Ultrasonic electromagnetic vibrations have a range. What is ultrasound? The use of ultrasound in technology and medicine. Quick scan tools

With the development of acoustics in late XIX century, ultrasound was discovered, at the same time the first studies of ultrasound began, but the foundations for its application were laid only in the first third of the 20th century.

Ultrasound and its properties

In nature, ultrasound is found as a component of many natural noises: in the noise of wind, waterfall, rain, sea pebbles rolled by the surf, in lightning discharges. Many mammals, such as cats and dogs, have the ability to perceive ultrasound with a frequency of up to 100 kHz, and the location abilities of bats, nocturnal insects and marine animals are well known to everyone.

Ultrasound- mechanical vibrations above the frequency range audible to the human ear (typically 20 kHz). Ultrasonic vibrations travel in a waveform, similar to the propagation of light. However, unlike light waves, which can travel in a vacuum, ultrasound requires an elastic medium such as a gas, liquid, or solid.

The main parameters of a wave are wavelength, frequency and period. Ultrasonic waves by their nature do not differ from the waves of the audible range and obey the same physical laws. But, ultrasound has specific features that have determined its widespread use in science and technology. Here are the main ones:

• 1. Short wavelength. For the lowest ultrasonic range, the wavelength does not exceed a few centimeters in most media. The short wavelength determines the ray nature of the propagation of ultrasonic waves. Near the emitter, ultrasound propagates in the form of beams close in size to the size of the emitter. When hitting inhomogeneities in a medium, an ultrasonic beam behaves like a light beam, experiencing reflection, refraction, and scattering, which makes it possible to form sound images in optically opaque media using purely optical effects (focusing, diffraction, etc.).
• 2. A short oscillation period, which makes it possible to emit ultrasound in the form of pulses and to carry out precise temporal selection of propagating signals in the medium.

The possibility of obtaining high values ​​of vibration energy at a small amplitude, because the energy of the oscillations is proportional to the square of the frequency. This makes it possible to create ultrasonic beams and fields with high level energy without requiring large equipment.

Significant acoustic currents develop in the ultrasonic field. Therefore, the impact of ultrasound on the environment generates specific effects: physical, chemical, biological and medical. Such as cavitation, sound-capillary effect, dispersion, emulsification, degassing, disinfection, local heating and many others.

The needs of the navy of the leading powers - England and France, to study the depths of the sea, aroused the interest of many scientists in the field of acoustics, because. this is the only type of signal that can travel far in water. So in 1826, the French scientist Colladon determined the speed of sound in water. 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 results of the experiment were disappointing. The sound of the bell gave too weak an echo, almost inaudible among other sounds of the sea. It was necessary to go to the region of higher frequencies, which would make it possible to create directed sound beams.

The first ultrasound generator was made in 1883 by 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" by Greek word, meaning "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. From now on, it became technically possible to manufacture small-sized emitters and receivers of ultrasound.

The death of the Titanic from a collision with an iceberg, the need to fight a new weapon - submarines required the rapid development of ultrasonic hydroacoustics. In 1914, the French physicist Paul Langevin, together with a talented Russian emigre scientist, Konstantin Vasilievich Shilovsky, first developed a sonar consisting of an ultrasound emitter and a hydrophone - a receiver of ultrasonic vibrations based on the piezoelectric effect. Sonar Langevin - Shilovsky, was the first ultrasonic device applied in practice. At the same time, the Russian scientist S.Ya.Sokolov developed the fundamentals of ultrasonic flaw detection in industry. In 1937, the German psychiatrist Karl Dussik, together with his brother Friedrich, a physicist, first used ultrasound to detect brain tumors, but the results they obtained were unreliable. In medical practice, ultrasound was first used only in the 50s of the 20th century in the United States.

Ultrasound- This elastic vibrations and waves with a frequency above 20 kHz, not audible to the human ear. Currently, it is possible to obtain ultrasonic vibrations with a frequency of up to 10 GHz. According to the indicated frequency ranges, the length region ultrasonic waves in air is from 1.6 to 0.3 × 10 - 4 cm , in liquids - from 6.0 to 1.2?10- 4 cm and in solids - from 20.0 to 4.0?10- 4 cm.

Ultrasonic waves by their nature do not differ from elastic waves in the audible range. The propagation of ultrasound obeys the basic laws common to acoustic waves of any frequency range. The basic laws of ultrasound propagation include the laws of reflection and refraction at the boundaries of various media, diffraction and scattering of ultrasound in the presence of obstacles and inhomogeneities at the boundaries, the laws of waveguide propagation in limited areas of the medium.

At the same time, the high frequency of ultrasonic vibrations and the short wavelength determine a number of specific properties that are unique to ultrasound.

Thus, it is possible to visually observe ultrasonic waves using optical methods. Due to the short wavelength, the ultrasonic waves are well focused and, consequently, it is possible to obtain directed radiation. Another very important feature of ultrasound is the possibility of obtaining high intensity values ​​at relatively small oscillation amplitudes.

A decrease in the amplitude and intensity of an ultrasonic wave as it propagates in a given direction, i.e. attenuation is determined by the scattering and absorption of ultrasound, the transition of ultrasonic energy into other forms, for example, into thermal energy.

Sources of ultrasound in the workplace. Man-made sources of ultrasound include all types of ultrasonic technological equipment, ultrasonic devices and equipment for industrial, medical and household purposes, which

generate ultrasonic vibrations in the frequency range from 20 kHz to 100 MHz and above. The source of ultrasound can also be equipment, during the operation of which ultrasonic vibrations occur as a concomitant factor.

The main elements of ultrasonic technology are ultrasonic transducers and generators. Ultrasonic transducers depending on the type of energy consumed, they are divided into mechanical (ultrasonic whistles, sirens) and electromechanical (magnetostrictive, piezoelectric, electrodynamic). Mechanical and magnetostrictive transducers are used to generate low-frequency ultrasound, and piezoelectric transducers make it possible to obtain high-frequency ultrasounds - up to 10 9 Hz.

Ultrasonic generators designed to convert industrial frequency current into high frequency current and to power electroacoustic systems - both piezoelectric and magnetostrictive transducers.

At present, ultrasound is widely used in mechanical engineering, metallurgy, chemistry, radio electronics, construction, geology, light and food industries, fisheries, medicine, etc.

