The 21st century is the century of radio electronics, the atom, space exploration and ultrasound. The science of ultrasound is relatively young today. At the end of the 19th century, P. N. Lebedev, a Russian physiologist, conducted his first studies. After that, many eminent scientists began to study ultrasound.
What is ultrasound?
Ultrasound is a wave-like propagating effect of medium particles. It has its own characteristics, in which it differs from the sounds of the audible range. It is relatively easy to obtain directed radiation in the ultrasonic range. In addition, it is well focused, and as a result of this, the intensity of the oscillations made increases. When distributed in solids, liquids and gases, ultrasound gives rise to interesting phenomena that have found practical use in many areas of technology and science. This is what ultrasound is, the role of which in various spheres of life today is very large.
The role of ultrasound in science and practice
Ultrasound in last years began to play an increasingly important role in scientific research. Experimental and theoretical studies in the field of acoustic flows and ultrasonic cavitation were successfully carried out, which allowed scientists to develop technological processes that occur when exposed to ultrasound in the liquid phase. It is a powerful method for studying various phenomena in such a field of knowledge as physics. Ultrasound is used, for example, in semiconductor and solid state physics. Today, a separate branch of chemistry is being formed, called "ultrasonic chemistry". Its application allows accelerating many chemical-technological processes. Molecular acoustics was also born - a new section of acoustics that studies molecular interaction with matter. New areas of application of ultrasound appeared: holography, introscopy, acoustoelectronics, ultrasonic phase measurement, quantum acoustics.
In addition to experimental and theoretical works in this area, a lot of practical work has been done today. Special and universal ultrasonic machines, installations that operate under increased static pressure, etc. have been developed. Automatic ultrasonic installations included in production lines have been introduced into production, which can significantly increase labor productivity.
More about ultrasound
Let's talk more about what ultrasound is. We have already said that these are elastic waves and ultrasound is over 15-20 kHz. The subjective properties of our hearing determine the lower limit of ultrasonic frequencies, which separates it from the frequency of audible sound. This boundary, therefore, is conditional, and each of us differently defines what ultrasound is. The upper limit is indicated by elastic waves, their physical nature. They propagate only in a material medium, that is, the wavelength must be significantly greater than the mean free path of the molecules present in the gas or the interatomic distances in solids and liquids. At normal pressure in gases, the upper limit of ultrasonic frequencies is 10 9 Hz, and in solids and liquids - 10 12 -10 13 Hz.
Sources of ultrasound
Ultrasound is found in nature both as a component of many natural noises (waterfall, wind, rain, pebbles rolled by the surf, as well as in the sounds accompanying thunderstorm discharges, etc.), and as an integral part of the animal world. Some species of animals use it for orientation in space, detection of obstacles. It is also known that dolphins use ultrasound in nature (mainly frequencies from 80 to 100 kHz). In this case, the power of the location signals emitted by them can be very large. Dolphins are known to be able to detect those up to a kilometer away from them.
Emitters (sources) of ultrasound are divided into 2 large groups. The first is generators, in which oscillations are excited due to the presence of obstacles in them installed in the path of a constant flow - a jet of liquid or gas. The second group into which ultrasound sources can be combined is electro-acoustic transducers, which convert given current or electrical voltage fluctuations into a mechanical vibration performed by a solid body that radiates acoustic waves into the environment.
Ultrasound receivers
On medium and ultrasonic receivers, electro-acoustic transducers are most often of the piezoelectric type. They can reproduce the form of the received acoustic signal, represented as a time dependence of the sound pressure. Devices can be either broadband or resonant, depending on the application conditions they are intended for. Thermal receivers are used to obtain time-averaged sound field characteristics. They are thermistors or thermocouples coated with a sound-absorbing substance. Sound pressure and intensity can also be estimated by optical methods such as light diffraction by ultrasound.
Where is ultrasound used?
There are many areas of its application, with the use of various features ultrasound. These areas can be roughly divided into three areas. The first of them is connected with obtaining various information by means of ultrasonic waves. The second direction is its active influence on the substance. And the third is connected with the transmission and processing of signals. US specific is used in each case. We will cover only a few of the many areas in which it has found its application.
Ultrasonic cleaning
The quality of such cleaning cannot be compared with other methods. When rinsing parts, for example, up to 80% of contaminants remain on their surface, about 55% - with vibration cleaning, about 20% - with manual cleaning, and with ultrasonic cleaning, no more than 0.5% of contaminants remain. Details that have a complex shape can be cleaned well only with the help of ultrasound. An important advantage of its use is high productivity, as well as low costs of physical labor. Moreover, it is possible to replace expensive and flammable organic solvents with cheap and safe ones. aqueous solutions, apply liquid freon, etc.
A serious problem is air pollution with soot, smoke, dust, metal oxides, etc. You can use the ultrasonic method of cleaning air and gas in gas outlets, regardless of ambient humidity and temperature. If an ultrasonic emitter is placed in a dust settling chamber, its efficiency will increase hundreds of times. What is the essence of such purification? Dust particles moving randomly in the air hit each other stronger and more often under the influence of ultrasonic vibrations. At the same time, their size increases due to the fact that they merge. Coagulation is the process of particle enlargement. Their weighted and enlarged accumulations are caught by special filters.
Machining of brittle and superhard materials
If you enter between the workpiece and the working surface of the tool that uses ultrasound, then the abrasive particles during the operation of the emitter will affect the surface of this part. In this case, the material is destroyed and removed, subjected to processing under the action of a variety of directed micro-impacts. The kinematics of processing consists of the main movement - cutting, that is, the longitudinal vibrations made by the tool, and the auxiliary one - the feed movement that the apparatus performs.
Ultrasound can do a variety of jobs. For abrasive grains, the source of energy is longitudinal vibrations. They destroy the processed material. The feed movement (auxiliary) can be circular, transverse and longitudinal. Ultrasonic processing is more precise. Depending on the grain size of the abrasive, it ranges from 50 to 1 micron. Using tools of various shapes, you can make not only holes, but also complex cuts, curved axes, engrave, grind, make matrices and even drill a diamond. The materials used as an abrasive are corundum, diamond, quartz sand, flint.
Ultrasound in radio electronics
Ultrasound in engineering is often used in the field of radio electronics. In this area, it often becomes necessary to delay an electrical signal relative to some other one. Scientists have found a good solution by suggesting the use of ultrasonic delay lines (LZ for short). Their action is based on the fact that electrical impulses are converted into ultrasonic. How does this happen? The fact is that the speed of ultrasound is significantly less than that developed by electromagnetic oscillations. The voltage pulse after the inverse transformation into electrical mechanical oscillations will be delayed at the output of the line relative to the input pulse.
Piezoelectric and magnetostrictive transducers are used to convert electrical to mechanical vibrations and vice versa. LZ, respectively, are divided into piezoelectric and magnetostrictive.
Ultrasound in medicine
Various types of ultrasound are used to influence living organisms. In medical practice, its use is now very popular. It is based on the effects that occur in biological tissues when ultrasound passes through them. The waves cause fluctuations in the particles of the medium, which creates a kind of tissue micromassage. And the absorption of ultrasound leads to their local heating. At the same time, certain physicochemical transformations occur in biological media. These phenomena in the case of moderate irreversible damage do not cause. They only improve the metabolism, and therefore contribute to the vital activity of the body exposed to them. Such phenomena are used in ultrasound therapy.
