Ticket number 1.
Physical foundations of ultrasonic flaw detection
The concept of acoustic oscillations and waves
· Acoustic waves Called the mechanical oscillations of the medium particles in the elastic medium.
When the wavelength moves, the particles do not move, but make oscillations near their equilibrium positions.
· The distance between the nearest particles, fluctuating in the same phase, is called wavelength .
The wavelength is associated with the speed of distribution. FROM and frequency f. (or period T. ) By the ratio
where: - the wavelength [m]; FROM - the propagation of the distribution [m / s];
T. - period [s]; f. - frequency Hz].
For example for air: FROM\u003d 330 m / s
f.\u003d 20 Hz ® \u003d 16.5 m;
f.\u003d 20000 Hz ® \u003d 1.65 cm;
f.\u003d 20000000 Hz ® \u003d 0.165 mm;
Depending on the direction of particle oscillations relative to the direction of distribution of the wave distinguishes: longitudinal, transverse, surface and normal waves (waves in plates).
In the longitudinal wave, the particle fluctuates along the direction of the wave propagation. Oscillations can spread in solid, liquid and gaseous media.
If the direction of oscillations of particles of the medium is perpendicular to the direction of distribution, then such oscillations are called transverse (or shift). They can spread only in a medium that has elasticity of form.
Longitudinal and transverse waves can spread in pure form only in an unlimited medium (¥ or ¥ / 2) or in the body, the dimensions of which in directions that do not coincide with the direction of the wave propagation, significantly exceed the last length. Schematically, longitudinal and transverse waves are presented in Fig. one.
Fig. 1 Distribution of longitudinal and transverse waves
On a free surface can spread surface waves (Rayleigh waves).In the surface wave of a particle simultaneously perform oscillations in the direction of distribution and perpendicular to it, describing elliptic or more complex trajectories. The amplitude of oscillations as it removes deployed from the surface of the exponent, therefore the wave is localized in a thin surface layer with a thickness into one - one and a half wavelength and follows the bends of the surface. 2.
Fig. 2 Spreading surface waves
When the wave is propagated in flat bodies with a constant thickness (sheets, thin plates, wire), normal waves or lamb waves may occur. At the same time, particles make oscillations on the same trajectories as in the surface wave, but on the entire thickness of the sheet, the shell plate. Typically, independently two normal waves symmetric (compression wave or stretching) and antisymmetric (bending wave) rice are arising and distributed. 3.
Fig. 3 waves in the plates
a - symmetric, b - asymmetric
The speed of propagation of the longitudinal, transverse and surface waves is determined by the elastic properties of the material (modules of elasticity and shift, Poisson coefficients) and its density. The speed of propagation of normal waves, in contrast to the speed of the propagation of other types of waves, does not depend on the properties of the material, but also from the frequency of sound oscillations and thickness of the product.
With l\u003e with t\u003e with s; With t ~ 0.55 c L with s ~ 0.93 s t.
Acoustic waves also differ in the form of a wave front or wave surface.
· Front of waves This is a geometric location of the points of the medium in which the wave phase of the wave has the same meaning.
If a short-term perturbation (impulse) is distributed in the environment, front of the wave It is called the border between the perturbed and unperturbed areas of the medium.
The front or wave surface is continuously moved in the medium and at the same time deformed. In an unlimited isotropic medium, the spread of elastic waves has a spatial character, and, depending on the form of the front, the waves may be flat, sphericaland cylindricalfigure 4.
Fig. 4 flat, spherical, cylindrical waves
· Flat waves The plate is excited if its transverse dimensions are far exceeding the wavelength. Wave surfaces of a flat wave have the form of parallel planes.
· Spherical waves They are excited by a point source or oscillating ball body, the dimensions of which are small. The wave surfaces of the spherical wave have the form of concentric spheres.
· Cylindrical waves They are excited by a cylindrical body (rod, cylinder, etc.) the length of which is significantly its transverse dimensions. Wave surfaces have the form of concentric cylinders.