Among the variety of ways to use ultrasound in terms of assessing their possible adverse influence on the body of workers, it is advisable to allocate two main directions:

1. Application low-frequency (up to 100 kHz) ultrasonic vibrations, spreading by contact and air ways, for active influence on substances and technological processes - cleaning, disinfection, welding, soldering, mechanical and thermal processing of materials (superhard alloys, diamonds, ceramics, etc.), coagulation of aerosols; in medicine - ultrasonic surgical instruments, devices for sterilizing the hands of medical staff, various items and etc.

2. Application high-frequency (100 kHz - 100 MHz and higher) ultrasonic vibrations propagating exclusively by contact for non-destructive testing and measurements; in medicine - diagnosis and treatment of various diseases.

An analysis of the prevalence and the prospect of using a variety of ultrasonic sources showed that 60-70% of all workers in conditions of adverse effects of ultrasonic

It consists of flaw detectors, operators of cleaning, welding, cutting units, doctors of ultrasound examinations (ultrasound), physiotherapists, surgeons, etc.

In order to unify the criteria and methods for assessing the degree of industrial effects of ultrasound, a “Hygienic classification of ultrasound affecting a human operator” has been developed. Classified signs of ultrasound affecting working people are: the method of propagation of the factor, the type of ultrasound source, the method and mode of generating oscillations, the frequency response of ultrasonic oscillations (Table 12.1).

Those working with technological and medical ultrasonic sources are exposed to a complex of unfavorable factors of the production environment, the leading of which is ultrasound with an oscillation frequency of 20 Hz - 20.0 MHz and an intensity of 50-160 dB.

So, stationary cleaning, welding, cutting installations generate constant ultrasonic vibrations with frequencies

Table 12.1.Hygienic classification of ultrasound affecting the operator

24.0-22.0 kHz, spreading by contact and air routes (25-30% of the work shift).

The intensity of ultrasound in the zone of contact with the hands of operators of cleaning, cutting and welding units is 0.03-1.4 W/cm 2 , i.e. levels range from practically normative values ​​up to 14-fold excess of the IMU. Sound pressure levels in the audible and ultrasonic frequency ranges at workplaces reach 80-101 dB with a maximum at the operating frequencies of the installations, which is normal.

Among the numerous methods of ultrasonic flaw detection, the most common is the pulse method (frequencies 0.5-20.0 MHz with a pulse repetition rate in the range of 300-4000 Hz; frequencies of 50 and 80 kHz with a pulse repetition rate in the range of 100-4000 Hz).

During ultrasonic testing of welded and reinforced concrete products, the operator is exposed to ultrasound during 72-75% of the working time, the intensity of ultrasound at the points of contact ranges from 1 × 10 -3 to 1.0 W/cm 2 , the levels of airborne ultrasound do not exceed the MPC.

The average shift time of exposure to contact ultrasound for workers depends on the type of ultrasonic source (manual or stationary), for which, as a rule, it is 2.5-3 times less.

The diagnostic units used in medical institutions operate in the frequency range of 0.8-20.0 MHz, the pulse repetition rate is 50-100 Hz. Diagnostic scanning is performed with a hand-held ultrasonic transducer. The duration of one study ranges from 15-20 minutes to 1-1.5 hours. The levels of high-frequency contact ultrasound affecting the doctor's hands range from 0.5-25.0-40.0 mW / cm 2 to 1.0 W / cm 2 for diagnostic studies, which occupy 70% of the working time.

In ultrasonic surgical equipment, the oscillation frequency is 26.6-44.0-66.0-88.0 kHz. During the work of surgeons, contact transmission of ultrasound to the hands was noted, the duration of ultrasound exposure does not exceed 14% of the working time. The intensity of contact ultrasound is in the range of 0.07-1.5 W/cm 2 , the levels of air ultrasound at surgeons' workplaces are below the permissible level - 80-89 dB.

Ultrasonic physiotherapy equipment generates oscillations with frequencies of 0.88 and 2.64 MHz. The levels of constant and pulsed contact ultrasound affecting the hands of the medical staff, propagating through the side surface of the hand-held emitter, are 0.02-1.5 W/cm. The duration of one procedure does not exceed 15 minutes, the contact time with ultrasound is 33% per shift.

Biological action of ultrasound. Ultrasonic waves can cause multidirectional biological effects, the nature of which is determined by many factors: the intensity of ultrasonic vibrations, frequency, temporal parameters of oscillations (constant, pulsed), duration of exposure, tissue sensitivity.

In particular, the frequency of ultrasonic vibrations determines the penetration depth of the factor: the higher the frequency, the most of energy is absorbed by the tissues, but ultrasonic vibrations penetrate to a shallower depth. It should be noted that the absorption of ultrasound in biological tissues does not follow general laws. According to available data, in biological tissues there is not a quadratic, but linear dependence absorption versus frequency. This is due to the large heterogeneity of body tissues. The heterogeneity of biological tissues is also due to the different degree of absorption of ultrasound. The lowest absorption is observed in the fat layer and almost twice as much in the muscle tissue. The gray matter of the brain absorbs ultrasound twice as much as the white matter; little absorbs ultrasonic energy cerebrospinal fluid. The greatest absorption is observed in the bone tissue (Table 12.2).

With the systematic impact of intense low-frequency ultrasound with levels exceeding the maximum allowable, workers may experience functional changes in the central and peripheral nervous systems, cardiovascular, endocrine systems, auditory and vestibular analyzers, and humoral disorders.

Exposure to ultrasonic vibrations of 130 dB at a frequency of 25 kHz revealed changes in heart rate, blood picture, endocrine function and brain electrogenesis (EEG flattening); fatigue, increased fatigue, decreased ability to work are noted.

Table 12.2.Absorption of ultrasound by tissues of the human body

Under the action of ultrasound with a frequency of 20 kHz with sound pressure levels of 120, 110 and 100 dB, there were no noticeable shifts in the thresholds of auditory sensitivity after an hour of exposure.

The most characteristic is the presence of vegetovascular dystonia and asthenic syndrome. faces, long time servicing low-frequency ultrasound equipment, complain of headache, dizziness, general weakness, fatigue, sleep disturbance, daytime sleepiness, irritability, memory impairment, increased sensitivity to sounds, fear of bright light. There are complaints of a decrease in the temperature of the extremities, bouts of pallor or redness of the face, complaints of a dyspeptic nature are not uncommon.