Ultrasound in surgery
Cavitation and strong heating at high intensities lead to tissue destruction. This effect is used today in surgery. Focused ultrasound is used for surgical operations, which allows local destruction in the deepest structures (for example, the brain), without damaging the surrounding ones. In surgery, ultrasonic instruments are also used, in which the working end looks like a file, scalpel, needle. The vibrations imposed on them give new qualities to these devices. The required force is significantly reduced, therefore, the traumatism of the operation is reduced. In addition, an analgesic and hemostatic effect is manifested. Impact with a blunt instrument using ultrasound is used to destroy certain types of neoplasms that have appeared in the body.
Impact on biological tissues is carried out to destroy microorganisms and is used in the processes of sterilization of medicines and medical instruments.
Examination of internal organs
Basically, we are talking about the study of the abdominal cavity. For this purpose, a special one can be used to find and recognize various anomalies of tissues and anatomical structures. The task is often as follows: there is a suspicion of a malignant formation and it is required to distinguish it from a benign or infectious formation.
Ultrasound is useful in examining the liver and for other tasks, which include detecting obstructions and diseases of the bile ducts, as well as examining the gallbladder to detect the presence of stones and other pathologies in it. In addition, testing for cirrhosis and other diffuse benign liver diseases may be used.
In the field of gynecology, especially in the analysis of the ovaries and uterus, the use of ultrasound has long been the main direction in which it is carried out with particular success. Often, differentiation of benign and malignant formations is also needed here, which usually requires the best contrast and spatial resolution. Similar conclusions can be useful in the study of many other internal organs.
The use of ultrasound in dentistry
Ultrasound has also found its way into dentistry, where it is used to remove tartar. It allows you to quickly, bloodlessly and painlessly remove plaque and stone. At the same time, the oral mucosa is not injured, and the "pockets" of the cavity are disinfected. Instead of pain, the patient experiences a sensation of warmth.
Ultrasound………………………………………………………………….4
Ultrasound as elastic waves………………………………………..4
Specific features of ultrasound………………………………..5
Sources and receivers of ultrasound………………………………………..7
Mechanical emitters…………………………………………...7
Electroacoustic transducers…………………………….9
Ultrasound receivers……………………………………………..11
The use of ultrasound…………………………………………………...11
Ultrasonic cleaning……………………………………………...11
Machining of superhard and brittle
materials………………………………………………………………13
Ultrasonic welding………………………………………………….14
Ultrasonic soldering and tinning……………………………………14
Acceleration of production processes………………..…………15
Ultrasonic flaw detection…………………………..…………15
Ultrasound in radio electronics………………………..……………17
Ultrasound in medicine………………………………..……………..18
Literature…………………………………………………..……………….19
conducting.The twenty-first century is the century of the atom, the conquest of space, radio electronics and ultrasound. The science of ultrasound is relatively young. First laboratory works on the study of ultrasound were carried out by the great Russian physicist P. N. Lebedev in late XIX, and then many prominent scientists were engaged in ultrasound.
Ultrasound is a wave-like oscillatory motion of medium particles. Ultrasound has some features in comparison with the sounds of the audible range. In the ultrasonic range, it is relatively easy to obtain directional radiation; it lends itself well to focusing, as a result of which the intensity of ultrasonic vibrations increases. When propagating in gases, liquids and solids, ultrasound generates interesting phenomena, many of which have found practical application in various fields of science and technology.
In recent years, ultrasound has begun to play an increasingly important role in scientific research. Theoretical and experimental studies in the field of ultrasonic cavitation and acoustic flows have been successfully carried out, which made it possible to develop new technological processes that occur under the action of ultrasound in the liquid phase. At present, a new direction in chemistry is being formed - ultrasonic chemistry, which allows accelerating many chemical and technological processes. Scientific research contributed to the emergence of a new branch of acoustics - molecular acoustics, which studies the molecular interaction of sound waves with matter. New areas of application of ultrasound have emerged: introscopy, holography, quantum acoustics, ultrasonic phase measurement, acoustoelectronics.
Along with theoretical and experimental studies much work has been done in the field of ultrasound practical work. Universal and special ultrasonic machines, installations operating under increased static pressure, ultrasonic mechanized installations for cleaning parts, generators with increased frequency and new system cooling, converters with a uniformly distributed field. Automatic ultrasonic installations have been created and introduced into production, which are included in production lines, which make it possible to significantly increase labor productivity.
ultrasound.Ultrasound (US) - elastic vibrations and waves, the frequency of which exceeds 15 - 20 kHz. The lower boundary of the ultrasonic frequency region, separating it from the region of audible sound, is determined by the subjective properties of human hearing and is conditional, since the upper boundary auditory perception each person has his own. The upper limit of ultrasonic frequencies is due to the physical nature of elastic waves, which can propagate only in a material medium, i.e. provided that the wavelength is much greater than the mean free path of molecules in a gas or interatomic distances in liquids and solids. In gases at normal pressure, the upper limit of ultrasonic frequencies is » 10 9 Hz; in liquids and solids, the cutoff frequency reaches 10 12 -10 13 Hz. Depending on the wavelength and frequency, ultrasound has various specific features of radiation, reception, propagation and application, therefore, the area of ultrasound frequencies is divided into three areas:
· low ultrasonic frequencies (1.5×10 4 - 10 5 Hz);
medium (10 5 - 10 7 Hz);
high (10 7 - 10 9 Hz).
Elastic waves with frequencies of 10 9 - 10 13 Hz are usually called hypersound.
Ultrasound as elastic waves.
Ultrasonic waves (inaudible sound) by their nature do not differ from elastic waves in the audible range. Only propagated in gases and liquids longitudinal waves, and in solids - longitudinal and shear s.
The propagation of ultrasound obeys the basic laws common to acoustic waves of any frequency range. The basic laws of distribution are laws of sound reflection and sound refraction at boundaries various environments, sound diffraction and sound scattering in the presence of obstacles and inhomogeneities in the medium and irregularities at the boundaries, laws of waveguide propagation in limited areas of the environment. An important role is played by the ratio between the sound wavelength l and the geometric dimension D, i.e., the size of the sound source or obstacle in the path of the wave, and the size of the inhomogeneities of the medium. When D>>l sound propagation near obstacles occurs mainly according to the laws of geometric acoustics (you can use the laws of reflection and refraction). The degree of deviation from the geometric pattern of propagation and the need to take into account diffraction phenomena are determined by the parameter
, where r is the distance from the observation point to the object causing diffraction.The speed of propagation of ultrasonic waves in an unlimited medium is determined by the characteristics of elasticity and density of the medium. In limited media, the wave propagation velocity is affected by the presence and nature of the boundaries, which leads to a frequency dependence of the velocity (dispersion of the speed of sound). The decrease in the amplitude and intensity of the ultrasonic wave as it propagates in a given direction, that is, the attenuation of sound, is caused, as for waves of any frequency, by the divergence of the wave front with distance from the source, scattering and absorption of sound. At all frequencies, both audible and inaudible ranges, the so-called "classical" absorption occurs, caused by the shear viscosity (internal friction) of the medium. In addition, there is an additional (relaxation) absorption, which often significantly exceeds the "classical" absorption.
With a significant intensity of sound waves, nonlinear effects appear:
the principle of superposition is violated and the interaction of waves occurs, leading to the appearance of tones;
· the waveform changes, its spectrum is enriched with higher harmonics and, accordingly, the absorption increases;
· when a certain threshold value of the ultrasonic intensity is reached, cavitation occurs in the liquid (see below).
The criterion for the applicability of the laws of linear acoustics and the possibility of neglecting nonlinear effects is: M<< 1, где М = v/c, v – колебательная скорость частиц в волне, с – скорость распространения волны.
The parameter M is called the "Mach number".
specific features of ultrasoundAlthough the physical nature of ultrasound and the basic laws that determine its propagation are the same as for sound waves of any frequency range, it has a number of specific features. These features are due to relatively high US frequencies.