At very large distances, spherical and cylindrical waves go to flat.
Depending on the frequencies, the following waves differ:
· Infrasound f \u003d up to 16-20 Hz;
· Sound F \u003d 16 - 20000 Hz;
· Ultrasound f \u003d 20 kHz - 1000 MHz;
· Hypersonic F\u003e 1000 MHz.
For defectoscopy purposes, waves of various ranges are used:
Sound f \u003d 1-8 kHz;
Ultrasonic f \u003d 20 kHz - 50 MHz;
Currently, work is underway and it is possible to obtain frequencies up to 1000 MHz.
The wavelength of hypersonic oscillations is comparable to the wavelength of visible light waves. This makes them similar in its properties with the properties of light rays, so many tasks are considered from the point of view of geometric acoustics.
· Geometric acoustics - Simplified theory of propagation of sound, neglecting diffraction phenomena.
Geometric acoustics is based on the presentation of sound rays, along each of which sound energy extends regardless of the adjacent rays. In a homogeneous environment, sound rays are straight lines.
From a mathematical point of view, geometric acoustics have an extreme case of the wave theory of sound propagation when the wavelength is striving to 0 and in this respect, similar to geometric optics in the theory of light propagation.
Shortwave ultrasound - oscillations are distributed as directed rays. Like light rays, they can reflect, refracted, focus, interfer, and with them not only with themselves, but also with light, test diffraction and fade as you distribute.
The wavelength of hypersonic waves can be comparable to the sizes of atoms. In this case, the quantum character of such a wave begins to appear and, by analogy with a light flux, such a flow of sound energy is possible to be considered as a flow of particles (phonons), which can no longer interact with finite volumes of the substance or crystals, but already with an atom electrons. At the same time, various effects of such interaction arise, which allow you to study a wider range of physical characteristics of materials.
On the other hand, infrasonic waves have large lengths, pass over long distances, which makes it possible to control the physical properties of large solids (eg in geological exploration).
The acoustic waves of the ultrasound range have properties very much reflected from the border of the solid body - air. Calculations show that air layers with a thickness of 10 -5 mm and more at f \u003d 5 MHz 100% reflection of the sent energy, with a layer thickness<10 -5 мм отражение составляет ~ 90%, а слой толщиной 10-6 мм отражает ~ 80% посланной энергии. Благодаря этому свойству УЗ - колебания эффективно отражаются от трещин, воздушных полостей и т.д., что позволяет их легко обнаружить.
All the above has led to the widespread acoustic methods for controlling the quality of materials and products.
Oscillation - This is a movement around a certain average position with repeatability (for example, the oscillation of the pendulum). Any oscillating body seeks to the position of equilibrium.
Waves - oscillatory movements propagating in space: oscillations of one point are transmitted neighboring, etc.
Sound- These are mechanical oscillations that apply to an elastic medium (air, water, solid bodies).
Infrase< 16 Гц
Sound 16 - 20000 Hz
Ultrasound 20000 - 109 Hz
Hyperzvuk\u003e 109 Hz
Heat oscillations\u003e 1012 Hz
1kHz \u003d 103 Hz, 1 MHz \u003d 106 Hz
In ultrasonic flaw detection, frequencies are used from 0.6 to 10 MHz.
The process of propagation of ultrasound in space is wave.
Wave Front- This is a combination of particles to which hesitations have reached this time. The front geometry is distinguished by spherical (for example, a sound wave at a short distance from a point source), cylindrical (for example, a sound wave at a short distance from the sound source, which is a long cylinder of small diameter), flat waves (flat wave can emit an infinite fluctuating plate ).
