The effects of elastic waves on biological objects elastic waves in nature, science, engineering, technology, medicine

Wednesday	Density kg/m^3	Speed UZ m/s	Wave impedance Zkg/m^2 s x 10^6
Absorption coefficient dB/cm , 1 MHz
Air	1,3	330	0,429х10^-3	12
Water	1000	1430	1,43	0,002
Blood	1060	1570	1.59	0,2
Brain	1025	1510	1.58	0,9
Fat cloth	952	1450	1,38	0,6
Muscle (average)	1075	1580	1,70	2,3
Soft fabrics (average)	1060	1540	1,63	1,0
Bones (limits changes)	1400-1900	4080	5,6 - 7,8	13

Table 2.3 shows the absorption coefficient and depth of half-absorption H at different frequencies for different tissues:

Table 2.3
Absorption coefficient values

Fabric	f (MHz)	k (see^-1 )	H (cm)
Water	1	0,001	350
	2	0,004	90
Blood	1	0,01	17
Leather	3	0,85	0,41
	5	1,05	0,33
Muscle tissue	0,8		3,6
Bone	0,88	0,71	03
	2,64	63	0,055
	45	92	0,038

2.2 Properties of ultrasonic vibrations
Man lives in a world of elastic waves. Some of them are audible, others are not perceived by the hearing organ at all. But both audible and inaudible elastic waves play an important role in human life.
If in a continuous medium - gases, liquids or solids - the particles of the medium are taken out of their equilibrium position, the elastic forces acting on them from other particles will bring them back to their equilibrium position. In this case the particles will perform an oscillatory motion. The propagation of elastic vibrations in a solid medium is a wave-like process.
Vibrations with frequencies between one Hertz (Hz) and 20 Hz are called infrasound, with frequencies between 20 Hz and 16-20 kHz producing audible sounds. Ultrasonic vibrations correspond to frequencies from 16-20 kHz to 10⁸ Hz, and vibrations with frequencies greater than 10⁸ Hz are called hypersonic [111-113]. Figure 2.3 shows a logarithmic frequency scale based on the expression lg² f=1, 2, 3..., n, where 1, 2, 3..., n are octave numbers.



Figure 2.3 - Ranges of elastic vibrations in material media

Any wave as an oscillation propagating in time and space can be characterised by frequency, wavelength and amplitude (figure 2.4) [114]. In this case, the wavelength is related to the frequency f through the velocity of propagation of the wave in the given material c:

Figure 2.4 - Characteristics of the oscillating process

Frequency is the number of vibrations a system makes per unit time; Wavelength is the distance a wave travels in time equal to the period of vibration T (T=1/f ), i.e. in the time it takes to make one vibration; Amplitude of vibration is the maximum deviation of an oscillating system from its equilibrium position.

Sound and ultrasonic vibrations are not different in their physical nature. They are elastic vibrations in material media. Consider what parameters can characterise a wave.
The wavelength λ is the distance that a wave travels while a particle in the medium makes a single oscillating motion. The distance between neighbouring maxima or minima of a perturbation is called the wavelength.
The vibration amplitude A represents the maximum displacement of a particle from its position of equilibrium during its oscillatory motion, caused by the excitation of particles in the medium.
The oscillation frequency f is the number of vibrations made by a particle in the medium in one second. Sound waves generated by a medium are characterised by a continuous series or range of frequencies. The lowest frequency of the wave is called the fundamental, or natural frequency, and the others are harmonics, or overtones. The frequency of the second harmonic is twice the natural frequency of the system. Similarly, the frequency of the third harmonic is three times its own frequency, etc.
The period of oscillation T is the time it takes for a particle to make one oscillating movement. By definition, the time it takes for a wave to make f vibrations is 1 second.
Oscillation is a reciprocating movement from one extreme position to another and back through a position of equilibrium.
The phase of an oscillation φ is the ratio of the displacement of the oscillating particle at a given time to its amplitude value. If the points of an oscillating process are in the same phase (their phase difference is 2π), the distance between the two points is one wavelength λ.
The speed of propagation of vibrations C is the distance travelled by the wave in one second [111].
Distinguishing features of ultrasonic vibrations:
1. Ultrasonic vibrations, having high frequency f, in comparison with sound vibrations at the same speed of propagation, are characterised by significantly shorter wavelengths. Ultrasonic (US) vibrations in various media with wavelength not exceeding 1...10 mm are similar to light beams by their properties. This makes it possible not only to focus vibrations, but also to form directional radiation, i.e. to direct energy in the right direction and to concentrate it in the right volume.
2. Ultrasonic vibrations can propagate in all material media (transparent and opaque media, conductors and dielectrics, etc.), allowing them to be used to study and influence polymers, metals, liquids, gases, etc.
The power of ultrasonic vibrations propagated in media is proportional to the square of the frequency, and therefore, in contrast to the power of sound vibrations, is very high. Power of ultrasonic vibrations can reach hundreds of kilowatts, and intensity (energy propagated through the unit area in unit time) - 1...1000 W/cm. At such intensities of ultrasonic influence very high energy of mechanical vibrations can be propagated inside the material body. During wave propagation (in an oscillatory process) sound pressure drops exceeding tens of MPa [115].
The effectiveness of ultrasonic impacts on various technological processes has been confirmed by numerous studies and experiences, which have made it possible to establish the following:
1. Application of high intensity ultrasonic vibrations provides 10...1000-fold acceleration of processes occurring between two or more heterogeneous media (dissolution, purification, degreasing, degassing, dyeing, grinding, impregnation, emulsification, extraction, crystallisation, polymerisation, scale prevention, homogenisation, erosion, chemical and electrochemical reactions and many others). This increases the yield of useful products (e.g. extracts) and gives them additional properties (e.g. biological activity and sterility), as well as producing substances with new properties (e.g. fine emulsions and suspensions).
2. the use of ultrasonic vibrations makes it possible to carry out technological processes that are not realized or are difficult to realize by traditional methods - to provide size processing (drilling, chamfering, slotting) of brittle and hard materials such as ceramics, semiconductor materials, glass, gemstones, ferrites, super hard alloys and steel.
3. Ultrasonic vibrations can intensify many processes occurring at the interface of materials (welding of polymeric materials, gluing, impregnation of various materials), accelerating technological processes and improving the quality of obtained products.
The undoubted and unique advantages of ultrasonic technology should have ensured its widespread use in solving the complex problems of today's competitive product oriented industries [111].