The smallness of the wavelength determines ray character propagation of ultrasonic waves. Near the emitter, the waves propagate in the form of beams, the transverse size of which remains close to the size of the emitter. When such a beam (US beam) hits large obstacles, it undergoes reflection and refraction. When the beam hits small obstacles, a scattered wave arises, which makes it possible to detect small inhomogeneities in the medium (of the order of tenths and hundredths of a mm.). Reflection and scattering of ultrasound on inhomogeneities of the medium make it possible to form in optically opaque media sound images objects using sound focusing systems, similar to how it is done with light beams.
Focusing ultrasound allows not only to obtain sound images (sound imaging and acoustic holography systems), but also concentrate sound energy. With the help of ultrasonic focusing systems, it is possible to form predetermined directivity characteristics emitters and manage them.
A periodic change in the refractive index of light waves, associated with a change in density in the ultrasonic wave, causes diffraction of light by ultrasound observed at US frequencies in the megahertz-gigahertz range. In this case, the ultrasonic wave can be considered as a diffraction grating.
The most important nonlinear effect in the ultrasonic field is cavitation- the appearance in the liquid of a mass of pulsating bubbles filled with vapor, gas or a mixture thereof. The complex movement of bubbles, their collapse, merging with each other, etc. generate compression pulses (microshock waves) and microflows in the liquid, cause local heating of the medium, ionization. These effects affect the substance: the destruction of solids in the liquid occurs ( cavitation erosion), fluid mixing occurs, various physical and chemical processes are initiated or accelerated. By changing the conditions of cavitation, it is possible to enhance or weaken various cavitation effects, for example, with an increase in the frequency of ultrasound, the role of microflows increases and cavitation erosion decreases, with an increase in pressure in the liquid, the role of microimpact increases. An increase in frequency leads to an increase in the threshold intensity corresponding to the onset of cavitation, which depends on the type of liquid, its gas content, temperature, etc. For water at atmospheric pressure, it is usually 0.3–1.0 W/cm 2 . Cavitation is a complex set of phenomena. Ultrasonic waves propagating in a liquid form alternating areas of high and low pressures, creating zones of high compression and rarefaction zones. In a rarefied zone, the hydrostatic pressure decreases to such an extent that the forces acting on the molecules of the liquid become greater than the forces of intermolecular cohesion. As a result of a sharp change in hydrostatic equilibrium, the liquid "breaks", forming numerous tiny bubbles of gases and vapors. At the next moment, when a period of high pressure begins in the liquid, the bubbles formed earlier collapse. The process of bubble collapse is accompanied by the formation of shock waves with a very high local instantaneous pressure, reaching several hundred atmospheres.
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The twenty-first century is the century of the atom, the conquest of space, radio electronics and ultrasound. The science of ultrasound is relatively young. The first laboratory work on the study of ultrasound was carried out by a Russian scientist - P.N. Lebedev at the end of the 19th century, and then J.-D. Colladon, J. and P. Curie, F. Galton.
In today's world, ultrasound plays an increasingly important role in scientific research. Theoretical and experimental studies in the field of ultrasonic cavitation and acoustic flows have been successfully carried out, which made it possible to develop new technological processes that occur under the action of ultrasound in the liquid phase. At present, a new direction in chemistry is being formed - ultrasonic chemistry, which makes it possible to accelerate many chemical and technological processes. Scientific research contributed to the emergence of a new section of acoustics - molecular acoustics, which studies the molecular interaction of sound waves with matter. New fields of application of ultrasound have emerged. Along with theoretical and experimental research in the field of ultrasound, a lot of practical work has been done.
While visiting the hospital, I saw devices based on ultrasound. Such devices make it possible to detect various homogeneities or heterogeneities of a substance in human tissues, brain tumors and other formations, pathological conditions of the brain, and make it possible to control the rhythm of the heart. It became interesting to me how these installations work with the help of ultrasound, and in general, what ultrasound is. The school physics course does not say anything about ultrasound and its properties, and I decided to study ultrasonic phenomena myself.
Goal of the work: to study ultrasound, experimentally investigate its properties, explore the possibilities of using ultrasound in technology.
Tasks:
theoretically consider the causes of the formation of ultrasound;
receive ULTRASONIC FOUNTAIN;
explore the properties of ultrasonic waves in water;
to investigate the dependence of the height of the fountain on the concentration of the dissolved substance for different solutions (viscous and non-viscous);
to study modern applications of ultrasound in engineering.
Hypothesis: ultrasonic waves have the same properties as sound waves (reflection, refraction, interference), but due to the greater penetrating power in a substance, ultrasound has more applications in technology; as the solution concentration (liquid density) increases, the height of the ultrasonic fountain decreases.
Research methods:
Analysis and selection of theoretical information; advancement of the research hypothesis; experiment; hypothesis testing.
II. - Theoretical part.
1. The history of the emergence of ultrasound.
Attention to acoustics was caused by the needs of the navies of the leading powers - England and France, because. acoustic - the only type of signal that can travel far in water. In 1826, French scientists J.-D. Colladon and Sh.-F. Storm determined the speed of sound in water. Their experiment is considered the birth of modern hydroacoustics. The impact on the underwater bell in Lake Geneva occurred with the simultaneous ignition of gunpowder. A flash from gunpowder was observed by scientists at a distance of 10 miles. The sound of a bell was also heard with the help of an underwater auditory tube. By measuring the time interval between these two events, the speed of sound was calculated - 1435 m/sec. The difference with modern calculations is only 3 m/s.
In 1838, in the United States, sound was first used to determine the profile of the seabed in order to lay a telegraph cable. The source of the sound, as in Colladon's experiment, was a bell sounding under water, and the receiver was large auditory tubes that descended overboard the ship. The results of the experiment were disappointing. The sound of the bell (as, indeed, the explosion of powder cartridges in the water) gave a very weak echo, almost inaudible among other sounds of the sea. It was necessary to go to the region of higher frequencies, which allow creating directed sound beams, that is, to switch to ultrasound.
The first ultrasound generator was made in 1883 by the Englishman Francis Galton. Ultrasound was created like a whistle on the edge of a knife if you blow on it. The role of such a point in Galton's whistle was played by a cylinder with sharp edges. Air or other gas escaping under pressure through an annular nozzle with a diameter the same as the edge of the cylinder ran against the edge, and high-frequency oscillations occurred. Blowing the whistle with hydrogen, it was possible to obtain oscillations up to 170 kHz.
In 1880, Pierre and Jacques Curie made a decisive discovery for ultrasonic technology. The Curie brothers noticed that when pressure is applied to quartz crystals, an electrical charge is generated that is directly proportional to the force applied to the crystal. This phenomenon has been called "piezoelectricity" from the Greek word meaning "to press". In addition, they demonstrated an inverse piezoelectric effect, which occurs when a rapidly changing electrical potential is applied to a crystal, causing it to vibrate. This vibration occurred at an ultrasonic frequency. From now on, it became technically possible to manufacture small-sized emitters and receivers of ultrasound.
The phenomenon of electrostriction (reverse piezoelectric effect) is due to the orientation and dense packing of some water molecules around the ionic groups of amino acids and is accompanied by a decrease in the heat capacity and compressibility of solutions of bipolar ions. The phenomenon of electrostriction is the deformation of a given body in an electric field. Due to the phenomenon of electrostriction, mechanical forces arise inside the dielectric. Although the phenomena of electrostriction are observed in many dielectrics, they are weakly expressed in most crystals. In some crystals, such as Rochelle salt and barium titanate, the phenomenon of electrostriction proceeds very intensively.