General cerebral disorders are often combined with moderate vegetative polyneuritis of the hands. This is due to the fact that along with the general effect of ultrasound on the body working through the air, low-frequency ultrasound has a local effect when it comes into contact with workpieces and media in which vibrations are excited, or with manual sources.

When exposed to low-frequency ultrasound, vegetative-vascular disorders occur (with the same work experience), as a rule, earlier than when exposed to high-frequency ultrasound, and are characterized by the presence of trophic disorders that spread to muscle tissue, followed by hypertrophy of the muscles of the hand.

Systematic, even short-term contact with liquid and solid media, in which ultrasonic vibrations are excited, significantly enhances the effect of airborne ultrasound.

Compared to high-frequency noise, ultrasound has a weaker effect on auditory function, but causes more pronounced abnormalities in vestibular function.

The adverse effect of low-frequency ultrasound on the functional state of the central nervous system. In workers in the dynamics of the working day, the speed of execution of conditioned reflex reactions to external stimuli slows down, there is tension or a violation of thermoregulation and, accordingly, an increase in body temperature to 37.1-37.3 ° C, dissociation in the degree of increase in body and skin temperature, no correlation between indicators of sweating, pulse and body temperature. There are also: a tendency to reduce diastolic pressure, up to arterial hypotension, changes in the ECG according to the extracardiac type, an increase in the thresholds of auditory sensitivity, if sound pressure levels significantly exceed the MPD, noticeable shifts in vestibular function (according to stabilography).

These changes are clearly manifested in persons exposed to intense ultrasound (122-130 dB), and are much less pronounced when exposed to medium and low intensity ultrasound (92-115 dB).

The intensities of contact ultrasound used in industry, biology, medicine are usually divided into low(up to 1.5 W / cm 2), medium(1.5-3.0 W / cm 2) and high(3.0-10.0 W / cm 2).

Depending on the intensity of contact ultrasound, there are three main types of action:

1) low intensity ultrasound promotes acceleration of metabolic processes in the body, easy heating of tissues, micromassage, etc. Low intensities do not lead to morphological changes inside the cells, since variable sound pressure causes only some acceleration of biophysical processes, therefore low exposures of ultrasound are considered as a physiological catalyst;

2) medium intensity ultrasound by increasing the variable sound pressure, it causes reversible reactions of oppression, in particular, of the nervous tissue. The rate of restoration of functions depends on the intensity and time of irradiation with ultrasound;

3) high intensity ultrasound causes irreversible oppression, turning into the process of complete destruction of tissues.

Available data indicate that ultrasonic vibrations generated in a pulsed mode have a slightly different effect. biological action than constant fluctuations. The peculiarity of the physiological action of pulsed ultrasound is less pronounced, but greater softness and duration of the manifestation of effects. The softness of the action of pulsed contact ultrasound is associated with the predominance of the physicochemical effects of action over thermal and mechanical ones.

The impact of ultrasound on biological structures is due to a number of factors. effects, caused by ultrasound, conditionally divided into:

mechanical, caused by alternating displacement of the medium, radiation pressure, etc. So, at low intensities (up to 2-3 W/cm 2 at frequencies of the order of 10 5 -10 6 Hz), vibrations of the particles of the biological environment produce a kind of micro-massage of tissue elements, which contributes to a better metabolism;

physical and chemical, related to the acceleration of diffusion processes through biological membranes, change in the rate of biological reactions;

thermal, resulting from the release of heat during the absorption of ultrasonic energy by tissues, an increase in temperature at the boundaries of tissue structures, heating on gas bubbles;

Tissue related effects ultrasonic cavitation(formation with subsequent collapse of gas-vapor bubbles in the medium under the action of ultrasound). Cavitation leads to the breaking of molecular bonds. For example, water molecules decompose into free radicals OH - and H +, which is the root cause of the oxidizing effect of ultrasound. In a similar way, high-molecular compounds in biological objects, for example, nucleic acids, protein substances, are split under the action of ultrasound.

Information about the biological effect of low-frequency ultrasound is very limited. The available evidence suggests that low frequency ultrasound is a factor with a large

biological activity and capable of causing functional and organic disorders of the nervous, cardiovascular, hematopoietic, endocrine and other body systems.

Data on the effect of high-frequency ultrasound on the human body indicate polymorphic and complex changes occurring in almost all tissues, organs and systems.

The changes occurring under the influence of ultrasound (air and contact) are subject to general pattern: low intensity stimulates and activates, while medium and high intensity depresses, inhibits and can completely suppress functions.

Due to the small wavelength, high-frequency contact ultrasound practically does not propagate in the air and affects workers only when the ultrasound source comes into contact with the body surface. Changes caused by the action of contact ultrasound are usually more pronounced in the contact zone, more often these are fingers, hands, although the possibility of distal manifestations due to reflex and neurohumoral connections is not excluded.

Prolonged work with ultrasound during contact transfer to the hands causes damage to the peripheral neurovascular apparatus, and the degree of severity of changes depends on the intensity of ultrasound, the time of sounding and the area of ​​contact, i.e. ultrasonic exposure, and may increase in the presence of concomitant factors of the production environment that aggravate its effect (air ultrasound, local and general cooling, contact lubricants - various types of oils, static muscle tension, etc.).

Among those working with sources of contact ultrasound noted high percent complaints about the presence of paresthesias, increased sensitivity of the hands to cold, a feeling of weakness and pain in the hands at night, a decrease in tactile sensitivity, sweating of the palms. There are also complaints of headaches, dizziness, noise in the ears and head, general weakness, palpitations, pain in the region of the heart.

It has been established that high-frequency ultrasound, acting by contact for a long time, has an adverse effect, causing the development of vegetative-vascular lesions of the hands in operators-defectoscopists. The operators of ultrasonic flaw detection revealed an increased

This is due to hemodynamic disorders of the eye, mainly in the form of a hypotonic state, manifested by atony of the veins, venules and venous knees of the capillaries of the anterior part of the eyeball, a decrease in retinal pressure, and hypotonic angiopathy of the retina. The revealed vascular disorders of the eyes in this professional group should be interpreted as a manifestation of a general vegetovascular disorder associated with exposure to ultrasonic vibrations (0.5-5.0 MHz, intensity up to 1.0 W/cm2).

The adverse effect of contact ultrasound on medical personnel serving physiotherapeutic and diagnostic equipment, which is also manifested by the development of vegetative-vascular lesions of the hands, has been noted.