2.3 Application of ultrasonic vibrations

The range of applications for low-intensity ultrasonic vibrations (conventionally up to 1 W/cm² ) is very broad, and we have looked at several major applications of low-intensity ultrasonic vibrations in succession.

1. Ultrasonic devices for monitoring the chemical characteristics of various materials and media. They are all based on changes in the velocity of ultrasonic vibrations in a medium and allow the determination of:
- concentration of binary mixtures;
- the density of the solutions;
- the degree of polymerization of polymers;
- the presence of impurities, gas bubbles in the solutions;
- the speed at which chemical reactions take place;
- the fat content of milk, cream and sour cream;
- dispersion in heterogeneous systems, etc.
The resolution of modern ultrasonic devices is 0.05%, the accuracy of propagation velocity measurements on samples of 1 m length is 0.5-1 m/s (velocity in metal is over 5000 m/s). Virtually all measurements are made by comparison with a reference.
2. Instruments for checking physico-chemical characteristics, based on the measurement of ultrasound attenuation. Such instruments make it possible to measure viscosity, density, composition, impurities, gases, etc. The techniques used are also based on methods of comparison with a reference.
3. Ultrasonic flowmeters for liquids in pipelines. They are also based on measuring the propagation velocity of ultrasonic vibrations along the liquid flow and against the flow. By comparing the two velocities, the flow rate can be determined, and if the cross-section of the pipeline is known, the flow rate can be determined.
4. Level detectors. The principle of operation is based on the detection of the level of liquid or bulk solids by ultrasonic pulses passing through a gaseous medium and on the phenomenon of reflection of these pulses from the interface between the gas and the medium being monitored.
5. Ultrasonic gas analysers are based on the relationship between the ultrasonic velocity in a mixture of gases and the velocity in each of the constituent gases.
6. Intrusion ultrasonic devices are based on the measurement of various ultrasonic field parameters (amplitude of oscillations when the space between the transmitter and the receiver overlaps, change in frequency when reflecting from a moving object, etc.).
7. Gas temperature meters and fire alarms based on changes in propagation velocity when the temperature of the medium changes or when smoke appears.
8. Ultrasonic nondestructive testing devices. Non-destructive testing is one of the main techniques for ensuring the quality of materials and products. No product should be operated without inspection. It is possible to check by testing, but so you can test 1-10 products, but it is impossible to check 100% of all products, because to check - it means to spoil all products. Therefore, it is necessary to check without destroying.
Depending on the type of medium, processes are conventionally divided into processes in liquid, solid, thermoplastic materials and gaseous (air) media. In the following sections, processes and apparatus for intensifying processes in liquids, solids, thermoplastics and gaseous media will be discussed in more detail [115-116].
The following are examples of the main technologies implemented using high-energy ultrasonic vibrations [111].
1. Dimensional machining. Ultrasonic vibrations are used for machining brittle and extremely hard materials and metals.
The main technological processes intensified by ultrasonic vibrations are drilling, countersinking, threading, wire drawing, polishing, grinding, drilling of holes with complex shapes. These processes are intensified by the application of ultrasonic vibrations to the tool.
2. Ultrasonic cleaning. There are now many ways to clean surfaces from various contaminants. Ultrasonic cleaning is faster, provides high quality and cleans areas that are difficult to access. Highly toxic, flammable and expensive solvents are replaced with plain water.
3. Ultrasonic welding. Nowadays high intensity ultrasonic vibrations are used to weld polymeric thermoplastic materials. Welding of polyethylene tubes, boxes, jars provides excellent tightness. Contrary to other procedures, contaminated plastics, liquid-filled tubes etc. can be welded with ultrasound. In doing so, the contents are sterilised.
Ultrasonic welding is used to weld thin foil or wire to a metal part. And ultrasonic welding is cold welding because the seam is formed at a temperature below the melting point. In this way, aluminium, tantalum, zirconium, niobium, molybdenum etc. are welded together.
At present, ultrasonic welding has found the greatest application for high-speed packaging processes and the production of polymer packaging materials.
4. Soldering and tinning. High frequency ultrasonic vibrations are used to braze aluminium. With ultrasonic vibration it is possible to tin and then solder ceramics, glass, which was impossible before. Ferrites, the soldering of semi-conductor crystals with gold-plated housings is realised today using ultrasonic technology.
5. Ultrasound in modern chemistry. At present, a new trend in chemistry - ultrasonic chemistry - has been formed. By studying chemical transformations occurring under the action of ultrasound, scientists have found that ultrasound not only accelerates oxidation, but in some cases provides a reducing effect. In this way, iron is recovered from oxides and salts.
Positive results have been obtained in the intensification of the following chemical engineering processes: electrodeposition, polymerisation, depolymerisation, oxidation, reduction, dispersion, emulsification, aerosol coagulation, homogenisation, impregnation, dissolution, atomisation, drying, burning, tanning etc.