III. - Practical part.
Creation of ultrasonic fountains.
To obtain ultrasound, 2 different ultrasonic units were used in the work: 1) a school ultrasonic unit UD-1 and 2) an ultrasonic demonstration unit UD-6.
To obtain a fountain, a lens cup was taken and placed on top of the emitter so that no air bubbles formed between the bottom of the cup and the piezoelectric element, which greatly interfered with the experiments. To do this, the glass was placed by moving the bottom along the emitter cover until the glass hit the emitter ledge. Having set the lens cup correctly, we began to make observations. We poured ordinary drinking water into the lens cup.
Approximately one minute after the generator was powered from the mains, an ultrasonic fountain was observed (Appendix 1, Fig. 1), which is adjusted by the frequency adjustment knob and adjusting screws. By turning the frequency adjustment knob, we got a fountain of such a height that water began to splash over the edge of the glass (Appendix 1, Fig. 3, 12). Again, we turned the tuning capacitor with a screwdriver, reduced the fountain and continued to adjust the screw to a new maximum of the fountain (the maximum height of the fountain is 13-15 cm).
The lowering of the fountain with liquid splashing is explained by the departure of the liquid level plane in the vessel from the focus of the ultrasonic lens due to the level decrease. For long-term observation of the fountain, the latter was placed in a glass tube, along the inner wall of which the fountain liquid flows, so its level in the vessel does not change. To do this, we took a tube 50 cm high with a diameter not exceeding the inner diameter of the lens cup (d = 3 cm). When using a glass tube, liquid was poured into the lens cup 5 mm below the upper edge of the glass to maintain the liquid level, due to its splashing onto the inner wall of the tube (Appendix 1, Fig. 4, 5, 6).
Observation of the properties of ultrasound .
In order to obtain wave reflection, a flat metal plate was introduced into a cuvette with glycerin and water poured on top and placed at an angle of 45 0 to the water surface. The generator was turned on and the formation of standing waves was achieved (Appendix 1, Fig. 10), which are obtained as a result of wave reflection from the introduced plate and the cuvette wall. In this experiment, wave interference was simultaneously observed (Appendix 1, Fig. 8, 9). They carried out exactly the same experiment, but poured down a strong solution of potassium permanganate with water (Appendix 1, Fig. 11), then glycerin and water on top. In this experiment, the refraction of waves was also achieved: when ultrasonic waves passed through the interface between two liquids, a change in the length of the standing wave was observed, in glycerin its wave is larger than in water and manganese dissolved in it, which is explained by the difference in the speed of propagation of ultrasound in these liquids. The phenomenon of particle coagulation was also obtained: starch was added to a cuvette with clean water, thoroughly mixed; after turning on the generator, we saw how the particles collect in the nodes of standing waves and, after turning off the generator, fall down, purifying the water. Thus, in these experiments, reflection, refraction, ultrasound interference, and particle coagulation were observed.
Observation of the dependence of the height of the fountain on the size of the solute molecule and the type of solution.
We tested the hypothesis put forward about the dependence of the height of the ultrasonic fountain on the density of the liquid (concentration of the solution) and the size of the molecule. To do this, the density was changed by dissolving substances with different molecular sizes (starch, sugar, egg white) in it.
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The dependence of the height of the fountain on the size of the dissolved molecule particles and solution concentrations at constant frequency, voltage, liquid volume-25 ml (accurate to tenths) |
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Experience number |
Solvent |
Solute |
Solution concentration |
Observations |
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water + starch |
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Initial concentration, water swelling 2mm, rings appeared |
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The concentration is 2 times lower, the fountain is 5 cm high, water mist appeared |
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The concentration is 4 times lower, the fountain is 7-8 cm high, water mist appeared |
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The concentration is 8 times lower, the fountain is 12-13 cm high, water mist appeared |
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water + sugar |
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Initial concentration, 13-14 cm high fountain, water mist appeared |
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The concentration is 2 times lower, the fountain is 12-13 cm high, water mist appeared |
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The concentration is 8 times lower, the fountain is 6-7 cm high, water mist appeared |
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Egg white |
Water + egg white |
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Initial concentration, 3-4 cm high fountain, water mist appeared |
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The concentration is 2 times lower, the fountain is 6-7 cm high, water mist appeared |
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The concentration is 4 times lower, the fountain is 8-9 cm high, water mist appeared |
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The concentration is 8 times lower, the fountain is 10-11 cm high, water mist appeared |
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In order to find out how the height of the fountain depends on the density of the solution and the size of the solute molecule, the following experiments were carried out. At a constant frequency, voltage and volume of liquid (25 ml), water was irradiated with ultrasound, with starch, sugar, and egg white dissolved in it. For each substance, 4 experiments were carried out, with each subsequent one, the concentration of substances was reduced by 2 times, i.e., in the second experiment, the concentration was 2 times lower, in the third experiment, 4 times lower, in the fourth, 8 times lower. All observations were recorded and presented in the table above. Also in the appendix there is a diagram in which you can clearly see how the concentration of substances decreases (Appendix 2, Diagram 1).
Thus, we obtained the dependence of the height of the fountain on the concentration of substances (Appendix 2, diagram 2), and in experiments with egg white and starch, the height of the fountain increased, and in experiments with sugar it decreased.
This is due to the fact that starch and protein molecules are biological polymers (HMCs are high molecular weight compounds). When dissolved in water, they form colloidal solutions (colloidal particle diameter - 1-100 nm) with high viscosity. Due to the presence of a large number of hydroxo groups (-OH), hydrogen bonds are formed in the molecules of such substances (between the molecules of water and starch, water and protein), which contributes to a more uniform distribution of particles in the solution, which negatively affects the transmission of waves.
Sugar is a dimer (C 12 H 22 O 11) n, its dissolution leads to the formation of a true solution (the size of the particles of the solute is comparable to the size of the solvent molecules), non-viscous, with high penetrating power, such a structure of the solution contributes to a stronger transfer of wave energy.
Thus, for viscous liquids, with increasing solution concentration, the height of the ultrasonic fountain decreases, and for non-viscous liquids, with increasing solution concentration, the height of the ultrasonic fountain increases.
IV. -Technical applications of ultrasound.
The various applications of ultrasound can be divided into three areas:
obtaining information about the substance;
effect on the substance;
signal processing and transmission.
The dependence of the speed of propagation and attenuation of acoustic waves on the properties of the substance and the processes occurring in them is used in the following studies:
study of molecular processes in gases, liquids and polymers;
study of the structure of crystals and other solids;
control of the course of chemical reactions, phase transitions, polymerization, etc.;
determination of the concentration of solutions;
determination of strength characteristics and composition of materials;
determination of the presence of impurities;
determination of the flow velocity of liquid and gas.
Information about the molecular structure of a substance is provided by measuring the speed and absorption coefficient of sound in it. This makes it possible to measure the concentration of solutions and suspensions in pulps and liquids, to control the course of extraction, polymerization, aging, and the kinetics of chemical reactions. The accuracy of determining the composition of substances and the presence of impurities by ultrasound is very high and amounts to fractions of a percent.
Measuring the speed of sound in solids makes it possible to determine the elastic and strength characteristics of structural materials. Such an indirect method for determining strength is convenient due to its simplicity and the possibility of using it in real conditions.
Ultrasonic gas analyzers monitor the accumulation of hazardous impurities. The dependence of ultrasonic speed on temperature is used for non-contact thermometry of gases and liquids.
Ultrasonic flow meters operating on the K. Doppler effect are based on measuring the speed of sound in moving liquids and gases, including inhomogeneous ones (emulsions, suspensions, pulps). Similar apparatus is used to determine the rate and flow of blood in clinical studies.