Introduction
Elasticity is the property of solid bodies to restore its shape and volume (and liquids and gases - only volume) after the termination of the external forces. Wednesday with elasticity is called an elastic medium. Elastic oscillations are oscillations of mechanical systems, an elastic medium or its parts arising under the action of mechanical perturbation. Elastic or acoustic waves are mechanical perturbations propagating in an elastic environment. A special case of acoustic waves - a sound audible sound, hence the term acoustics, the term acoustics (from Greek. Akustikos- auditory) in a broad sense of the word - the doctrine of elastic waves, in a narrow - the doctrine of sound. Depending on the frequency, the elastic oscillations and waves are called differently.
Table 1 - frequency bands of elastic oscillations
Elastic oscillations and acoustic waves, especially the ultrasound range, are widely used in the technique. Powerful low-frequency ultrasound fluctuations are used for local destruction of fragile durable materials (ultrasound slobs); dispersion (fine grinding of solid or liquid bodies in any environment, such as fat in water); Coagulation (consolidation of particles of substance, such as smoke) and other purposes. Another area of \u200b\u200bapplication of acoustic oscillations and waves is control and measurement. This includes sound and ultrasound location, ultrasound medical diagnosis, fluid level control, flow rate, pressure, temperature in vessels and pipelines, as well as the use of acoustic oscillations and waves for non-destructive testing (NK).
In your test work, I plan to consider acoustic methods of controlling materials, their types and features.
1. Types of acoustic waves
Acoustic control methods are used by small amplitude waves. This is a region of linear acoustics, where the voltage (or pressure) is proportional to the deformation. The region of oscillations with large amplitudes or intensities, where there is no such proportionality, refers to nonlinear acoustics.
In an unlimited solid medium, there are two types of waves that apply to different speeds: longitudinal and transverse.
Fig. 1 - a schematic representation of longitudinal (a) and transverse (b) waves
Wave u L. Call longitian A wave or wave of compression expansion (Fig. 1. a), because the direction of oscillations in the wave coincides with the direction of its propagation.
Wave u T. Call transverse or shear wave (Fig. 1. b). The direction of oscillations in it is perpendicular to the direction of the spread of the wave, and the deformations in it shifts. There are no transverse waves in liquids and gases, since there is no elasticity of the form in these environments. Longitudinal and transverse waves (their generalized name - volumetric waves) Most widely used to control the materials. These waves are best detected by defects with a normal drop on their surface.
Along the surface of the solid is distributed surface (Rayleigh waves) and head (creeping, quasi-born) Waves .
Fig. 2 - a schematic representation of waves on a free surface of a solid: A - Raleevsky, b - head
The surface wave is successfully used to detect defects near the surface of the product. It selectively responds to defects depending on the depth of their occurrence. Defects located on the surface give the maximum reflection, and at a depth, the wavelength is almost not detected.
A quasi-cell (head) wave almost does not respond to surface defects and surface irregularities, at the same time, with its help, subsurface defects can be detected in the layer, ranging from the depth of about 1 ... 2 mm. Control of thin products with such waves interfere with side transverse waves, which are reflected from the opposite surface OK and give false signals.
If two hard environments are bordered (Fig. 3, c), the modules of elasticity and density of which are not much different, then along the border spreads wave Stonelli (or worsley), such waves are used to monitor the connection of bimetals.
Transverse waves propagating along the border of the section of two media and having horizontal polarization are called waves Lyava . They arise when on the surface of a solid half-space is a layer of solid material. The speed of propagation in which transverse waves is less than in the half-space. The depth of penetration of the wave in the half-space increases with a decrease in the thickness of the layer. In the absence of a layer of a wave of Lyava in the half-space turns into bulk, i.e. In a flat, horizontally polarized, transverse wave. Lyava waves are used to control the quality of coatings (plating) applied to the surface.
Fig. 3 - waves on the border of two environments: A - Flowering Rayleev type on the border of a solid body - liquid, b - weakly-tightened on the same border, B - Wave Stoneley on the border of two solids
If the solid has two free surfaces (plate), then it may exist specific types of elastic waves. They are called waves in the plates or waves Lamba and refer to K. normal waves, i.e. waves running (carrying energy) along plate, layer or rod, and stucking (not carrying energy) in perpendicular direction. Normal waves apply to the plate, as in a waveguide, over long distances. They are successfully used to control sheets, shells, pipes with a thickness of 3 ... 5 mm and less.
