators. What are magnetostrictive oscillators?

WHY DO TRANSFORMERS HUM?Transformers are widely used electrical apparatuses. The

simplest transformer consists of two coils of insulated wireon a common iron core.

When an alternating current of comparatively high intensity is flowing through the coils of a transformer, a soundof low tone can often be heard coming from the core.

The humming of the trans-former is due to the fact thatcertain metals and alloyschange their dimensionswhen magnetized.

This property, called magnetostriction, is especiallystrong in iron, nickel andtheir alloys. It is exception- ~ally high in an alloy 'consist- ~ing of 49 'per cent iron, 49 per ~ ~ ~cent cobalt and two per cent ~vanadium,If we place a rod of a magnetostrictive material in a coilwound of insulated wire(Fig. 14) and energize it withalternating current, the intensity of which keeps rising Fig. 14. Magnetostriction rod

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and falling, the rod will be alternately magnetized anddemagnetized, as a result of which its dimensions willkeep changing periodically.Just as with the quartz plate the changing of the dimen

sions of the rod ,gives rise to alternate compressions andrarefactions in the surrounding air, Le., to a sonic wave.If the frequency of the alternating current is not very high,the sound will be audible, as is the case with the bummingtransformers. If we increase th,e frequency of the alternating current supersonic sounds will result. That is how theyare produced in magnetostrictive vibrators.

To increase the 'amplitude ol the vibratlons, the rod ismade to change i ts dimensions at Its resonance frequency,just as in the case of the quartz plate.

The resonance vibrationfrequency of the rod dependson i ts length. The shorter therod, the higher its resonancefrequency.

If we pass the end of therod through the stopper in thebottom of a vessel filled witha liquid we can produce ahigh-frequency supersonicwave -in the latter (Fig. 15).When the rnagnetostric tivevibrations are very intense therod gets hot so quickly that ithas to be specially cooled. Apowerful magnetostrictive vibrator is shown in Fig. 16. It isabout one foot seven inches indiameter.Fig. 15. Magnetostriction rod Magnetostrictive and pi-in a vessel of liquid40


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ezoelectric vibrators supplement one another. Both aresources of sound of frequencies which in the great majority of cases coincide with theresonance frequency of theprojector, with that of a metalrod or of a quartz plate respectively.

It is difficult to make a mag- Fig-. 16. A powerful magneto-netostrictive vibrator with strictive vibratora very short rod. And sincethe resonance frequency of long rods is comparatively small,magnetostrictive oscillators are used to produce supersonicsounds of low frequencies approaching those of audiblesounds,

On the contrary, piezoelectric vibrators may be used toobtain high-frequency supersonic waves, Besides, piezoelectric and magnetostrlct ive vibrators differ greatly indesign.That is why both kinds of vibrators are used. In some

cases the magnetostrlctive vibrator is more suitable, inothers-the piezoelectric.

A RELIABLE SCOUT

The hydrophone not only detects submarines, shallowsor icebergs, but determines their exact location as well.

The sound used in hydro-phonics has an average frequency of between 15,000 and 30,000 vibrations per second.The duration of a single signal is about 0.1 second.

The moment of emission of the- signal is recorded on thescreen of a special apparatus called the oscillograph

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Fig'. 17. Oscillograph

(Fig. 17) by the appear-ance of a sharp toothlike indentation in theluminescent ray.

As soon as the signalhas been sent a speciairelay (Fig. 18) connectsthe transducer to areceiving device and thehydrophone waits sev-eral seconds for the reflected signal to return.

If an echo returns, aspecial apparatus firstamplifies it, then trans-forms the inaudible supersonic signals into. ordinary sounds whichcan be heard through aloudspeaker.

At the same time the signals received are transmittedalso to the oscillograph and a second tooth-like indenta-tion appears in the ray on the screen,The longer the time elapsed between the moments of

transmission .and Deception of the signal, the farther awayfrom each other will be the indentations in the r a y o n thescreen of the oscillograph. If the screen is furnished witha transparent scale, the distance to the obstruction whichreflected the signal can be determined directly by Justglancing at the apparatus.The transducer is usually enclosed in a special casing

and mounted on the ship's bottom. Rotating, the trans-ducer, so to say, "scans the horizon" (Fig. 19).