A large group of measurement methods is based on the reflection and scattering of ultrasound waves at the boundaries between media. These methods allow you to accurately locate foreign bodies in the environment and are used in such areas as:
sonar;
non-destructive testing and flaw detection;
medical diagnostics;
determination of the levels of liquids and loose bodies in closed containers;
determining the size of products;
visualization of sound fields - sound vision and acoustic holography.
Reflection, refraction and the possibility of focusing ultrasound are used in ultrasonic flaw detection, in ultrasonic acoustic microscopes, in medical diagnostics, to study macroinhomogeneities of a substance. The presence of inhomogeneities and their coordinates are determined by the reflected signals or by the structure of the shadow.
Measurement methods based on the dependence of the parameters of a resonant oscillatory system on the properties of the medium loading it (impedance) are used to continuously measure the viscosity and density of liquids, to measure the thickness of parts that can only be accessed from one side. The same principle underlies ultrasonic hardness testers, level gauges, level indicators. Advantages of ultrasonic testing methods: short measurement time, ability to control explosive, aggressive and toxic media, no impact of the tool on the controlled environment and processes.
V. - Conclusion:
In the process of doing research work, I theoretically considered the causes of the formation of ultrasound; studied the modern applications of ultrasound in technology: ultrasound allows you to find out the molecular structure of a substance, determine the elastic and strength characteristics of structural materials, monitor the accumulation of hazardous impurities; used in ultrasonic flaw detection, in ultrasonic acoustic microscopes, in medical diagnostics, to study macroinhomogeneities of a substance, to continuously measure the viscosity and density of liquids, to measure the thickness of parts that can only be accessed from one side. Experimentally received an ultrasonic fountain: found that the maximum height of the fountain is 13-15 cm, (depending on the water level in the glass, the frequency of ultrasound, the concentration of the solution, the viscosity of the solution). She experimentally investigated the properties of ultrasonic waves in water: she determined that the properties of an ultrasonic wave are the same as those of a sound wave, but all processes, due to the high frequency of ultrasound, occur with great penetration into the depth of the substance.
The experiments carried out proved that the ultrasonic fountain can be used to study the properties of solutions, such as concentration, density, transparency, size of dissolved particles. This research method is distinguished by its speed and ease of implementation, the accuracy of the study, and the ability to easily compare different solutions. Such studies are relevant in the implementation of environmental monitoring. For example, when studying the composition of a mining tailing dump in the city of Olenegorsk at various depths or for monitoring water at wastewater treatment plants.
Thus, I confirmed my hypothesis that ultrasonic waves have the same properties as sound waves (reflection, refraction, interference), but due to the greater penetrating power in a substance, ultrasound has more applications in technology. The hypothesis about the dependence of the height of the ultrasonic fountain on the density of the liquid was partially confirmed: when the concentration of the solute changes, the density changes and the height of the fountain changes, but the transfer of energy of the ultrasonic wave depends to a greater extent on the viscosity of the solution, therefore, for different liquids (viscous and inviscid), the dependence of the height of the fountain on concentration was different.
VI. - Bibliographic list:
Myasnikov L.L. Inaudible sound. Leningrad "Shipbuilding", 1967. 140 p.
Passport Installation ultrasonic demonstration UD-76 3.836.000 PS
Khorbenko I.G. Sound, ultrasound, infrasound. M., "Knowledge", 1978. 160 p. (Science and progress)
Annex 1
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Appendix 2
Diagram 1
Ultrasound is called elastic waves (waves propagating in liquid, solid and gaseous media due to the action of elastic forces), the frequency of which lies outside the range audible to humans - from approximately 20 kHz and above.
Useful Features of Ultrasonic Waves
And although physically ultrasound has the same nature as the audible sound, differing only conditionally (higher frequency), it is precisely because of the higher frequency that ultrasound is applicable in a number of useful areas. So, when measuring the speed of ultrasound in a solid, liquid or gaseous substance, very small errors are obtained when monitoring fast processes, when determining the specific heat capacity (gas), when measuring the elastic constants of solids.
High frequency at small amplitudes makes it possible to achieve increased energy flux densities, because the energy of an elastic wave is proportional to the square of its frequency. In addition, ultrasonic waves, used in the right way, allow you to get a number of very special acoustic effects and phenomena.
One of these unusual phenomena is acoustic cavitation, which occurs when a powerful ultrasonic wave is directed into a liquid. In a liquid, in the field of action of ultrasound, tiny bubbles of vapor or gas (submicroscopic size) begin to grow to fractions of millimeters in diameter, while pulsating with the frequency of the wave and collapsing in the positive phase of pressure.
The collapsing bubble generates a locally high pressure pulse measured in thousands of atmospheres, becoming a source of shock spherical waves. Acoustic micro-streams generated near such pulsating bubbles have been useful for making emulsions, cleaning parts, etc.
By focusing ultrasound, sound images are obtained in acoustic holography and in sound vision systems, sound energy is concentrated in order to form directional radiation with given and controlled directional characteristics.
Using an ultrasonic wave as a diffraction grating for light, it is possible to change the refractive indices of light for various purposes, since the density in an ultrasonic wave, as in an elastic wave, in principle, changes periodically.
Finally, the features associated with the speed of propagation of ultrasound. In inorganic media, ultrasound propagates at a rate that depends on the elasticity and density of the media.
As for organic media, here the boundaries and their nature affect the speed, that is, the phase velocity depends on the frequency (dispersion). Ultrasound attenuates with the removal of the wave front from the source - the front diverges, the ultrasound is scattered, absorbed.
The internal friction of the medium (shear viscosity) leads to the classical absorption of ultrasound; in addition, the relaxation absorption for ultrasound exceeds the classical one. In a gas, ultrasound attenuates more strongly, in solids and liquids - much weaker. In water, for example, it decays 1000 times slower than in air. Thus, the industrial fields of application of ultrasound are almost entirely associated with solid and liquid bodies.
Ultrasound in echolocation and sonar (food, defense, mining)
The first prototype of the sonar was created to prevent collisions of ships with ice floes and icebergs by the Russian engineer Shilovsky together with the French physicist Langevin back in 1912.
The device used the principle of reflection and reception of a sound wave. The signal was sent to a certain point, and by the delay of the response signal (echo), knowing the speed of sound, it was possible to judge the distance to the obstacle that reflected the sound.
Shilovsky and Langevin began to study hydroacoustics in depth, and soon created a device capable of detecting enemy submarines in the Mediterranean Sea at a distance of up to 2 kilometers. All modern sonars, including military ones, are descendants of the same device.
Modern echo sounders for studying the bottom relief consist of four blocks: a transmitter, a receiver, a transducer and a screen. The function of the transmitter is to send ultrasonic pulses (50 kHz, 192 kHz or 200 kHz) deep into the water, which propagate in the water at a speed of 1.5 km / s, where they are reflected from fish, stones, other objects and the bottom, then the echo reaches the receiver, is processed converter and the result is displayed on the display in a form convenient for visual perception.
Ultrasound in the electronics and electric power industry
Many areas of modern physics cannot do without ultrasound. The physics of solids and semiconductors, as well as acoustoelectronics, are in many respects closely related to ultrasonic research methods - with effects at a frequency of 20 kHz and higher. Acoustoelectronics occupies a special place here, where ultrasonic waves interact with electric fields and electrons inside solids.
Volumetric ultrasonic waves are used on delay lines and in quartz resonators to stabilize the frequency in modern radio-electronic systems for processing and transmitting information. Surface acoustic waves occupy a special place in bandpass filters for television, in frequency synthesizers, in acoustic wave charge transfer devices, in memory and image reading devices. Finally, correlators and convolvers use the transverse acoustoelectric effect in their work.