Also allocate a special kind of waves - ultrasound Waves. They do not differ in nature from the waves of the audible range and are subject to the same physical laws. But, ultrasound has specific features that have identified its widespread use in science and technology. The reflection, refraction and the possibility of focusing ultrasound is used in ultrasonic flaw detection, in ultrasound acoustic microscopes, in medical diagnostics, for the study of macro-generic substances. The presence of heterogeneities and their coordinates are determined by reflected signals or by the structure of the shadow.
2. refraction, reflection, diffraction, refraction of acoustic waves
Refraction - The phenomenon of changes to the path of the light beam (or other waves) arising on the boundary of the section of two transparent (permeable for these waves) of media or in the thickness of the medium with continuously changing properties.
Refraction of sound - change of distribution direction sound wave When it passes through the border of the section of two environments.
When falling on the border of the section of two homogeneous media (air - wall, air - aqueous surface, etc.) Flat sound wave can partially reflect And partially refracted (to go through the second Wednesday.
Prerequisite for refraction is the difference sound distribution speeds In both environments.
According to the refraction law, the refracted beam (OL ") lies in the same plane with a falling beam (OL) and a normal to the surface of the medium partition, conducted at the fall point O. The ratio of the sinus of the angle of the fall α To the sinus of the corner of refraction β Equal to the ratio of sound waves in the first and second media C 1 and C 2. (Snellius law):
sINα / SINβ \u003d C 1 / C 2
From the refractive law it follows that the higher the speed of the sound in a particular environment, the greater the refractive angle.
If the speed of sound in the second medium is smaller than in the first, then the refractive angle will be less than the angle of the fall, if the speed in the second medium is greater, then the refractive angle will be larger than the angle of the fall. If specific speaker Both media are close to each other, then almost all the energy will switch from one environment to another.
An important characteristic of the medium is a specific acoustic impedance determining the conditions of the refractiveness of the sound at its border. With a normal drop in the flat wave on the flat boundary of the sections of the two medium, the value of the refractive index is determined only by the ratio of the acoustic impedances of these environments. If acoustic impedances of the media are equal, then the wave passes the border without reflection. With a normal drop in the wave on the border of two environments. Passage coefficient W. Waves are defined only by acoustic impedances of media Z 1 \u003d ρ 1 with 1 and Z 2 \u003d ρ 2 c 2 . Frenelle formula (for normal fall) has the form:
W \u003d 2z 2 / (z 2 + z 1).
Frenelle formula for a wave falling on the border of the section at an angle:
W \u003d 2z 2 cosβ / (z 2 cosβ + z 1 cosα).
Reflection of sound - The phenomenon that occurs when the sound wave falls on the border of the separation of two elastic media and consisting of the formation of waves propagating from the border of the partition to the same environment from which the incident wave came. As a rule, the reflection is recorded by the formation of refracted waves in the second environment. A special case of reflecting sound is reflected from the free surface. It is usually considered reflection on the flat boundaries of the section, however, we can talk about the reflection of sound from the obstacles of an arbitrary form if the dimensions of the obstacle are much larger than the length of the sound wave. Otherwise takes place sound scattering or sound diffraction.
acoustic waves
Alternative descriptionsPhysical phenomenon caused by oscillations of air particles
The oscillatory movement of the elastic medium particles
What moves through the air at a speed of 330m / s?