Vegetative-sensory (angioneurosis) polyneuropathy of the hands, which develops under the influence of contact ultrasound, was first recognized as an occupational disease and included in the list of occupational diseases in 1989. subsequent inclusion of reflex, neurohumoral connections. It is determined by mechanical and physicochemical factors, since the role of thermal and cavitation components at levels created by ultrasound sources in contact media is insignificant.

Specific features of the impact on workers of contact ultrasound, due to its high biophysical activity, are manifested in sensory, vegetative-vascular disorders and changes in the musculoskeletal system of the upper extremities.

Along with changes in the neuromuscular apparatus in persons working with sources of contact ultrasound, changes in the bone structure are revealed in the form of osteoporosis, osteosclerosis of the distal phalanges of the hands, as well as some other changes of a degenerative-dystrophic nature. The method of X-ray densitophotometry.

The skin is the "entrance gate" for contact ultrasound, since when performing work with various ultrasonic

In the first place, the skin of the hands of workers is exposed to sound sources. The intensity of ultrasonic vibrations in the skin of the hands is closest to the intensity of ultrasound on the surfaces of the emitter.

The skin in different areas of the human body has different sensitivity: the skin of the face is more sensitive than the skin of the abdomen, and the skin of the abdomen is more sensitive than the skin of the extremities. Ultrasound intensity of 0.6 W/cm 2 (frequency 2.5 MHz) causes hyperemia of the skin, mild swelling of the dermis.

The impact of ultrasound with an intensity of 0.4 W / cm 2 (1-2 MHz) is accompanied by a natural decrease in the pH value of the skin surface, which indicates the predominant use of carbohydrates for energy metabolism, since acid metabolic products accumulate in the tissues during their enhanced transformations. It is possible that the change in the pH of the skin surface under the influence of ultrasound is associated with an increase in the functional activity of the sebaceous glands. When exposed to ultrasound, the number of active sweat glands increases, and, accordingly, the excretion of chlorides increases.

Clinical and laboratory examination of defectoscopists reveals the following skin diseases: hyperhidrosis of the palms and soles, dyshidrosis of the palms and soles, rubrophytosis and epidermophytosis of the feet and hands, seborrhea of ​​the scalp, etc. In most patients with hyperhidrosis, dyshidrosis, etc., a correlation with concomitant diseases in particular, with neurovascular disorders, manifested in the form of vegetative polyneuritis of the hands, vegetative-vascular dysfunctions. This makes it possible to associate skin pathology with ultrasound exposure.

When exposed to low-intensity ultrasound - 20-35 mW / cm 2 (frequency 1 MHz), the permeability of skin vessels increases, while local exposure to heat, leading to an increase in skin temperature by 0.8-1.0 ? effect on vascular permeability of the skin. Consequently, in the processes of changes in the vascular permeability of the skin under the influence of ultrasonic waves, not the thermal factor, but the mechanical effect plays an important role. At high ultrasound intensities, vascular permeability can also change through reflex mechanisms.

An important point in the action of ultrasound and its analgesic effect is, in addition to lowering the pH of the medium, local accumulation

tion of histamine, which contributes to the inhibition of impulse conduction in the synapses of sympathetic ganglia.

It is believed that ultrasonic stimulation, getting on the receptor apparatus of the skin, is transmitted in all directions to the peripheral and central formations of the sympathetic and parasympathetic nervous systems, both along specific and non-specific pathways.

Regularities in the change in cardiovascular activity under the influence of contact ultrasound were revealed. So, when sounding patients with therapeutic doses of ultrasound (2.46 MHz, 1 W / cm 2), an increase in heart rate with a change in the ECG is observed. An increase in the intensity of ultrasound leads to bradycardia, arrhythmias, and a decrease in biological activity. Similar reactions are observed when sounding not only the region of the heart, but also adjacent areas.

The study of the body's vascular reactions to the impact of ultrasound during contact transmission showed that low doses of high-frequency ultrasound (0.2-1.0 W/cm2) cause a vasodilating effect, and large doses (3 W/cm2 and above) cause a vasoconstrictor effect.

A decrease in vascular tone and vasodilation is noted not only in the area exposed to ultrasound, but also in symmetrical areas, which suggests the important role of neuroreflex mechanisms in the formation of a response to ultrasound.

The impact of ultrasound on the body is accompanied by biochemical changes: the amount of proteins in the blood serum decreases, the metabolism of carbohydrates intensifies, the content of bound bilirubin in the blood increases, the activity of enzymes, in particular blood catalase, decreases, and the level of pituitary adrenocorticotropic hormone (ACTH) in blood plasma increases. It is believed that ultrasound with an intensity of 0.1-0.3 W/cm 2 has an optimal stimulating effect on enzymatic processes in tissues.

The study of the antitumor effect of high-frequency ultrasound showed that high intensities of ultrasound (3.0-10.0 W/cm 2 ) contribute to the destruction of tumor cells and inhibit the growth of tumors.

Under the influence of high-frequency ultrasound on the bone tissue, a violation of mineral metabolism is noted - the content of calcium salts in the bones decreases.

Thus, under the influence of contact ultrasound, the development of generalized reflex-vascular changes is possible. However, the pathogenesis of changes detected in patients with severe manifestations of ultrasound pathology of the gastrointestinal tract, kidneys, and cardiovascular system has not yet been studied enough.

At present, a mathematical model has been developed for predicting the probability of developing occupational pathology in workers with sources of contact ultrasound of various frequencies, depending on the intensity and duration of contact, which makes it possible to determine the safe work experience in the profession, i.e. manage the risk of ill health through “time protection”. Estimated data on the probability of developing hand polyneuropathy of ultrasonic etiology are presented in tab. 12.3.

Hygienic regulation of air and contact ultrasound. When developing effective preventive measures aimed at optimizing and improving the working conditions of workers in ultrasonic professions, the issues of hygienic regulation of ultrasound as an unfavorable physical factor of the production environment and habitat are put forward in the first place.

The materials of the integrated research conducted at the State Research Institute of Occupational Medicine of the Russian Academy of Medical Sciences served as the basis for the development of a new system of hygienic regulation of ultrasound, which was reflected in the sanitary norms and rules "Hygienic requirements when working with sources of air and contact ultrasound for industrial, medical and domestic purposes."