Radio electronics and ultrasound
To delay one electrical signal relative to another, ultrasonic delay lines are useful. The electrical impulse is converted into a pulsed mechanical vibration of ultrasonic frequency, which propagates many times slower than the electromagnetic impulse; then the mechanical vibration is converted back into an electrical impulse, and a signal is obtained that is delayed relative to the originally applied one.
For such a conversion, piezoelectric or magnetostrictive transducers are usually used, and therefore the delay lines are called piezoelectric or magnetostrictive.
In a piezoelectric delay line, an electrical signal is applied to a quartz plate (piezoelectric transducer) rigidly connected to a metal rod.
A second piezoelectric transducer is attached to the other end of the rod. The input transducer receives a signal, creates mechanical vibrations propagating along the rod, and when the vibrations reach through the rod of the second transducer, an electrical signal is again obtained.
The speed of propagation of vibrations along the rod is much less than that of just an electrical signal, so the signal that passed through the rod is delayed relative to the input by an amount associated with the difference in the speeds of electromagnetic and ultrasonic vibrations.
The magnetostrictive delay line contains the input transducer, magnets, sound duct, output transducer and absorbers. The input signal is applied to the first coil, ultrasonic frequency oscillations - mechanical oscillations - begin in the rod sound duct made of magnetostrictive material - the magnet creates here a constant bias in the conversion zone and the initial magnetic induction.
Ultrasound in the manufacturing industry (cutting and welding)
An abrasive material (quartz sand, diamond, stone, etc.) is placed between the ultrasound source and the part. Ultrasound acts on abrasive particles, which, in turn, hit the workpiece with the frequency of ultrasound. The material of the part is destroyed under the influence of a huge number of tiny impacts of abrasive grains - this is how processing occurs.
Cutting is added to the feed movement, while the longitudinal cutting oscillations are the main ones. The accuracy of ultrasonic processing depends on the grain size of the abrasive, and reaches 1 micron. In this way, complex cuts are made, which are necessary in the manufacture of metal parts, grinding, engraving and drilling.
If it is necessary to weld dissimilar metals (or even polymers) or combine a thick part with a thin plate, ultrasound again comes to the rescue. This is the so-called. Under the action of ultrasound in the welding area, the metal becomes very ductile, the parts can be very easily rotated during the connection at any angle. And it is worth turning off the ultrasound - the parts will instantly connect, seize.
It is especially noteworthy that welding takes place at a temperature below the melting point of the parts, and their connection occurs in fact in a solid state. But this is how steel, titanium, and even molybdenum are welded. Thin sheets are the easiest to weld. This welding method does not involve special surface preparation of parts, this also applies to metals and polymers.
Ultrasound in metallurgy (ultrasonic flaw detection)
Ultrasonic flaw detection is one of the most effective methods for quality control of metal parts without destruction. In homogeneous media, ultrasound propagates directionally without rapid attenuation, and reflection is characteristic of it at the boundary of the media. So, metal parts are checked for the presence of cavities and cracks inside them (air-metal interface), and increased metal fatigue is detected.
Ultrasound is capable of penetrating a part to a depth of 10 meters, and the dimensions of the detected defects are of the order of 5 mm. There are: shadow, pulse, resonant, structural analysis, visualization - five methods of ultrasonic flaw detection.
The simplest method is shadow ultrasonic flaw detection, this method is based on the attenuation of the ultrasonic wave when it encounters a defect when passing through the part, since the defect creates an ultrasonic shadow. Two converters work: the first emits a wave, the second - receives.
This method is insensitive, a defect is detected only if its influence changes the signal by at least 15%, and it is also impossible to determine the depth where the defect is located in the part. More accurate results are given by the pulsed ultrasonic method, it also shows the depth.
Ultrasound is called elastic vibrations and waves, the frequencies of which exceed the frequencies of sound perceived by the human ear. This definition has developed historically, however, the lower limit of ultrasound, associated with the subjective sensations of a person, cannot be clear, since some people cannot hear sounds with frequencies of 10 kHz, but there are people who perceive frequencies of 25 kHz. To clarify the definition of the lower limit of ultrasound since 1983, it has been established to consider it equal to 11.12 kHz (GOST 12.1.001–83).
The upper limit of ultrasound is due to the physical nature of elastic waves, which can propagate in a medium only if the wavelength is greater than the mean free path of molecules in gases or interatomic distances in liquids and solids. Therefore, in gases, the upper limit of ultrasonic waves (US) is determined from the approximate equality of the sound wave length and the mean free path of gas molecules (~10 –6 m), which gives a frequency of the order of 1 GHz (109 Hz). The distance between atoms and molecules in the crystal lattice of a solid is approximately 10–10 m. Assuming that the wavelength of ultrasound is of the same order of magnitude, we obtain a frequency of 10–13 Hz. Elastic waves with frequencies above 1 GHz are called hypersonic.
Ultrasonic waves by their nature do not differ from the waves of the audible range or infrasound, and the propagation of ultrasound obeys the laws common to all acoustic waves (laws of reflection, refraction, scattering, etc.). The propagation velocities of ultrasonic waves are approximately the same as the speeds of audible sound (see Table 4), and therefore the wavelengths of ultrasonic waves are much shorter. So, when propagating in water ( With= 1500 m / s) ultrasound with a frequency of 1 MHz wavelength l = 1500/10 6 \u003d 1.5 10 -3 m \u003d 1.5 mm. Due to the short wavelength, the diffraction of ultrasound occurs on objects smaller than for audible sound. Therefore, in many cases, the laws of geometric optics can be applied to ultrasound and ultrasonic focusing systems can be manufactured: convex and concave mirrors and lenses, which are used to obtain sound images in sound recording systems and acoustic holography. In addition, focusing ultrasound allows you to concentrate sound energy, while obtaining high intensities.
The absorption of ultrasound in a substance, even in air, is very significant, due to its short wavelength. However, as for ordinary sound, the attenuation of ultrasound is determined not only by its absorption, but also by reflection at the interfaces of media that differ in their acoustic resistance. This factor is of great importance in the propagation of ultrasound in living organisms, whose tissues have a wide variety of acoustic resistances (for example, at the boundaries of the muscle - periosteum - bone, on the surfaces of hollow organs, etc.). Since the acoustic resistance of biological tissues is, on average, hundreds of times higher than the acoustic resistance of air, almost complete reflection of ultrasound occurs at the air-tissue interface. This creates certain difficulties in ultrasound therapy, since the air layer of only 0.01 mm between the vibrator and the skin is an insurmountable obstacle to ultrasound. Since it is impossible to avoid air layers between the skin and the emitter, special contact substances are used to fill the irregularities between them, which must meet certain requirements: have an acoustic resistance close to the acoustic resistance of the skin and the emitter, have a low ultrasound absorption coefficient, have a significant viscosity and wet the skin, be non-toxic to the body. Vaseline oil, glycerin, lanolin and even water are usually used as contact substances.
RECEIVING AND REGISTRATION OF ULTRASOUND
To obtain ultrasound, mechanical and electromechanical generators are used.
Mechanical generators include gas-jet emitters and sirens. In gas-jet emitters (whistles and membrane generators), the source of ultrasonic energy is the kinetic energy of the gas jet. The first ultrasonic generator was the Galton whistle - a short, closed at one end tube with sharp edges, to which an air stream is directed from an annular nozzle. Jet breaks at the sharp ends of the tube cause air oscillations, the frequency of which is determined by the length of the tube. Galton whistles allow you to receive ultrasound with a frequency of up to 50 kHz. It is interesting that poachers used similar whistles in the last century, calling hunting dogs with signals that are not audible to humans.