What is heard is perceived by hearing
Silence killer
Acoustics, audio
Wave at a speed of 330 m / s
Wave
Ears perceived waves
Perceived ear
All that hears
Vowel or consonant
It is measured in decibels
We perceive it with hearing
His ear hears
Mixer mixes it
He catches his ear
Information for ears
Air fluctuations
M. All that hears the ear, which comes to hearing. Star. trash, stone scrap, litter. Sound, sound, publish, produce hum, sound, ringing. This piano sounds especially good. Sonic in Klepalo. I called, revealed the string, it was sounded only, it was walked and fell silent, did not come. I played yet. She called me tired. Sounding cf. Status on the verb. Sound, relating to sound. Sound shamehans, waves. Double, stingy, loud, mild, ringing, sound sounding. Tellness The condition of the sonor, or the property of sounding. Sound Cognition, Cancellation, Cancellation of Wed. Acoustics, science of sounds, part of physics. Sound M. Shell for measuring sounds or numbers of sound objects. Soundstroat CP. Pigeon, sounding sounds. Saw-making cf. The action of someone imites any sounds: the similarity of the word, speech, dialect, voices with any other sound. Thunder, crash, whistling, words of sound-resistant. Sound-eyed cf. consent, respectfulness, mutual slightness of sounds
Silent silver movie
The object of studying phonetics
The basis of "s" in ultrasound
Reflected echo
Add it, but not heard
Product of labor speakers
Specifies from speakers
Cross
What we catch the ears
What hears the ear
Hears
What catches your ear
Silence killer
His ear hears
Missering element of speech
What first appeared in the movie "Don Juan" (USA, 1926)
Which writes phonograph
What is removed from the string
What does microphone writes
What he hears the ear
What catches our ears
What enhances MegaFon
Rustle or roar
Rustle, crash or knock
Subject of phonetics
The oscillatory movement of the elastic medium particles
What is heard is perceived by hearing
Hearing physical phenomenon
Add it, and then not hear
What first appeared in the movie "Don Juan" (USA, 1926)?
What does the phonograph write down?
What are removed from the string?
Object of studying acoustics
What is measured in decibels?
What studies acoustics?
Enhanced by a shore
Shore and roar
What are acoustics investigate?
Acoustic wave
Wave with a frequency of 1000 hertz
Violaste silence
Hear
Waves for the ear
What does the microphone writes?
What is strengthened with a mouthpiece?
The basis of "s" in ultrasound
What does the ear hears?
What strengthens megaphone?
Wave captured by the ear
What catches our ears?
Surface acoustic waves (Surfactant) - elastic waves propagating along the surface of a solid body or along the border with other environments. Pav is divided into two types: with vertical polarization and with horizontal polarization ( waves Lyava).
The most commonly encountered special cases of surface waves include the following:
- Railey waves (or Rayleigh), in a classical understanding that extending along the boundaries of an elastic half-space with a vacuum or a fairly rarefied gas medium.
- On the border of the solid with liquid.
- running along the border of liquid and solid
- Wave Stonellipropagating along the flat boundary of two hard media, the modulus of elasticity and density of which are not widely different.
- Waves Lyava - Surface waves with horizontal polarization (SH type), which can be distributed in the structure of the elastic layer on an elastic half-space.
Encyclopedic YouTube.
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✪ Seismic waves
✪ Longitudinal and transverse waves. Sound waves. Lesson 120.