Sanitary norms and rules establish a hygienic classification of ultrasound affecting a human operator; normalized parameters and maximum permissible levels of ultrasound for workers and the public; requirements for the control of air and contact ultrasound, preventive measures. It should be noted that these rules and regulations do not apply to persons (patients) exposed to ultrasound for medical and diagnostic purposes.

Table 12.3.Probability of developing polyneuropathy of hands working with sources of contact ultrasound propagating in liquid and solid media

Normalized parameters air ultrasound are sound pressure levels in decibels in one-third octave bands with geometric mean frequencies of 12.5; 16; 20; 25; 31.5; 40; 50; 63; 80; 100 kHz.

Normalized parameters contact ultrasound are the peak values ​​of vibration velocity or its logarithmic levels in dB in octave bands with geometric mean frequencies of 16; 31.5; 63; 125; 250; 500; 1000; 2000; 4000; 8000; 16,000; 31 500 kHz, determined by the formula:

L v = 20 logV/V0,

Where:

V - peak value of vibration velocity, m/s;

V0 - reference value of vibration velocity, equal to 5?10 -8 m/s.

IN tab. 12.4 the maximum permissible levels of airborne ultrasound at workplaces and contact ultrasound in the areas of contact of hands or other parts of the body working with sources of ultrasonic vibrations or media in which they propagate are presented.

The new standards are built according to the spectral principle, taking into account the combined effect of contact and air ultrasound by establishing a downward correction equal to 5 dB to the limit value of contact ultrasound, which has a higher biological activity.

When using household ultrasonic sources (washing machines, devices for repelling insects, rodents, dogs, burglar alarms, etc.), as a rule, operating at frequencies below 100 kHz, the standard levels of air and contact ultrasound affecting a person do not must exceed 75 dB at the operating frequency.

In addition to sanitary rules and norms, a number of regulatory and methodological documents have been developed that regulate, in particular, the working conditions of medical workers using ultrasonic sources in the form of apparatus, equipment or tools.

Table 12.4.Maximum permissible levels of ultrasound in the workplace

Note. 1 The maximum allowable levels of contact ultrasound should be taken 5 dB below the tabular data with a combined effect on working air and contact ultrasound.

ultrasound diagnostics, organization and conduct of diagnostic studies, as well as sanitary and hygienic and medical preventive measures to limit the adverse effects of contact ultrasound on medical staff. For example, in accordance with hygienic recommendations, the area of ​​​​the cabinet for conducting ultrasound examinations (ultrasound) should be at least 20 m 2, provided that one ultrasound diagnostic unit is placed in it. The room for ultrasound should have natural and artificial lighting, a sink with cold and hot water supply, a general exchange supply and exhaust ventilation system with an air exchange rate of 1:3, and installation of air conditioners is allowed. Certain parameters of the microclimate should be maintained indoors: air temperature - 22 ?С, relative humidity 40-60%, air velocity not higher than 0.16 m/s.

When measuring air and contact ultrasound generated by household appliances and equipment, follow the guidelines

comply with the requirements set forth in the current sanitary norms and rules.

Preventive actions. Measures to protect workers from the adverse effects of contact ultrasound and related factors of the working environment and the labor process include:

1. Medical and biological screening when applying for a job, taking into account subjective (individual) and objective (occupational) risk factors.

2. The use of various modes of work (shift and sliding weekly, ten-day, monthly, quarterly, etc.) and a contract system for conducting work for the period of the predicted length of safety experience.

3. Hygienic, including exposure, and clinical and physiological monitoring.

4. Measures of a medical and preventive nature for the improvement of workers.

Medical and biological screening when applying for a job should be carried out in several stages:

Stage I - social selection. According to the current hygienic norms and rules, the main contraindication for working in conditions of exposure to ultrasound is the age of under 18 years.

II stage - medical selection, including a preliminary medical examination and functional studies, taking into account the specifics of the action of contact ultrasound and risk factors (both identified individual and specific vocational, established during certification or licensing of the workplace for which employment is expected).

A preliminary medical examination is carried out in accordance with the current order. When conducting preliminary medical examinations, one should take into account contraindications for work in "ultrasound" professions, which, along with general medical contraindications for admission to work in contact with harmful, hazardous substances and production factors, include chronic diseases of the peripheral nervous system, obliterating diseases of the arteries and peripheral angiospasm.

In addition to medical contraindications, individual and objective risk factors have been identified that can aggravate the impact of contact ultrasound. Subjective (personal) risk factors include hereditary burden of vascular diseases, asthenic type of constitution, cold allergy, injuries of the extremities and their history of frostbite, autonomic lability, predominantly with a predominance of the tone of the sympathetic nervous system, long work experience in the profession, etc.

Objective or occupational risk factors are high levels of contact and air ultrasound, transmission of ultrasonic vibrations through a liquid medium, a large area of ​​contact with the source, hand contamination with contact lubricants, hand cooling, high ultrasonic source index, static load on the muscles of the fingers and hands, forced posture, cooling microclimate, high levels of the total index of a one-digit assessment of the complex impact of factors, etc.

Of great importance in the prevention of ultrasonic exposure are rational work regimes established for a particular workplace or source of vibrations. When developing work regimes, it is necessary to be guided by the following principles:

Reducing the total contact time and reducing the exposure of ultrasonic sounding when the standards are exceeded;

Conducting work with regularly interrupted ultrasonic impacts;

Organization of two regulated breaks, the first - 10 minutes long, the second - 15 minutes for outdoor activities, a special complex of industrial gymnastics, physio-prophylactic procedures, etc. It is rational to arrange the first break 1.5-2 hours after the start of the shift, the second - 1.5 hours after the lunch break;

Lunch break of at least 30 minutes. In addition to shift modes of work, it is advisable to introduce sliding modes - weekly, ten-day, monthly, quarterly, etc. These modern forms modes of work are most acceptable for medical workers, when the ultrasonic load on workers, exceeding the allowable one, can be evenly spaced in time.

Measures aimed at increasing the body's resistance, including those under the influence of contact ultrasound, include various types of physioprophylactic procedures, reflex prophylaxis, industrial gymnastics, rational balanced nutrition, vitaminization, and psychophysiological unloading.

Introductory gymnastics is carried out before work and is recommended to all workers without exception. Its main task is to raise the general tone of the body, activate the activity of organs and systems, help to quickly get involved in the working rhythm and reduce the period of workability. The complex includes 7-9 exercises and is performed for 5-7 minutes before starting work.