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Cenprator(in tAp !olVt'r IIydroacou.rlic comparlm!I1tJr-------------------,,I ~ I: Cttnpra!or II oj e/ecrrtca! II vitrallons I

.' ( I 5 ~ k c . ) I: I. J!-fof/nl'/oslr/cliVI? !rgf?Sdl;C(','oin s/l'eom!iIlP cosmobelow

ShlP:S lee!, serves alsoasarecetrer oj reflected slflnals

li'eceiYlIr and control unit(on bridfJ")r - - - - - - - - l . ? i i d ~ P ~

,-....--.....--_/:III,

InC/leo/tnt; I oscl l/oqroplJ IIl1 ~ ~ m l. - ~ C U : ~ : : / : V : : : S = / : / ~ - - _.Jto /rasssnsston or receatro

Fig'. 18. Schematic diagram of a hydrophoneThe distance to the object detected by the hydrophone

can be determined by observing the reflected signal bymeans of the oscillograph,When, however, the ship is in motion, and the distancebetween the observer and the detected object is continuously changing, it is sometimes impossible to determinethe cause of the echo by its shape. The cause may be asubmarine or a reef, or even a large fish.

In coming to a decision on this' question it is often helpful to listen to the reflected signal made audible.

An! experienced operator lean draw many valuable 'conclusions from the sound ol the reflected signal. For instance, by the tone of the sound he can tell whether theobstacle which reflected the signal is in 'motion or sta-tionary, whether it is approaching or receding.All of our readers have probably noticed that the tone

of a locomotive whistle is higher when the locomotive isapproaching than when it is moving away from us.The explanation of this is very simple. SUPPos'e that at

the moment the engineer pulled the whistle the locomotive43


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was 1,088 feet away from the observer. As we alreadyknow, the sound of the whistle is an alternation of compressions and rarefactions. It is they that cause the sensation of sound when they fallon the ear.

The tone of the sound depends on the number of compressions or rarefactions of the air per second.

Suppose the whistle causes 200 compressions per sec ..ond, IIf the locomotive I)) and the observer are stationary,the compressions will follow one after another every 200thof a second and will give the observer a sound sensation ofa certain tone.

But it is different if the locomotive is approaching theobserver. The first compression will take one second toreach the observer, but the next one will reach him sooner,since the locomotive has come a little closer by the timeit is remitted. The same will be true ,of all the subsequentcompressions, as a result of which more than 200 compressions will reach the observer every second, Le., thetone of the sound will be higher,

If the locomotive is receding, the second compressionwill have to cover a longer path than the first to reach theobserver, and the interval of time between them will increase. The ear of the observer will receive less than 200compressions per second, and the sound will consequentlybe -of a lower tone,

The faster the locomotive is moving, the more perceptible the change in tone at the moment the appro-achingsound source comes past us and begins to recede.

It is by this change in the tone of the echo that the operator can determine the movement of the object which reflected the signal sent. By observing how the reflectedsignal first grows louder and then fades away an experiencedoperator 'can get an idea of the nature of the obstacle de ..tected in, the sea,

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Fig. 19. How the hydrophone is mounted on the ship


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The range of the hydrophone varies from severa1 hun-dred yards to several miles, depending on the conditionsexistent in the water at the moment of observation, Oneof the 'main factors in this connection is the difference intemperature of different water layers, which tends to dis-tort the path of the sound nay. Under such conditions thesupersonic signal does 'not travel in a straight line, butdescribes a curve bending in the direction of the cooler lay-ers. Another hindrance is the tiny air bubbles liberatedby the innumerous microscopic organisms living in seawater. Water layers saturated with air bubbles are veryeffective sound absorbers, and in some cases 'may evenreflect the s ignal.

Apparatuses similar to hydrophones may be used forunderwater communication, say, between two submergedsubmarines.

Supersonics may be employed also to transmit signalsthrough the air. True, in this case the range of transmis-sion is greatly limited due to rapid damping of the super-sonic waves.

A MECHANICA.L WATCHMAN

There is an Arabian story .about !a magic door whichopened only in answer to the words: "Open Sesame!" Su-personics enables us to do even more wonderful things.For instance, we can make garage doors open by them-selves only when the owner's Icar approaches,For this purpose the car is equipped with a supersonictransducer which emits an inaudible signal as the car ap-preaches the garage. This signal is received by a special

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apparatus which turns on a mechanism to open the door.On the approach of any other car not equipped with a supersonic signaller, the doors will remain 'closed.