Sirens allow you to receive ultrasound with a frequency of up to 500 kHz. Gas-jet radiators and sirens are almost the only sources of powerful acoustic oscillations in gaseous media, into which, due to the low acoustic resistance, radiators with a solid oscillating surface cannot transmit high-intensity ultrasound. The disadvantage of mechanical generators is the wide range of frequencies emitted by them, which limits their scope in biology.
Electromechanical sources of ultrasound convert the electrical energy supplied to them into the energy of acoustic vibrations. The most widespread are piezoelectric and magnetostrictive emitters.
In 1880, French scientists Pierre and Jacques Curie discovered a phenomenon called piezoelectric effect(gr. piezo- I press). If cut in a certain way from crystals of certain substances (quartz, Rochelle salt); plate and compress it, then opposite electric charges will appear on its faces. When compression is replaced by tension, the signs of the charges change. The piezoelectric effect is reversible. This means that if a crystal is placed in an electric field, it will stretch or shrink depending on the direction of the electric field strength vector. In an alternating electric field, the crystal will deform in time with changes in the directions of the intensity vector and act on the surrounding substance as a piston, creating compression and rarefaction, i.e., a longitudinal acoustic wave.
The direct piezoelectric effect is used in ultrasound receivers, in which acoustic vibrations are converted into electrical ones. But if an alternating voltage of the corresponding frequency is applied to such a receiver, then it is converted into ultrasonic vibrations and the receiver works as a radiator. Consequently, one and the same crystal can serve as both a receiver and an ultrasound emitter in turn. Such a device is called an ultrasonic acoustic transducer (Fig.). Due to the fact that the use of ultrasound in various fields of science, technology, medicine and veterinary medicine is increasing every year, an increasing number of ultrasonic transducers are required, but the reserves of natural quartz cannot satisfy the growing demand for it. The most suitable substitute for quartz turned out to be barium titanate, which is an amorphous mixture of two mineral substances - barium carbonate and titanium dioxide. To give it the desired properties, the amorphous mass is heated to a high temperature, at which it softens, and placed in an electric field. In this case, the polarization of dipole molecules occurs. After the substance is cooled in an electric field, the molecules are fixed in the orientation position and the substance acquires a certain electric dipole moment. Barium titanate has a piezoelectric effect 50 times stronger than quartz, and its cost is low.
Other type converters are based on the phenomenon magnetostriction(lat. strictura - contraction). This phenomenon lies in the fact that during magnetization, the ferromagnetic rod is compressed or stretched, depending on the direction of magnetization. If the rod is placed in an alternating magnetic field, then its length will change in time with changes in the electric current that creates the magnetic field. The deformation of the rod creates an acoustic wave in the environment.
For the manufacture of magnetostrictive transducers, permendur, nickel, iron-aluminum alloys - alsifers are used. They have large values of relative deformations, high mechanical density and less sensitivity to temperature effects.
Both types of transducers are used in modern ultrasonic equipment. Piezoelectric ones are used to obtain high-frequency ultrasound (above 100 kHz), magnetostrictive - to obtain lower frequency ultrasound. For medical and veterinary purposes, generators of small power (10–20 W) are usually used (Fig.).
INTERACTION OF ULTRASOUND WITH SUBSTANCE
Let us consider what parameters of oscillatory motion have to deal with when ultrasound propagates in a substance. Let the emitter create a wave with intensity I\u003d 10 5 W / m 2 and a frequency of 10 5 Hz. I= 0,5rcA 2 w 2 = 2cA 2 rp 2 n 2. From here
Substituting in the formula the values of the quantities included in it, we obtain that the displacement amplitude of water particles under the given conditions A= 0.6 µm. The amplitude value of the acceleration of water particles a m = Ah 2 \u003d 2 4 10 5 m / s 2, which is 24,000 times greater than the acceleration of gravity. Peak value of acoustic pressure R a = rcAw\u003d 5.6 10 5 Pa @ 6 atm. When focusing ultrasound, even higher pressures are obtained.
When an ultrasonic wave propagates in a liquid during rarefaction half-periods, tensile forces arise, which can lead to a rupture of the liquid in a given place and the formation of bubbles filled with the vapor of this liquid. This phenomenon is called cavitation(lat. cavum - emptiness). Cavitation bubbles form when the tensile stress in a liquid becomes greater than some critical value called the cavitation threshold. For pure water, the theoretical value of the cavitation threshold r to\u003d 1.5 10 8 Pa \u003d 1500 atm. Real liquids are less durable due to the fact that they always contain cavitation nuclei - microscopic gas bubbles, solid particles with cracks filled with gas, etc. Electric charges often appear on the surface of the bubbles. The collapse of cavitation bubbles is accompanied by a strong heating of their contents, as well as the release of gases containing atomic and ionized components. As a result, the substance in the cavitation region is subjected to intense influences. This manifests itself in cavitation erosion, i.e., in the destruction of the surface of solids. Even such strong substances as steel and quartz are destroyed under the action of microshock hydrodynamic waves arising from the collapse of bubbles, not to mention biological objects in the liquid, such as microorganisms. This is used to clean the surface of metals from scale, fatty films, as well as to disperse solids and obtain emulsions of immiscible liquids.
When the intensity of ultrasound is less than 0.3-10 4 W/m 2 cavitation in the tissues does not occur, and ultrasound causes a number of other effects. Thus, acoustic flows, or “sonic wind”, arise in a liquid, the speed of which reaches tens of centimeters per second. Acoustic flows mix the irradiated liquids and change the physical properties of suspensions. If there are particles in the liquid that have opposite electric charges and different masses, then in the ultrasonic wave these particles will deviate from the equilibrium position at different distances and a variable potential difference arises in the wave field (the Debye effect). Such a phenomenon occurs, for example, in a solution of table salt containing H + ions and 35 times heavier C1 - ions. With large differences in masses, the Debye potential can reach tens and hundreds of mV.
The absorption of ultrasound by a substance is accompanied by the transition of mechanical energy into thermal energy. Heat is generated in areas adjacent to the interface between two media with different acoustic impedances. When ultrasound is reflected, the intensity of the wave near the boundary increases and, accordingly, the amount of absorbed energy increases. It is easy to verify this by pressing the emitter to a wet hand. Soon, a pain sensation arises on the opposite side of the arm, similar to the pain from a burn, caused by ultrasound reflected at the skin-air interface. However, the thermal effect of ultrasound at the intensities used in therapy is very small.
In an ultrasonic field, both oxidative and reduction reactions can occur, and even those that are not feasible under normal conditions. One of the characteristic reactions is the splitting of a water molecule into H + and OH - radicals, followed by the formation of hydrogen peroxide H 2 O 2 and some fatty acids. Ultrasound has a significant effect on some biochemical compounds: amino acid molecules are torn off from protein molecules, proteins are denatured, etc. All these reactions are stimulated, obviously, by the colossal pressures that arise in shock cavitation waves, but at present there is still no complete theory of sound-chemical reactions. exists.
Ultrasound causes the glow of water and some other liquids (US luminescence). This glow is very weak and is usually recorded by photomultipliers. The reason for the glow is mainly that when the cavitation bubbles collapse, a strong adiabatic heating of the vapor enclosed in them occurs. The temperature inside the bubbles can reach 10 4 K, which leads to the excitation of gas atoms and the emission of light quanta by them. The intensity of ultrasonic luminescence depends on the amount of gas in the bubble, on the properties of the liquid, and on the intensity of ultrasound. This phenomenon carries information about the nature and kinetics of the processes that occur when a liquid is irradiated with ultrasound. As was shown by V. B. Akopyan and A. I. Zhuravlev, in some diseases of ultrasound, the luminescence of a number of biological fluids changes, which can form the basis for the diagnosis of these diseases.