As a result of numerous experimental studies, the most effective ways to protect the hands of workers from the effects of low-frequency and high-frequency ultrasound propagating in solid and liquid media have been selected.

Working with low frequency sources

When vibrations propagate in a solid medium - two pairs of thick cotton gloves;

When vibrations propagate in a liquid medium, two pairs of gloves are used: the lower ones are cotton and the upper ones are dense rubber.

Working with high frequency sources contact ultrasound is recommended to use:

When vibrations propagate in a solid medium - one pair of cotton gloves, or cotton gloves with a waterproof palm surface (made, for example, from waterproof synthetic materials), or cotton fingertips;

When vibrations propagate in a liquid medium, two pairs of gloves are used: the lower ones are cotton and the upper ones are rubber.

As a means of individual protection against the effects of noise and airborne ultrasound, workers should use anti-noise - earplugs, headphones.

Among the measures to protect workers from ultrasonic exposure, an important place is occupied by the training of workers in the basics of labor protection legislation, the rules of technology

safety and preventive measures when working with sources of contact ultrasound; health education among workers, promotion of a healthy lifestyle.

Measures to form and manage the quality of the production environment at workplaces with ultrasound sources in order to reduce the risk of health problems for workers. An important role in managing the quality of the production environment is assigned to the means and methods of collective protection of workers. The most effective in this respect are organizational and technical measures in the source, reducing the levels of contact ultrasound affecting workers, reducing the time of contact with it and limiting the influence of adverse concomitant factors of the production environment, in particular:

Development and implementation of new, improved equipment with improved ultrasonic characteristics;

Creation of automatic ultrasonic equipment, for example, for cleaning parts, flaw detection, mechanical processing of materials, etc.;

Creation of installations with remote control;

The use of low-power ultrasonic generators in equipment, if this does not contradict the requirements of technological processes;

Designing ultrasonic installations with operating frequencies as far as possible from the audible frequency range (not lower than 22 kHz) in order to avoid the effect of pronounced high-frequency noise;

Blocking, i.e. automatic shutdown of equipment, devices when performing auxiliary operations for loading and unloading products, applying contact lubricants, etc.;

Design of seekers and sensors held by hands, taking into account the need to ensure minimal tension of the muscles of the hand;

The use of grids and various devices equipped with handles when loading and unloading parts from ultrasonic waves or special devices (clamps, tripods, hooks, etc.) to hold the workpieces or the ultrasound source;

Lining of the places where the operator's hands come into contact with the source (scanning devices of flaw detectors and diagnostic equipment,

handheld ultrasonic instrument handles, etc.) with insulating material;

Implementation of control over the timeliness of preventive and current repairs of ultrasonic equipment and equipment;

Equipment of ultrasonic installations with soundproof devices (casings, screen) made of sheet steel or duralumin, covering them with rubber, anti-noise mastic or other materials, equipment of soundproof booths, boxes;

Screening of feeder lines;

Efficient ventilation equipment.

In addition, when designing and developing new ultrasonic equipment with video terminal devices, the following must be observed: technical and hygienic requirements:

The brightness of the screen glow is not less than 100 cd/m 2 ;

The minimum size of a luminous dot for a monochrome display is 0.4 mm, for a color display - 0.6 mm;

The contrast of the image of signs is not less than 0.8;

Low-frequency image jitter in the range of 0.05-1.0 Hz within 0.1 mm;

The frequency of image regeneration when working with positive contrast is at least 72 Hz;

The presence of an anti-glare screen coating.

Optimization of the factors that determine the severity of work is achieved as a result of the correct choice of posture due to the rational layout of the workplace. To do this, first of all, it is necessary to select production equipment and work furniture that correspond to anthropometric data and psychophysiological capabilities of a person.

It is necessary to maintain the dimensions of the working area, including the space in which the controls for equipment, workpieces, parts, tools are located, i.e. everything you need to get the job done.

In the process of performing labor operations, it is advisable to exclude, if possible, static loads that arise when maintaining, for example, workpieces, parts, etc. due to the arrangement of workbenches, stands for workpieces, as well as the use of manipulators, carts, various small-scale mechanization tools to reduce the dynamic load and overstrain of the musculoskeletal system.

In the complex of activities for scientific organization labor a special place is occupied by recommendations for the rationalization of labor movements and efforts.

For factor optimization, determining the intensity of labor, it is advisable:

Creation of a rational lighting system in each specific case (or, conversely, dimming, for example, during flaw detection and ultrasound diagnostics), correct placement of lamps;

Fight against gloss of screens of the ultrasonic equipment;

Creation of the necessary color climate in industrial premises;

Device for light and sound indication of defects during ultrasonic flaw detection;

The introduction of work and rest regimes (gymnastics for the eyes, industrial gymnastics, psychophysiological unloading, etc.).

Ultrasound - these are elastic mechanical oscillations with a frequency exceeding 18 kHz, which is the upper threshold of audibility of the human ear. Due to the increased frequency, ultrasonic vibrations (USO) have a number of specific features (the possibility of focusing and radiation directivity), which makes it possible to concentrate acoustic energy over small areas of the emitted surface.

From the source of vibrations, ultrasound is transmitted in the medium in the form of elastic waves and can be represented as a wave equation for a longitudinal plane wave:

Where L- displacement of the oscillating particle; t- time; X- distance from the vibration source; With is the speed of sound in the medium.

The speed of sound is different for each medium and depends on its density and elasticity. Particular types of the wave equation make it possible to describe wave propagation for many practical cases.

Shape of ultrasonic waves

Ultrasonic waves from the vibration source propagate in all directions. Near each particle of the medium there are other particles oscillating with it in the same phase. A set of points that have the same phase of oscillation is called wave surface.

The distance over which a wave propagates in a time equal to the period of oscillation of the particles of the medium is called wavelength.

Where T - oscillation period; / - oscillation frequency.

wave front is the set of points up to which the vibrations reach certain moment time. There is only one wave front at any moment of time, and it moves all the time, while the wave surfaces remain motionless.

Depending on the shape of the wave surface, plane, cylindrical and spherical waves are distinguished. In the simplest case, the wave surfaces are flat and the waves are called flat, and the source of their excitation is the plane. Cylindrical called waves, in which the wave surfaces are concentric cylinders. The sources of excitation of such waves appear in the form of a straight line or a cylinder. Spherical waves are created by point or spherical sources, the radii of which are much smaller than the wavelength. If the radius exceeds the wavelength, then it can be considered flat.

