Ultrasound

ULTRASOUND, elastic vibrations and waves with frequencies approximately from 1,5-2*104 hz (15-20 kHz) and up to 109 Hz (1 Ghz); the frequency range from 109 to 1012-13 Hz is assumed to be. hypersound. The frequency range of the UHF can be divided into three subdomains: the low frequency range (1.5*104-10s Hz) - UHF, the medium frequency range (105 - 107 Hz) - UHF and the high frequency range of the UHF (10G-109 Hz) - UHF. Each of these subdomains is characterized by its own specifics. features of generation, reception, distribution and application.

Physical properties and features of ultrasound propagation. By its physical nature. in nature, U. represents elastic waves and in this it does not differ from sound. The frequency boundary between sound and ultrasonic waves is therefore conditional; it is determined by the subjective properties of the human. hearing and corresponds to the average upper limit of the audible sound. However, due to higher frequencies and, consequently, small wavelengths, there are a number of features of the propagation of Y. So, for UHF, the wavelengths in air are 3.4 *10-3-3.4*10-5 cm, in water 1.5*10-2-1.5 * 10-4 cm and in steel 5*10-2- 5 • 10-4 see U. in gases and, in particular, in air, it propagates with a large attenuation (see Sound absorption). Liquids and solids (especially single crystals) are, as a rule, good conductors of U, attenuation in k is much less. So, for example, in water, the attenuation of U. all other things being equal, approx. 1000 times less than in the air. Therefore, the areas of use of UHF and UHF relate almost exclusively to liquids and solids, and only UHF is used in air and gases. Due to the small wavelength of U. the nature of its propagation is affected by the molecular structure of the medium, therefore, measuring the velocity of U. with and absorption coefficient a, it is possible to judge the molecular properties of the substance. Molecular acoustics deals with these issues. A characteristic feature of the propagation of sound in gases and liquids is the existence of distinct areas of dispersion, accompanied by a sharp increase in its absorption (see Sound dispersion). The coefficient. The absorption of U in a number of liquids significantly exceeds that calculated according to classical theory and does not detect the increase predicted by this theory proportional to the square of the frequency. All these effects are explained in the relaxation theory (see Relaxation), which describes the propagation of U. in any media and is theoretical. the basis of modern molecular acoustics, and the main experiment. The method is to measure the dependence of c and especially a on frequency and on external conditions (temperature, pressure, etc.).

The combination of seals and rarefactions accompanying the propagation of an ultrasonic wave is a kind of lattice, the diffraction of light waves on which can be observed in optically transparent bodies. The small length of ultrasonic waves is the basis for considering their propagation in a number of cases by methods

geometric acoustics. Physically, this leads to a radiation pattern of propagation. This implies such properties of U as the possibility of geometric. reflections and refractions, as well as focusing of sound (fig. 1).

The next important feature of U. is the possibility of obtaining high intensity even with relatively small amplitudes of oscillations, because at a given amplitude, the energy flux density is proportional to the square of the frequency. Ultrasonic waves of high intensity are accompanied by a number of effects that can be described.only the laws of nonlinear acoustics. Thus, the propagation of ultrasonic waves in gases and liquids is accompanied by the movement of the medium, which is called acoustic techeniem (fig. 2). The speed of acoustic. the flow rate depends on the viscosity of the medium, the intensity of the current and its frequency; generally speaking, it is small and amounts to fractions % from the speed of Y .

Among the important nonlinear phenomena that occur during the propagation of intense U. in liquids, it is acoustic. cavitation is the growth in the ultrasonic field of bubbles from the available submicroscopic. germs of gas or steam in liquids up to the size of fractions of mm, which begin to pulsate with a frequency of U. and slam shut in the posit. the pressure phase. When gas bubbles close, large local pressures of the order of thousands of atmospheres arise, spherical particles are formed. shock waves. Acoustic waves are formed near pulsating bubbles. micro-streams. Phenomena in the cavitation field lead to a number of both useful (obtaining emulsions, cleaning of contaminated parts, etc.) and harmful (erosion of emitters) phenomena. The frequency of U., in which ultrasonic cavitation is used in the technological. goals, lie in the area of the UNC. The intensity corresponding to the cavitation threshold depends on the type of liquid, sound frequency, tempo, and other factors. In water at a frequency of 20 kHz, it is about 0.3 w / cm2. At frequencies in the UHF range in an ultrasonic field with an intensity of several w/cm2, liquid gushing may occur (Fig. 3) and spraying it with the formation of a very fine mist.