If necessary supersonic apparatuses can be used to keepclose watch over any room and they have definite advantages in this respect over any other apparatuses designedfor the same purpose, Any movement that may arise inthe room protected by supersonics will immediately be detected. For this purpose the room is filled with supersonic waves travelling in all directions, which are repeatedlyreflected from the various objects in the room. If there isno movement in the room all the echo-signals formed inthis way will be of the same frequency. But this conditionchanges if a moving object appears in th'e 'room. The echoproduced by reflection f:rom a moving object will differ infrequency from all the other echo-signals, A special highlysensitive echo-signal receiver set up in the same room willreact instantly to the appearance of oscillations of a different frequency by switching on an alarm designating thatthere is some movement in rthe room. An article in an American magazine tells of a large jewellery shop which wasrobbed, though equipped with a conventional electric burglar alarm. Instead of using the door or the window wherethe alarms were installed, the thieves had brokenthrough the brick wall of the shop. After that the shopowner had .a supersonic (burglar alarm installed and a fewmonths later supersonics helped to catch the thieves whohad chosen the ceiling as the entranceway this time. Thecriminals protested against their arrest on the groundsthat they had been caught "unfairly," having taken allmeasures against conventional alarms, which had not operated as a matter of fact, but never having heard of supersonics,

Now supersonic alarms make it possible to protect large48


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rooms up to about 10,000 cu. ft. in volume. Supersonicalarms can be employed at some plants to keep men outof danger zones or forbidden zones.

The above supersonic alarms trigger off automaticallyin case of fire. The rising column of hot air coming fromthe flame is an excellent reflector of supersonic waves andgives echo-signals whose frequency differs from that ofthe basic signals, Several cases of- fires being averted atAmerican enterprises equipped with supersonic alarmsystems have already been registered.

What w,e have mentioned here does not by any meansexhaust the possibilities which came up before mankindwhen the properties of supersonic sound became known.t h e most daring dreams become realities as a result ofthe use of inaudible sounds and their peculiarities.

THE SUPERSONIC ECHO SOUNDERIn our days automatic 'Supersonic echo sounder is used

not only to measure the depth of the ocean and investigate the relief of its bottom, but to -detect various objects there as well.

This device is similar to the hydrophone.A supersonic magnetostrictive vibrator 4 (Fig. 20) ,mounted on the ship's body, emits short signals at definite

time intervals, usually once a second; the signal is recorded automatically on special tape. All the operationsin the echo sounder are automatic. When the supersonic echo returns after reflection from the bottom, it isreceived by the magnetostrictlve receiver 3, passes throughan amplifier 2 and is registered on the tape. Thus, two linesappear on the moving tape: one of them, 0, correspondsto the emission of the signals, Le., to the ship's bottom,and the other, D, to the return of the echo, Le., to the sea4-132 49


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Fig. 20. Diagram showing howth e echo sounder works

bottom. The greater thedistance between thesetwo lines, the greaterthe depth of the oceanat the point measured.If asp eciaI sea le isap-plied to the tape thedepth of the ocean canbe measured in feet.Such depth records arecalled bathograrns.

Modern sounding ap-paratus are designed sothat a neon light flasheson at a point on a spe-cial scale correspondingto the depth of the oceanunder the ship. Bywatching this scale thenavigator can alwaysknow the depth of theocean at the point the

ship is passing. The echo sounder not only gives warn-ing of reefs and shallows, but makes it possible to deter-mine the bearings of the ship as well.Very detailed mapsof the ocean floor have been compiled. By using such a mapand a bathogram, the ship's bearings can be determinedeven when it is impossible for some reason or other to doso by other m ea ns .

An echo sounder helped to find one of the deepest spotsin the ocean, a depth in the Pacific Ocean amounting to35,630 feet.The advantage of supersonic echo sounder is that it

can be used in almost any weather w it ho ut s lo wi ng down50


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the ship's headway and for measuring both very great andvery small depths.The accuracy with which the echo sounder deter-mines the relief of the ocean's bottom is so high that itrnay be used to find sunken ships. Fig. 21 shows the sil-houette of the Lusitania at a depth of 100 metres, recordedby means of an echo sounder.

The use of this device in fishing. is of great economicimportance.