✪ Lecture Seventh: Waves
Subtitles
In this video, I want to discuss seismic waves a little. We write the topic. First, they are very interesting in themselves and, secondly, very important to understand the structure of the Earth. You have already seen my video about the layers of the Earth, and it is thanks to the seismic waves we made a conclusion from which layers our planet consists. And, although the seismic waves are usually associated with earthquakes, in fact these are any waves traveling on the ground. They may arise from an earthquake, a strong explosion, anything, which is able to send a lot of energy directly to the ground and stone. So, there are two main types of seismic waves. And we will focus more on one of them. First - surface waves. We write. The second is bulk waves. Surface waves are just waves spread over the surface of something. In our case, on the surface of the Earth. Here, on the illustration, it can be seen how the surface waves look like. They look like ripples that can be seen on the surface of the water. Surface waves are two types: Rayleigh waves and waves of Lyava. I will not spread, but here it can be seen that Rayleigh waves move up and down. Here the earth moves up and down. There is moving down. Here - up. And here - again down. Looks like a wave running around the ground. Waves of Lyava, in turn, move to the sides. That is, here the wave is not moving up and down, but if you look in the direction of the wave, it moves to the left. It moves here to the right. Here - left. Here - again right. In both cases, the wave movement is perpendicular to the direction of its movement. Sometimes such waves are called transverse. And they, as I said, look like waves in the water. Much more interesting volume waves, because, firstly, these are the fastest waves. And, moreover, it is these waves that are used to study the structure of the Earth. Volga waves are two types. There are P-waves, or primary waves. And S-waves, or secondary. They can be seen here. Such waves are the energy moving inside the body. And not just on its surface. So, in this picture, which I downloaded from Wikipedia, it can be seen how the hammer beaten by the big stone. And when the hammer hits the stone ... Let me redraw more more. Here I will have a stone, and I beat him with a hammer. He will be squeezed there, where he fell. Then the energy from the blow will push the molecule that will stay in the molecules in the neighborhood. And these molecules will die into the molecules behind them, and those, in turn, in the molecules nearby. It turns out that this compressed part of the stone moves the wave. These are compressed molecules, they will die in the molecules nearby and then here the stone will become denser. The first molecules, those that started all the movement will return to the place. Therefore, the compression has shifted, and further will move. It turns out a compression wave. You beat the hammer here and get a changing density that moves towards the wave. In our case, the molecules move forward and backward along one axis. Parallel to the direction of the wave. This is a r-waves. R-waves can spread in the air. Essentially, sound waves are the compression waves. They can move both in liquids and in solids. And, depending on the medium, they move with different speeds. In the air, they move at a speed of 330 m / s, which is not so slow for everyday life. In the liquid, they move at a speed of 1,500 m / s. And in granite, from which most of the earth's surface consists, they move at a speed of 5,000 m / s. Let me write it. 5,000 meters, or 5 km / s in granite. And the S-waves, now I will draw, because this is too small. If you hit the hammer here, the blow strength will temporarily move the stone to the side. It is a bit deformed and pulls behind the neighboring area of \u200b\u200bthe stone. Then this stone will be pulled down from above, and the stone, which was originally hit, will return up. And approximately through the millisecond layer of stone from above, a little deform to the right. And on, over time, the deformation will move up. Note that in this case the wave is also moving up. But the movement of the material is now not parallel to the axis, as in the R-waves, and perpendicularly. These perpendicular waves are also called transverse oscillations. Movement of particles perpendicular to the axis of the wave movement. This is the S-waves. They move slightly slower p-waves. Therefore, if the earthquake suddenly happens, you first feel the r-wave. And then, approximately 60% of the s-waves will come. So, for understanding the structure of the Earth it is important to remember that the S-waves can only move in solids. We write it. You could say that we saw lateral waves on the water. But there were superficial waves. And we are discussing bulk waves. Waves that pass inside the volume of water. To make it easier to imagine, I paint a little water, say, here is the pool here. In context. Something like that. Yes, I could also draw better. So, there will be a swimming pool in the context, and I hope that you will understand what happens in it. And if I squeeze a part of the water, for example, by hitting it with something very large so that the water is quickly clenched. The R-wave can move, because the water molecules will die in the molecules in the neighborhood, which will stay in the molecules behind them. And this compression, this r-wave will move towards my blow. It can be seen that the p-wave can move both in liquids and, for example, in the air. Okay. And remember that we are talking about underwater waves. Not about surfaces. Our waves are moving in the volume of water. Suppose we took the hammer and hit this volume of water from the side. And this will only arise a compression wave in this direction. And nothing more. The transverse wave will not arise, because the wave does not have that elasticity that allows its parts to range from side to side. For the S-wave, such elasticity is needed, which only happens in solids. In the future, we will use the properties of p-waves that can move in the air, liquids and solids, and the properties of the S-waves to find out what the land consists of. Subtitles by The Amara.org Community
Railey waves
Flowing waves of Rayleev type
Flowing waves of Rayleev type on the boundary of the solid with liquid.