Ultrasound generation. To generate ultrasonic vibrations, a variety of devices are used, which can be divided into 2 main groups - mechanical, in which the source of U is mechanical. the energy of the gas or liquid flow, and electromechanical, in which ultrasonic energy is obtained by converting electrical. Mechanical emitters of U.- air and liquid whistles and sirens - differ in comparison. simplicity of the device and operation, do not require expensive electricity. high frequency energy, their efficiency is 10-20%. The main drawback of all is mechanical. ultrasonic emitters - a relatively wide range of radiated frequencies and instability of frequency and amplitude, which does not allow them to be used for control and measurement. they are used mainly in industrial ultrasonic technology and partially as signaling devices.

The main method of U. radiation is the conversion of electricity in one way or another. the vibrations in the vibrations are mechanical. In the UHF range, it is possible to use electrodynamic and electrostatic emitters. U. emitters using the magnetostrictive effect (see Magnetostriction) in nickel and in a number of special alloys, as well as in ferrites, have found wide application in this frequency range. The phenomenon of piezoelectricity is used for the emission of UHF and UHF, mainly by piezoelectric materials for emitters. piezoquartz, lithium niobate, potassium dihydrophosphate serve, and in the range of UHF and UHF-mainly various piezo-ceramic materials. Magnetostrictive emitters are a core of a rod or ring shape with a winding through which alternating current flows, and piezoelectric emitters are a plate (Fig. 4) or a rod made of piezoelectric. of a material with metallic electrodes, to which alternating electricity is applied. voltage. In the UHF range, composite piezo-emitters, in which piezoceramic plates are clamped between metals, have become widespread. blocks. As a rule, vibrations of magnetostrictive and piezoelectric elements at their own resonant frequency are used to increase the amplitude of vibrations and the power radiated into the medium.

The maximum intensity of the radiation is determined by the strength and nonlinear properties of the material of the emitters, as well as the features of the use of emitters. The intensity range when generating Y . in the USF region, it is extremely wide: intensities from 10-14-10-15 w/cm2 to 0.1 w/cm2 are considered small. For many purposes, it is necessary to obtain much higher intensities than those that can be obtained from the surface of the emitter. In these cases, you can use the focusby rov U . So, in the focus of the paraboloid, the inner walls of which are made of a mosaic of quartz plates or of piezoceramics of barium titanate, at a frequency of 0.5 mhz it is possible to obtain in water intensities of U greater than 105 w / cm2. To increase the amplitude of vibrations of solids in the range of UNF, rod ultrasonic concentrators are often used (see Acoustic concentrator), which allow to obtain displacement amplitudes up to 10-4 cm.

The choice of the method of generating the U. depends on the frequency range of the U., the nature of the medium (gas, liquid, solid), the type of elastic waves and the required radiation intensity.

Ultrasound reception and detection. Due to the reversibility of the piezoelectric effect , it is widely used for the reception of U . The study of the ultrasonic field can also be performed optically. methods: U., spreading in the K.-L. environment, causes a change in its optical properties. refractive index, so that it can be visualized if the medium is transparent to light. The related field of acoustics and optics (acousto-optics) has been greatly developed, especially after the advent of continuous-acting gas lasers; research has developed on light diffraction on U. and its various applications.

Ultrasound applications.

The applications of U. are extremely diverse. U. serves as a powerful method of studying various phenomena in many areas of physics. For example, ultrasonic methods are used in solid state physics and semiconductor physics; a whole new field of physics has emerged - acoustoelectronics, on the basis of the achievements of which various devices for processing signal information in microelectronics are being developed. U. plays an important role in the study of matter. Along with the methods of molecular acoustics for liquids and gases, in the field of the study of solids, the measurement of velocity with and coefficient.

absorption a is used to determine the elastic modulus and dissipative characteristics of a substance. Quantum acoustics has been developed, studying the interaction of quanta of elastic perturbations - phonons - with electrons, magnons, etc. quasiparticles and elementary excitations in solids. U. is widely used in engineering, as well as ultrasonic methods are increasingly penetrating biology and medicine.