The swimming bladders of fish are filled with air andtherefore reflect supersonic signals very well, makingpossible to detect schools of fish by means of a soundingmachine. Fig. 22 shows the tape of an echo sounderwith the record of a detected school of herring on it. Theupper boundary 1 corresponds to the surface of the sea.The lower zigzag line 2 corresponds to the sea bottom,The line 3 recorded by the sounding device between thesurface and the bottom of the sea was caused by reflection

Fig. 21. Silhouette of the Lusitania4 51


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of the supersonic signals from the school of herring. Rec-ords of this kind make it possible to estimate not onlythe location of the school but its size as well.

rrhe uSle of supersonics in searching for schools of fishgreatly increases catches and shortens the duration offishing expeditions.

In the 'near future echo sounders will undoubtedly findeven wider use in fishing.

In hydrophones and sounding 'devices supersonic soundsare detected by their action on a special receiver.

At present several methods have been developed to ren-d er s up er so ni c sounds visible as well, offering an interest-ing opportunity of watchin g t he course of the supersonicbeam.

SUPERSONIC WAVES BECOME VISIBLEOn hot summer days streams of air heated by the sur-

face of the ground can be seen rising above the road. Thesestreams become visible due to the air expanding whenheated, decreasing its density and changing its opticalproperties, viz., lowering its coefficient ot refraction, Fora s im il ar r ea so n streams of compressed air, the density ofwhich is greater than that of the surrounding air, wouldalso be visible,

The same phenomenon can be observed in liquids. Poursome warm water into a glass, put a book behind it andcarefully add some cold, a nd , c on se qu en tl y, denser water.Streams with different optical p ro pe rt ie s i mm ed ia te ly ap-pear and the letters 'On the page observed t hr oug h t he glassof water will seem to fluctuate an,d become diffused.If the glass is illuminated by a candle so that its shad-ow falls on a clean sheet of paper, these streams will beclearly visible on the shadow.

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Fig. 22. A school of herring registeredby a sounding apparatus


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When a sound wave is propagated alternate compressions and rarefactions, Le., changes analogical to thosementioned in the above experiment, occur in the air. Hence,a shadow image of the sound wave can be obtained inthe same way as the image of the water s treams with different temperatures. It should be remembered, however,that in a passing sonic wave the -compressions and rarefactions alternate very rapidly. If w,e wish to get an imageof the wave we must illuminate it for a very short periodof time, before the pressure distribution has had a chanceto change appreciably. In practice, to obtain an image ofsound waves intermittent light is used which flashes onand off at the same frequency as the quartz plate vibrates.The light flashes coincide with the same position of thevibrating plate each time, making the image of the sonicwave on the screen "rigid" andclear-cut.

If the screen is replaced by aphotographic plate the wave canbe photographed.

All this made it possible forthe Soviet scientists S. N. Rzhevkin and S. I. Krechrner to employ supersonics in a modelstudy of the acoustic propertiesof various structures: concerthalls, lecture rooms, etc.

Fig. 23 shows the propagationof a wave with a column in itspath. The "acoustic shadow," adark zone behind the column,can readil y be seen. In the Fig. 23. Acoustic shadowacoustic shadow zone the sound of a 'Column

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will be weaker. This method can be used to solve variouskinds of problems in architectural acoustics.When studying the acoustic properties of a projectedconcert hall or theatre on a small model, ordinary sonicwaves cannot be employed. The behaviour of the w;ave as

goes through an aperture in an obstruction or passesan obstacle in its path, as w,e know now, depends on theratio between its wavelength and the dimensions of theaperture or obstacle: For this reason, in modelling it isnecessary to reduce th e wa ve le ng th of the sound in pro-portion to the decrease in the dimensions of the structure.If we use supersonics, Le., very short wavelengths, themodels can also be made of small dimensions.

But how to obtain an intermittent illumination of a highenough frequency to correspond to the supersonic waves?

If the brightness of the light does not have to changetoo rapidly, .an ordinary electric lamp can be employed,by varying the voltage of the current feeding it. If theb ri gh tn es s m us t vary' very rapidly, this method is useless,since the hot filament of the lamp will not have timeenough to cool down and the brightness of the light willr em ai n c on st an t.