ACTION OF ULTRASOUND ON BIOLOGICAL OBJECTS
Ultrasound, like other physical factors, has a perturbing effect on living organisms, resulting in adaptive reactions of the body. The mechanism of the disturbing action of ultrasound has not yet been sufficiently studied, but it can be argued that it is determined by a combination of mechanical, thermal, and physicochemical actions. The effectiveness of these factors depends on the frequency and intensity of the ultrasound. Above, the amplitude values of the acoustic pressure and acceleration of the particles of the medium in the ultrasonic wave were calculated, which turned out to be very large, but they do not give an idea of the mechanical forces per cell. The calculation of the forces acting on a cell in an ultrasonic field was carried out by V. B. Akopyan, who showed that if ultrasound with a frequency of 1 MHz and an intensity of 10 4 W/m 2 acts on a cell of 5 10 - 5 m, then the maximum difference tensile and compressive forces at opposite ends of the cell does not exceed 10–13 N. Such forces cannot have a noticeable effect on the cell, not to mention its destruction. Therefore, tensile and compressive forces acting on a cell in an ultrasonic wave can hardly lead to tangible biological consequences.
Apparently, acoustic flows leading to the transfer of matter and mixing of the liquid are more efficient. Inside a cell with a complex internal structure, microflows may well change the mutual arrangement of cell organelles, mix the cytoplasm and change its viscosity, detach biological macromolecules (enzymes, hormones, antigens) from cell membranes, change the surface charge, membranes and their permeability, affecting cell viability. If the membranes are not damaged, then after some time the macromolecules that have passed into the extracellular medium or into the cytoplasm return back to the surface of the membranes, although it is not known whether they get exactly to the places from which they were torn out, and if not, whether this leads to what or disturbances in cell physiology.
Membrane destruction occurs at sufficiently high ultrasound intensities, however, different cells have different resistance: some cells are already destroyed at intensities of the order of 0.1 10 4 W / m 2, while others withstand intensity up to 25 10 4 W / m 2 and higher . As a rule, cells of animal tissues are more sensitive and plant cells protected by a strong membrane are less sensitive. The different ultrasonic resistance of erythrocytes was discussed in Chapter I. Ultrasonic irradiation with an intensity of more than 0.3·10 4 W / m 2 (ie, above the cavitation threshold) is used to destroy bacteria and viruses present in the liquid. This is how typhoid and tubercle bacilli, streptococci, etc. are destroyed. It should be noted that irradiation with ultrasound with an intensity less than the cavitation threshold can lead to an increase in the vital activity of cells and an increase in the number of these microorganisms, which instead of a positive effect will lead to a negative one. Ultrasound used in therapy and diagnostics does not cause cavitation in tissues. This is due either to deliberately low intensities (from 0.05 to 0.1 W / cm 2), or the use of intense (up to 1 kW / cm 2), but short pulses (from 1 to 10 μs) for echolocation of internal organs. The time-averaged intensity of ultrasound is also in this case not higher than 0.1-10 4 W/m 2 , which is insufficient for cavitation to occur.
Heating of tissues during their irradiation with therapeutic ultrasound is very insignificant. Thus, when individual organs of cows are irradiated at the site of exposure to ultrasound, the skin temperature rises by no more than 1 °C at an intensity of 10 4 W/m 2 . When irradiated with ultrasound, heat is mainly released not in the bulk of the tissue, but at the interfaces between tissues with different acoustic resistances, or in the same tissue at the inhomogeneities of its structure. It is possible that this explains the fact that tissues with a complex structure (lungs) are more sensitive to ultrasound than homogeneous tissues (liver, etc.). Relatively much heat is released at the border of soft tissues and bone.
No less significant may be the effects associated with the Debye potential. Diagnostic ultrasound pulses can cause Debye potential up to hundreds of mV in tissues, which is comparable in order of magnitude to the potentials of cell membranes, and this can cause membrane depolarization and increase their permeability with respect to ions involved in cell metabolism. It should be noted that a change in the permeability of cell membranes is a universal response to ultrasonic exposure, regardless of which of the ultrasonic factors acting on cells prevails in one case or another.
Thus, the biological action of ultrasound is due to many interconnected processes, some of which have not been sufficiently studied to date and the description of which is not included in the task of the textbook. According to V.B. Hakobyan, ultrasound causes the following chain of transformations in biological objects: ultrasonic action ® microflows in the cell ® increase in the permeability of cell membranes ® change in the composition of the intracellular environment ® violation of optimal conditions for enzymatic processes ® suppression of enzymatic reactions in the cell ® synthesis of new enzymes in the cell, etc. The threshold for the biological effect of ultrasound will be such a value of its intensity at which there is no violation of the permeability of cell membranes, i.e., the intensity is not higher than 0.01·10 4 W/m 2 .
Ultrasound, which has a strong biological property, can be used in agriculture. The experiments of recent years have shown the promise of the impact of low-frequency ultrasound on the seeds of cereals and horticultural crops, fodder and ornamental plants.
ULTRASOUND IN THE ANIMAL WORLD
Some nocturnal birds use the sounds of the audible range for echolocation (nightjars, salangan swifts). Nightjars, for example, make sharp, staccato calls with a frequency of 7 kHz. After each call, the bird picks up the sound reflected from the obstacle, and recognizes the location of this obstacle in the direction from which the echo came. Knowing the speed of sound propagation and the time elapsed from its emission to reception, it is possible to calculate the distance to the obstacle. Of course, the bird does not make such calculations, but somehow its brain allows it to navigate well in space.
Ultrasonic echolocation organs have reached the greatest perfection in bats. Since insects serve as food for them, i.e., objects of small size, to reduce diffraction on such objects, it is necessary to use oscillations with a small wavelength. Indeed, if we assume that the size of the insect is 3 mm, then the diffraction on it will be insignificant at a wavelength of the same order of magnitude, and for this the oscillation frequency should be at least equal to n = c/l= 340/3 10 –3 » 10 5 Hz = 100 kHz. This implies the need to use ultrasound for echolocation, and, indeed, bats emit signals with frequencies of the order of 100 kHz. The process of echolocation is as follows. The animal emits a signal with a duration of 1–2 ms, and during this time its sensitive ears are closed by special muscles. Then the signal stops, the ears open, and the bat hears the reflected signal. During the hunt, the signals follow one after another up to 250 times per second.
The sensitivity of the echolocation apparatus of bats is very high. So, for example, Griffin stretched in a dark room a grid of metal wires with a diameter of 0.12 mm with a distance between the wires of 30 cm, which only slightly exceeded the wingspan of bats. Nevertheless, the animals flew freely around the room without touching the wires. The power of the signal perceived by them, reflected from the wire, was about 10–17 W. The ability of bats to isolate the desired signal from the chaos of sounds is also amazing. During the hunt, each bat perceives only those ultrasonic signals that it emits itself. Obviously, the organs of these animals have a strict resonant tuning to signals of a certain frequency, and they do not respond to signals that differ from their own by only a fraction of a hertz. So far, no location device created by man has such selectivity and sensitivity. Dolphins widely use ultrasonic location. The sensitivity of their locator is so great that they can detect a pellet lowered into the water at a distance of 20–30 m. The frequency range emitted by dolphins ranges from a few tens of hertz to 250 kHz, but the intensity is maximum at 20–60 kHz. For intraspecific communication, dolphins use sounds in the range audible to humans, up to about 400 Hz.