Impeachment wave with vertical polarization
Impeachment wave with vertical polarization, running along the boundary of the liquid and solid at the speed of sound in this environment.
Acoustic waves (sound waves), perturbation of an elastic material environment (gaseous, liquid or solid), propagating in space. The perturbations are local deviations of density and pressure in medium from equilibrium values, displacement of the medium particles on the equilibrium position. These changes in the state of the medium transmitted from some particles of the substance to others characterize the audio field. In the acoustic waves, the energy transfer and the amount of movement without the transfer of the substance itself is carried out.
In gaseous and liquid media with volumetric elasticity, only longitudinal acoustic waves can be distributed, in which particle displacements coincide in the direction of the wave propagation. The sound pressure is a scalar value. In unlimited solid media, which, in addition to volume, also, and shear elasticity, along with longitudinal, transverse (shear) acoustic waves can be distributed; In them, the directions of displacements of particles and the spread of the wave are mutually perpendicular. Analogue of sound pressure in solid media is a mechanical stress tensor. If there are boundaries in solid bodies, other types of acoustic waves arise (see elastic waves).
In accordance with the type of dependence of the characteristics of the sound field on time, acoustic waves may have a different form. Of particular importance are harmonic acoustic waves, in which the characteristics of the sound field are changed over time and in the space according to the sinusoidal law (see the waves). Acoustic waves of any form can be represented as a sum (in the limiting case - integral) of harmonic waves of different frequencies. As a result of the wave decomposition on simple harmonic components (see sound analysis), the spectrum of sound is obtained.
The frequency range of acoustic waves from the bottom is practically not limited - in nature there are acoustic waves with a frequency equal to the hundredth and thousandths of Hertz. The upper limit of the range of acoustic waves is due to the physical nature of their interaction with the substance: in the gases, the wavelength should be larger than the length of the molecules, and in liquids and solid bodies there are more intermolecular or interatomic distance. On this basis, the value of 10 9 Hz was adopted for the upper frequency boundary in the gases, in liquids 10 10-10 11 Hz, in solid bodies 10 12 -10 13 Hz. In the total range, acoustic waves allocate the area of \u200b\u200bsound actually perceived by a person for rumor; The conditional boundaries of this area are 16 Hz - 20 kHz (the term "sound" is used often to acoustic waves in the entire frequency range). Below is an infrasound area, above - ultrasound (2 · 10 4 Hz - 10 9 Hz) and hypersonic (10 9 Hz - 10 13 Hz). Hypersonic waves in crystals are sometimes viewed from the standpoint of a quantum theory, comparing them phonons.
The propagation of acoustic waves is characterized primarily by sound speed. Under certain conditions, the dispersion of sound is observed - the dependence of the speed of acoustic waves from the frequency. As the propagation is used, there is a gradual attenuation of sound, i.e., a decrease in the intensity of acoustic waves. It is due to a large extent to the absorption of sound associated with an irreversible transition of an acoustic wave energy in heat. The propagation of acoustic waves is considered by the methods of wave acoustics or geometric acoustics. With a large intensity of acoustic waves, there is a distortion of their shapes and other nonlinear effects (see nonlinear acoustics).
Sound waves of the audio range serve as a means of communication of people, as well as various representatives of the animal world. Acoustic waves are used to obtain information about the properties and structure of different environments and various objects. With their help, natural environments are studied - the atmosphere, the Earth Cora, the World Ocean, the features of the structure of the substance on the microscopic level are found. In human practical activity, acoustic waves serve to detect defects in products, are used as one of the methods of medical diagnostics, are used to affect the substance to change its properties.
Lit.: Krasilnikov V. A. Sound and ultrasonic waves in the air, water and solids. 3rd ed. M., 1960; Isakovich M. A. Total acoustics. M., 1973; Bread E. Basics of acoustics: in 2 tons. M., 1976. I. P. Golovna.
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