The voltage of the electric current usually employed inlighting falls to zero 100 times a second, without anychanges in brightness being noticed. Even this relativelylon:g interval of time is 'not enough for the filament of thelamp to cool down,

The necessity of rapidly varying or modulating, as it iscalled, the in tens ity of light arises often in practice: insound recording, in television, in investigating the oper-ation of r ap id ly m ov in g machine parts, etc.This problem can also be solved by applying supersonics.To vary t he b ri gh tn es s of light rapidly use clan be made

of t he c ha ng es cau sed in the optical properties of substance56


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lJiverled rars

Fig. 24. Diagram of supersonic modulation of lightby the passage of sound through it. Fig. 24 shows oneof the possible arrangements for the supersonic modulation of light. The lIens 0 1 gathers the light rays divergingfrom the light source L into a beam of parallel rays,which, after passing through the glass bath K, is converged by the lens O2 to the focus F. The further progressof the rays is stopped by the screen S. The bath K contains a transparent liquid with a piezoelectric plate submerged in it. If the 'plate is set vibrating and emitting supersonic waves into the liquid, the latter will become optically non-homogeneous, This will change the paths of thelight rays. A certain par t of the rays will fail to convergeat F and will therefore not be stopped by the screen.The higher the intensity of the sound, the more rays willmiss the screen. In its turn, the intensity of the sound depends on the voltage applied to the piezoelectric projector.If the voltage is varied the intensity of the supersonic vibrations will vary with it, and thus modulate the brightnessof the illumination beyond the screen,Not long ago supersonic Hight modulation was used insignalling devic-es for sen-ding secret messages, Thechanges in light intensity caused by supersonic vibrationswere transmitted to an observer armed with a telescope,In the telescope the light rays fell on a photoelectric cellwhich converted them into electric current. The greater the

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intensity of the light, the stronger the current. The messagewas deciphered by the changes in current. In the day ..time such signals could be transmitted almost two milesand at night, about three.With the help of supersonics very powerful sources ofmodulated light 'Can be obtained with intensities varyingat almost a-ny frequency.If such a ray is run oVler a screen, scanning it line byline, it may be employed to obtain a television image,The visible image of the light wave makes it possibleto draw conclusions as to the absorption olf sound by va-rious materials on the bas-is of the intensity of waves refleeted from and transmitted through obstacles,Experience has shown that the study of various undula-tory processes by the use of models permits the reproduc-tion of a detailed picture of the phenomena observed therein.If we photograph the picture observed we can clearlytrace en the snapshot the wave corning from the source,its meeting with the obstacle, the appearance of the r'efleeted wave, the interaction of the latter with the incidentwave, etc. All this is difficult, and sometimes altogetherimpossible, to observe without the help of inaudible sounds.The peculiarities of supersonic sound mentioned above areof great importance at school. In this way the pupils canbe shown the laws of propagation of sonic and supersonicwaves, rendering instruction more graphical and convincing.

- ---" --- ~ - .',. ==-----~ ~ ~ , : : : : . . .i


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CHAPTER 3SUPERSONICS AND LIVE CREATURES

TIlE RIDDLE OF BATSWithout suspecting it we are constantly encountering

inaudible sounds in everyday life.Having built sensitive supersonic receivers, scientistsfound that even familiar sounds, such as, for instance,

the ringing of a telephone, the t icking of a clock, the roarof an aeroplane, contain inaudible sounds along with theusual audible ones,Placing special apparatuses in the woods, investigators

found that the apparently sleeping forest, submerged inthe stillness of night, was actually full of the squeaks andcries of its numerous inhabitants.Some domestic animals 'can readily sense or "hear" su-

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Cats ,cain hear supersonic sounds of not very high tone.In various languages cats are called by uttering soundssimilar to th e E nglis h "puss, puss." It has been found thatthis combination of sounds contains supersonic sounds aswell as the vibrations sensed by the ear.

Some species of birds a!f,e sensitive to supersonic sounds.There has even been a. suggestion to use supersonic ap-paratuses to frighten away the gulls contaminating freshwater reservoirs.The part played by supersonics in the life of bats has beenstudied in especially great detail, Bats have very poor eye-sight, but this does not keep them :from orientating them-selves excellently and never missing when they attack thesmall insects they feed on.

It might have been assumed that in hunting for food batsare guided by their excellent sense of hearing and not byeyesight; but then it remained quite a riddle how bat's man ..aged to detect in the dark even such small obstacles intheir path as thin tree twigs or telegraph wires.

About two h un dr ed y ea rs ago the Italian scientist Spal-lanzani first s tudi ed t he se peculiarities of bats in detail. Inthe attempt to find out which of the bat's sense organshelps it to orientate itself in flying, he alternately deprivedit of the senses of seeing, taste, smell and touch. He'found that a blind bat could fly just as well as one witheyes, Depriving the bat of the senses of smell, taste andtouch did not c ha ng e a ny th ing either. It remained to .as-sume that the bat orientated itself by ear. Indeed, as soonas its ears were plugged it began to dash about helpless ..ly from side to side, bumping into various objects.