The equation of a plane wave propagating along an axis x, if the excitation source harmonic vibrations with angular frequency ω and amplitude L 0 has the form

The initial phase a of the wave is determined by the choice of the origin of the coordinate X and time t.

When analyzing the passage of one wave, the reference point is usually chosen in such a way that A= 0. Then equation (3.2) can be written as

The last equation describes a traveling wave propagating towards increasing (+) or decreasing (-) values. It is one of the solutions of the wave equation (3.1) for a plane wave.

Depending on the direction of oscillation of the particles of the medium relative to the direction of wave propagation, several types of ultrasonic waves are distinguished (Fig. 3.1).

If the particles of the medium oscillate along a line coinciding with the direction of wave propagation, then such waves are called longitudinal(Fig. 3.1, A). When the displacement of the particles of the medium occurs in a direction perpendicular to the direction of wave propagation, the waves are called transverse(Fig. 3.1, b).

Rice. 3.1. Scheme of oscillatory displacements of particles of the medium for various types of waves: A- longitudinal; b- transverse; V- bending

In liquids and gases can only propagate longitudinal waves, since elastic deformations in them occur during compression and do not occur during shear. In solids, both longitudinal and transverse waves can propagate, since solids have shape elasticity, i.e. tend to retain their shape when subjected to mechanical forces. Elastic deformations and stresses arise in them not only during compression, but also during shear.

In small solids, such as rods and plates, the pattern of wave propagation is more complex. In such bodies, waves arise, which are a combination of two main types: torsional, bending, surface.

The type of wave in a solid depends on the nature of the excitation of vibrations, the shape solid body, its dimensions in relation to the wavelength, and under certain conditions, waves of several types can simultaneously exist. A schematic representation of a flexural wave is shown in fig. 3.1, c. As can be seen, the displacement of the particles of the medium occurs both perpendicular to the direction of wave propagation and along it. Thus, the flexural wave has common features both longitudinal and transverse waves.

Ultrasound represents longitudinal waves that have an oscillation frequency of more than 20 kHz. This is more than the frequency of vibrations perceived by the human hearing aid. A person can perceive frequencies within the range of 16-20 kHz, they are called sound. Ultrasonic waves look like a series of condensations and rarefaction of a substance or medium. Due to their properties, they are widely used in many areas.

What is this

Frequencies ranging from 20 thousand to several billion hertz fall into the ultrasonic range. These are high frequency vibrations that are beyond the audibility of the human ear. However, some types of animals perceive ultrasonic waves quite well. These are dolphins, whales, rats and other mammals.

By physical properties Ultrasonic waves are elastic, so they do not differ from sound waves. As a result, the difference between sound and ultrasonic vibrations is very conditional, because it depends on the subjective perception of a person's hearing and is equal to the upper level of audible sound.

But the presence of higher frequencies, and hence a small wavelength, gives ultrasonic vibrations certain features:

• Ultrasonic frequencies have different speeds of movement through various substances, due to which it is possible to determine with high accuracy the properties of ongoing processes, the specific thermal capacity of gases, as well as the characteristics of a solid body.
• Waves of significant intensity have certain effects that are subject to nonlinear acoustics.
• When ultrasonic waves move with significant power in a liquid medium, the phenomenon of acoustic cavitation occurs. This phenomenon is very important, because as a result, a field of bubbles is created, which are formed from submicroscopic particles of gas or vapor in an aqueous or other medium. They pulsate with some frequency and slam shut with tremendous local pressure. This creates spherical shock waves, which leads to the appearance of acoustic microscopic flows. Through the use of this phenomenon, scientists have learned how to clean contaminated parts, as well as create torpedoes that move in water. faster speed sound.
• Ultrasound can be focused and concentrated, allowing sound patterns to be created. This property is successfully used in holography and sound vision.
• An ultrasonic wave may well act as a diffraction grating.

Properties

Ultrasonic waves are similar in properties to sound waves, but they also have specific features:

• Small wavelength. Even for a low border, the length is less than a few centimeters. Such a small size of the length leads to the radial nature of the movement of ultrasonic vibrations. Directly next to the emitter, the wave travels in the form of a beam, which approaches the parameters of the emitter. However, when placed in an inhomogeneous medium, the beam moves like a beam of light. It can also be reflected, scattered, refracted.
• A small period of oscillation, which makes it possible to use ultrasonic vibrations in the form of pulses.
• Ultrasound cannot be heard and does not create an irritating effect.
• When exposed to ultrasonic vibrations on certain media, specific effects can be achieved. For example, you can create local heating, degassing, disinfect the environment, cavitation, and many other effects.

Operating principle

Various devices are used to create ultrasonic vibrations:

• Mechanical, where the source is the energy of a liquid or gas.
• Electromechanical, where ultrasonic energy is created from electrical.

Whistles and sirens operating with the help of air or liquid can act as mechanical emitters. They are convenient and simple, but they have their drawbacks. So their efficiency is in the range of 10-20 percent. They create a wide spectrum of frequencies with unstable amplitude and frequency. This leads to the fact that such devices cannot be used in conditions where accuracy is required. Most often they are used as a means of signaling.

Electromechanical devices use the principle of the piezoelectric effect. Its peculiarity is that during the formation of electric charges on the faces of the crystal, it is compressed and stretched. As a result, oscillations are created with a frequency that depends on the period of potential change on the crystal surfaces.

In addition to transducers based on the piezoelectric effect, magnetostrictive transducers can also be used. They are used to create a powerful ultrasonic beam. The core, which is made of a magnetostrictive material, placed in a conductive winding, changes its own length according to the shape of the electrical signal supplied to the winding.

Application

Ultrasound is widely used in a wide variety of fields.

Most often it is used in the following areas:

• Obtaining data on a specific substance.
• Processing and transmission of signals.
• effect on the substance.

So with the help of ultrasonic waves they study:

• Molecular processes in various structures.
• Determination of the concentration of substances in solutions.
• Definition, composition, strength characteristics of materials and so on.

In ultrasonic treatment, the cavitation method is often used:

• Metallization.
• Ultrasonic cleaning.
• Degassing of liquids.
• Dispersion.
• Obtaining aerosols.
• Ultrasonic sterilization.
• Destruction of microorganisms.
• Intensification of electrochemical processes.