These experiments showed clearly that bats found theirway in flight by ear. But it is impossible to orientate one-self correctly with the help of audible sounds! The riddleof bats has been solved only in our days!

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When it was found that inaudible sounds travel in nar-row beams, making it possible to detect objects in theirpaths, scientists began to wonder whether it was not supersonics that substituted eyesight in bats.By 'using special piezoelectric receivers it was estab-lished that during its flight the bat emits short 'Supersonicsignals at definite time intervals. These signals have evenbeen recorded on a film. Clever experiments have led scientists to the firm belief that the bat hears the sounds it'emits.

The ratio between the wavelength and the dimensionsof the bat's mouth when open, the mouth being the projector of the sound, makes the signal sent by the bat directional, like the signals of a hydrophone.

When the bat is at rest, it emits 5 to 10 signals per secand. In flight, however, it cries out more often, emittingan average of 30 signals per second.After sending its supersonic signal the bat listens care-fully to catch theecho of the signal with its immense ears.As soon as the echo comes back it sends the next signal.The closer the obstacle, the more often the bat cries out. Ata distance of sixty-five feet it emits about eight signals persecond, increasing this number to sixty as it comes withinthree feet of the obstacle,Careful observation of the behaviour of bats has broughtscientists to the conviction that these animals also orientat,e themselves by the way the signals become more intense as they come closer to the obstacle.lIt has recently been found that some species O ~ bats

make use of both methods of orientation simultaneously.Supersonic sound damps very quickly in air, and thisgreatly limits the bats' 'capacity for orientation, narrow-

ing their "horizon." They are probably incapable of detecting objects more t h ' a ~ ~ sixty-five feet away,

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By reproducing the signals usually sent by bats withspecial projectors, investigators have found that mothsand other insects on which bats feed also sense, or "hear,"these sounds. When, for instance, a supersonic beam wasdirected at some moths, their behaviour changed abruptly:a moth which had been flying along tranquilly, darted aside,as if fleeing, another unexpectedly folded its wings andfell to the ground, "playing dead."

Doubtlessly this sensitivity to supersonic sound is a pro-tective adaptation of these insects.Various studies carried out with a view to elucidating the

role of supersonics in live nature suggested a wonderfulinvention to man, in which he attempted to make use ofwhat he had found in nature.

ALONG THE PATH POINTED OUT BY NATUREOne of the worst misfortunes that 'can befall a man is

loss of eyesight. In numerous legends and tales man hasexpressed his dream of learning to conquer blindness. InAshik-Kerib, a story by Lerrnontov, the powerful magiciangives Ashik a lump of earth and says: "If they fail to be-lieve the truth of thy words, tell them to bring thee a blindwoman who hath been so for seven years; smear her eyeswith this and she will begin to s'ee again." This was tohave been a miracle which would prove the omnipotenceof the magician and the truth of Ashik's words.Modern medicine has made great progress in combat-

ing blindness. In our days returned eyesight is by nomeans a rarity. Soviet physicians under V. P. Fi-latov have carried out many thousands of such opera-tions. But sometimes physicians are powerless in thisrespect.

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The fact that the bat can find its way through space verywell with the aid of supersonics led scientists to theidea of making it possible for a blind man also to detectobstacles in his way when walking along the streets of acity without having- to resort to the aid of his fellow creatures.

In one of the apparatuses designed to enable a personto find his way with the help of supersonics the projectorsends about ten short signals, inaudible to the human ear,each second, A few instants after sending the signal theapparatus automatically switches over to reception andlistens in for the return echo. A special device convertsthis echo into .audible sound, which oan be heard by theblind man.

By the intensity of the echo the main can determine thedistance to the object it was reflected from: the greater thedistance, the weaker the echo.In another apparatus the time interval between the emission of the signal and the switching on of the receiver canbe changed as desired by means of a special regulator. Ifthe interval is increased the echo will arrive before the receiver is turned on and will not be heard. Gradually chang-ing- the moment oi switching on the receiver, thelatter canbe made to switch on, in imitation of the bat, exactly atthe moment the echo arrives. In this case the distance tothe obstacle the signal was reflected from can be estimatedby the position of the regulator knob: the later the echoreturns, the farther off is the 0 bstacle.

Experimental specimens of these apparatuses permittedtheir operators to distinguish objects at a distance of several yards. It should be noted that supersonic appara-tuses have a very "sharp eyesight": they can even discerna string stretched D'Ut about a foot away.These experiments are but a first attempt to approach

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a still distant goal. But we can say for sure that the daring tho-ught, persistence and clearness of purpose of scientists will overcome all difficulties, and that in the end 'suchan apparatus will be created.

Of course, vain hopes should not be engendered in thisconnection. Even when such lain apparatus is ereated, itwill not be of much help in finding one's way along acrowded city street with its endless stream of hurryingpedestrians, and with i ts automobiles, trolleybuses andtrams shooting by.

The apparatus would register so many echo-signals eachmoment that it would be practically impossible to understand them, However, at horne or in rural districtsvwhercthe traffic is not so great, the supersonic ranger can be ofgreat help to a man deprived of the sense of seeing.

ACTION OF SUPERSONICSON SIMPLEST LIVING CREATURES

The very first experiments with powerful supersonic radiations showed that th'e simplest living creatures arequickly destroyed by supersonic rays. .The Soviet scientists G. B. Dolivo-Dobrovolsky andS. I. Kuznetsov discovered that the unicellular organisms

and infusoria which inhabit almost all bodies of waterperish very quickly under the action of supersonicwaves.

A microscopic investigation of water treated with supersonic waves did not reveal a single live infusorian.

If a special apparatus taking 1,200 photographs per second is connected to the microscope, all the stages of destruction of the microorganisms by supersonics can bephotographed.

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Experiment showed that the time required to rupture 3single cell is less than 1/1,200 of a second: in one photo ..graph the cell could be seen yet uninjured; in the next itwas already completely demolished.The reason for the death of the simplest organisms underthe influence of supersonics has not been established exact ..ly as yet, but certain assumptions may be made.We know that supersonic waves consist of alternatecompressions and rarefactions. Wh,ena powerful supersonic wave is propagated through water, the rarefactionsmay be so great that the water cannot withstand thetensions which arise and is torn asunder. In the placeswhere the ruptures occur tiny bubbles appear, filledwith the vapours of the liquid and the gases dissolvedin it.The occurrence of such microscopic ruptures is calledcavitation. 'The greater the power of the sound, the higherthe intensity of the cavitation incurred. After making itsappearance the bubble lasts for a very short time landthen collapses and disappears,The collapse of the cavitation bubbles gives rise to immense pressures amounting to thousands of atmospheres,which undoubtedly plays a great part in the biological

effect of supersonics.The importance of cavitation can be seen from the following example: in some cases it was enough to preventthe occurrence of cavitation bubbles by raising the external pressure for the supersonic waves to lose their deadlyeffect on the cells,The destruction of various disease bacteria by the actionof powerful supersonic waves is of great interest. Bacteriaexcreted from a diseased organism and placed in a vesselwith a substratum perish rapidly when treated with supersonic waves; even such stable bacteria as tubercle ba-5-132 65


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cill' perish. Diphtheria bacteria are completelydestroyedin a f.ew minutes,A study of preparations of various bacteria after super-sonic treatment under the electron microscope which givesmagnifications of tens of thousands of times, made itpossible to reveal the exact changes caused in them bysupersonics.

Figs. a and b, Plate II, are photographs of tubercle ba-cilli. In a we see the intact tubercle bacilli magnified30,100 times. In b the destruction of the tubercle bacilliunder the action of supersonic vibrations can be seen.It should be noted that even prolonged treatmentdid not lead to complete destruction of all the tuberclebacilli.

Figs. g and h, Plate II, are photographs ol one of thespecies of bacteria magnified 10,850X. Figure g showsthe microorganism before treatment, and figure h, after10 minutes' treatment with supersonic waves of a frequen-cy of 700,000 vibrations per second. It can be seen thatunder the action of the supersonic rays the bacter ia havelost their clear-cut outline and have acquired a sort of"atmosphere" with irregular and diffuse boundaries.

Various kinds of viruses are also subject to the destructive action of supersonic waves.Figs. c and d, Plate II, are photographs of the virus offeline pneumonia, magnified 13,570X before and after su-personic treatment,

After treatment the virus ceases to cause the disease.Figs. e and t. Plate II, show the changes that take place

in one of man's most dangerous enemies-the grippevirus-under the action of supersonic vibrations, Althoughthe changes in this case are not 'So clearly defined, anhour's treatment with supersonic waves decreases theactivity of the virus a thousandfold.