The impact of ultrasonic waves in industry produces the following technological operations:

• Coagulation.
• Combustion in an ultrasonic environment.
• Drying.
• Welding.

In medicine, ultrasonic waves are used in therapy and diagnostics. In diagnostics, location methods using pulsed radiation are used. These include ultrasound cardiography, echoencephalography and a number of other methods. In therapy, ultrasonic waves are used as methods based on thermal and mechanical effects on tissues. For example, quite often during operations, an ultrasonic scalpel is used.

Ultrasonic vibrations are also carried out:

• Micromassage of tissue structures using vibration.
• Stimulation of cell regeneration, as well as intercellular exchange.
• Increasing the permeability of tissue membranes.

Ultrasound can act on tissues by oppression, stimulation or destruction. All this depends on the applied dose of ultrasonic vibrations and their power. However, not all areas of the human body are allowed to use such waves. So, with a certain caution, they affect the heart muscle and a number of endocrine organs. On the brain, cervical vertebrae, scrotum and a number of other organs, the impact is not used at all.

Ultrasonic vibrations are used in cases where it is impossible to use X-rays in:

• Traumatology uses the echography method, which easily detects internal bleeding.
• Obstetrics waves are used to assess the development of the fetus, as well as its parameters.
• Cardiology, they allow you to examine the cardiovascular system.

Ultrasound in the future

Currently, ultrasound is widely used in various fields, but in the future it will find even more applications. Already today it is planned to create devices that are fantastic for today.

• For medical purposes, the technology of ultrasonic acoustic hologram is being developed. This technology involves the arrangement of microparticles in space to create the desired image.
• Scientists are working on the creation of technology for contactless devices that will have to replace touch devices. For example, gaming devices have already been created today that recognize human movements without direct contact. Technologies are being developed that involve the creation of invisible buttons that can be felt and controlled by hands. The development of such technologies will make it possible to create contactless smartphones or tablets. In addition, this technology will expand the possibilities of virtual reality.
• With the help of ultrasonic waves, it is already possible today to make small objects levitate. In the future, machines may appear that will soar above the ground due to waves and, in the absence of friction, move at great speed.
• Scientists suggest that in the future, ultrasound will teach blind people to see. This confidence is based on the fact that bats recognize objects using reflected ultrasonic waves. A helmet has already been created that converts reflected waves into audible sound.
• Already today, people are planning to mine minerals in space, because everything is there. So astronomers found diamond planet full of gems. But how to mine such solid materials in space. It is ultrasound that will have to help in drilling dense materials. Such processes are quite possible even in the absence of an atmosphere. Such drilling technologies will allow collecting samples, conducting research and extracting minerals where it is considered impossible today.

The method of ultrasonic flaw detection of metals and other materials was first developed and practically implemented in the Soviet Union in 1928-1930. prof. S. Ya. Sokolov.

Ultrasonic waves are elastic vibrations of the material medium, the frequency of which lies beyond the limits of hearing in the range from 20 kHz (low frequency waves) to 500 MHz (high frequency waves).

Ultrasonic vibrations are longitudinal and transverse. If the particles of the medium move parallel to the direction of wave propagation, then such a wave is longitudinal, if it is perpendicular to transverse. To find defects in welds, transverse waves are mainly used, directed at an angle to the surface of the parts to be welded.

Ultrasonic waves are capable of penetrating into material media to a great depth, refracting and reflecting when they hit the boundary of two materials with different sound permeability. It is this ability of ultrasonic waves that is used in ultrasonic flaw detection of welded joints.

Ultrasonic vibrations can propagate in the most various environments- air, gases, wood, metal, liquids.

The propagation velocity of ultrasonic waves C is determined by the formula:

where f is the oscillation frequency, Hz; λ - wavelength, cm.

To detect small defects in welds, short-wave ultrasonic vibrations should be used, since a wave whose length is greater than the size of the defect may not detect it.

Receiving ultrasonic waves

Ultrasonic waves are obtained by mechanical, thermal, magnetostrictive (Magnetostriction - change in body size during magnetization) and piezoelectric (The prefix "piezo" means "press") methods.

The most common is the latter method, based on the piezoelectric effect of some crystals (quartz, Rochelle salt, barium titanate): if the opposite faces of a plate cut from a crystal are charged with opposite electricity with a frequency above 20,000 Hz, then the plate will vibrate in time with changes in the signs of charges , transmitting mechanical vibrations to the environment in the form of an ultrasonic wave. Thus, electrical vibrations are converted into mechanical ones.

In various systems of ultrasonic flaw detectors, high-frequency generators are used that set electrical oscillations from hundreds of thousands to several million hertz to piezoelectric plates.

Piezoelectric plates can serve not only as emitters, but also as receivers of ultrasound. In this case, under the action of ultrasonic waves, electric charges small value, which are recorded by special amplifying devices.

Methods for detecting defects with ultrasound

There are basically two methods of ultrasonic flaw detection: shadow and pulse echo (method of reflected vibrations.)

Rice. 41. Schemes for conducting ultrasonic flaw detection a - shadow; b - echo pulse method; 1 - probe-emitter; 2 - the item under study; 3 - probe receiver; 4 - defect

With the shadow method (Fig. 41, a), ultrasonic waves traveling through the weld from a source of ultrasonic vibrations (probe-emitter), when meeting with a defect, do not penetrate through it, since the defect boundary is the boundary of two heterogeneous media (metal - slag or metal - gas). Behind the defect, an area of ​​the so-called "sound shadow" is formed. The intensity of ultrasonic vibrations received by the probe-receiver drops sharply, and a change in the magnitude of the pulses on the screen of the cathode-ray tube of the flaw detector indicates the presence of defects. This method is of limited use, since bilateral access to the seam is required, and in some cases it is necessary to remove the seam reinforcement.

With the echo-pulse method (Fig. 41.6), the probe-emitter sends pulses of ultrasonic waves through the weld, which, when they encounter a defect, are reflected from it and captured by the probe-receiver. These pulses are recorded on the screen of the cathode ray tube of the flaw detector in the form of peaks, indicating the presence of a defect. By measuring the time from the moment of sending the pulse to receiving the return signal, one can also determine the depth of the defects. The main advantage of this method is that inspection can be carried out with one-sided access to the weld without removing the reinforcement or pre-treatment of the weld. This method has been most widely used in ultrasonic flaw detection of welds.