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Plate IIa, b-effect of supersonics all tubercle bacilli: c. d-e-effect of supersonicson the feline pneumonia virus ; e. f-effect of supersonics on the grippevirus; fl, h-effect of supersonics on bacteria


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V. A. Kuteishchlkov, a Soviet scientist, has succeeded inweakening the virus of spotted fever by subjecting it tothe action of supersonic waves for 30 seconds,

The capacity of supersonics f.or killing microorganismsmakes it possible to use it to sterilize water, milk and va-rious other foodstuffs.

Very interesting attempts have been 'made to employsupersonics for the extraction of various important biolog-ical substances from microorganisms: toxins, enzymes,etc. Various serums have recently been successfully pre-pared with the aid oi supersonics,

By treating whooping 'Dough bacteria with supersonicsscientists have isolated the poison they deposit, known asendotoxin. If this endotoxin is kept in the cold for sometime, it loses its toxic properties, becomes harmless, butretains its capacity 'of imparting 'immunity to 'animals, Le.,making them not subject to the disease. The advantage ofthe supersonic method of producing toxins, enzymes andother biological substances is that during the short time ittakes to destroy the cell its contents do not get a chanceto undergo chemical change, and they pass 11nto the SUT-rounding medium without alteration.

There is no doubt that in the near future supersonicswill be widely used for the production of various biologi-cal preparations.

WHAT HAPPENED TO THE FISH?Here is a vessel of water with little fishes darting gaily

about in it.But what 'has happened?Why have the movements of the fishes suddenly become

erratic? Why do they float helplessly on the surface under-s i Q ~ up, trying in vain to right themselves?

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This is due to the supersonic vibrations which have ap-peared in the water. Switch off the supersonic source andthe fishes will resume their gay darting around, as if noth-ing had happened to them. But if the power of the soundis increased, the fishes will be killed.The effect of. supersonics on tadpoles is much the same.

True, unlike the fishes, which when subjected to super-sonic vibrations still try to right themselves, the tadpolescompletely lose their ability to move.Alongside their property of destroying live organisms,

supersonics may in some cases stimulate vital processes.For instance, if the seeds of sweet peas are treated withsupersonic waves they begin to germinate violently. Butexperiments of this kind should be carried out very care-fully. Intense supersonic waves act very severely on theorganism.

Men operating powerful sirens noticed that whenevertheir hand got into the path of the sound ray their fingersbecame unbearably hot in a few seconds, This is probablydue to the Iact that the heat generated by the action ofsound arises in the very tissues of the organism, insteadof being conducted to them as is the case when warmingin the usual way.Once when the operator of a powerful siren accidental-ly got into the way of the sonic wave for a few moments,

he immediately began to feel nauseous and to lose hisbalance, in spite of the fact that he had special sound ab-sorbers over his ears.Supersonic waves of smaller power behave differently,

Their action on the human organism may be beneficial. Inthis respect supersonics may be compared with sunlight,which causes sunburn when used immoderately, but if ap-plied correctly, restores people to health.In recent years scientists have succeeded in developing

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Fig. 25. Instrument for determining- the power of supersonicvibrations

several methods of treating various disordersby means of supersonics. In one of thesemethods the su personicsource is pressed firmlyto the part of the humanbody which is to betreated with high-frequency vibrations. Inanother the projector ispassed to and fro overthe surface of the skinin the place to be irradiated. The skin is firstsmeared with paraffinoil to make it conductthe sound vibrationsbetter.One of the main difficulties connected with the use of

supersonic vibrations in medicine is the lack of welldeveloped methods of dosing them. Fig. 25 shows one ofthe instruments for determining the intensity of a supersonic beam.Scientists are also persistently investigating the processes that take place in live organisms under the influenceof supersonics.There is no doubt that besides cavitation, which rup

tures the simplest organisms, the chemical action of supersonics on the complex organic substances present in livecells must also be taken into account.Under the action of powerful supersonic vibrations large

protein molecules break up, forming part icles of smaller size.

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Supersonics is also capable of changing the chemicaland biochemical properties of 'molecules less complex instructure and more stable in nature than pr

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Sounds We Cannot Hear