Basic Principles Of Ultrasound

Basic Principles Of Ultrasound

Date:Jun 06, 2018

Ultrasonic is part of the sound waves, is the human ear can't hear the sound waves, frequency is higher than 20 KHZ, it and sound waves have in common, which are produced by material and vibration, and is only transmitted in the medium; At the same time, it also widely exist in nature, many animals can transmit and receive ultrasonic, of which the most bats is outstanding, it make use of the ultrasonic echo of weak flight and catch food in the dark. But ultrasound also has special properties, such as higher frequencies and shorter wavelengths, so it is similar to light waves with shorter wavelengths.

features

Ultrasonic wave is an elastic mechanical vibration wave, which has some characteristics compared with audible sound. The acceleration of vibration at the mass point of the transmission medium is very large. Cavitation occurs in liquid media when the ultrasonic intensity reaches a certain value.

Beam characteristics

Sound waves from a sound source travel in a direction (weak in other directions) called a beam. Because of its short wavelength, ultrasonic waves show a concentrated beam of radiation moving in a certain direction as they pass through the hole, which is larger than the wave length. Because of the strong direction of ultrasound, the information can be collected. Also, when the diameter of an obstacle is larger than the wavelength in the direction of ultrasonic propagation, the "sound shadow" will be generated behind the obstacle. These are like light passing through holes and obstacles, so ultrasonic waves have beam characteristics similar to light waves.

The beam quality of ultrasonic wave is generally measured by the size of the divergence Angle (customarily)

This is shown as a semi-transmitting acetabulum. Taking plane circular piston type sound source as an example, its size determines

Basic principles of ultrasound

Basic principles of ultrasound (4 photos)

The suitable diameter (D) of the sound source and the wavelength of the sound wave are shown below. Thus, to make the sound body emits a directional good ultrasound, must make theta Angle is small, as far as possible direct spasm, D emitter (source) must be large or frequency f also must be high to get fired, otherwise will backfire. As the wavelength of ultrasound, shorter than the wavelength of audible sound, so it has better than audible sound wave beam characteristics, the higher the frequency of ultrasound, the shorter the wavelength is, the propagation characteristics is significant to a certain direction.

Absorption characteristics

When ultrasonic waves travel in various media, with the increase of the propagation distance, the ultrasonic intensity will gradually weaken and the energy will gradually be consumed. This kind of energy is absorbed by the media, which is called sound absorption. 1845 Stoke. G. G.) Found: when sound waves through the liquid, due to the liquid particle relative motion and the internal friction (that is, the viscous effect) lead to sound absorption, thus deduced caused by internal friction of the medium or viscous liquid in sound absorption formula. Also, when sound waves travel through liquid media, the temperature of the compression zone will be higher than the average temperature. On the contrary, the temperature is lower than the average temperature of sparse area, therefore, due to the heat transfer between the compression and sparse part of the sound waves to heat exchange, thus the decrease of the acoustic energy in 1868 Kirchhoff (Kirchhoff g.) caused by the sound absorption of heat conduction formula are deduced.

It can be seen that the absorption coefficient a is proportional to the square of the sound wave frequency, and when the frequency increases by 10 times, the absorption coefficient increases by 100 times. That is, the higher the frequency, the greater the absorption, so the sound wave propagation distance is smaller. In gas, Einstein proposed in 1920 by the audio frequency dispersion to determine the reaction rate of associated gas, thus promote the uptake of gas molecular thermal relaxation mechanism extends to the liquid, because the molecules in medium is obtained by the collisions between molecules absorb thermal relaxation. So low frequency sound waves can travel a long distance in the air, and high frequency sound waves decay rapidly in the air.

In solids, sound absorption depends largely on the actual structure of solids.

Caused by the above to see some of the reasons for different medium on the sound absorption, but the main reason is that the medium viscosity, heat conduction, the actual structure of the medium and the medium of microscopic dynamics caused by the relaxation effect, etc., in the process of sound absorption of the medium changes with the frequency of the sound. Ultrasonic wave is a high frequency sound wave, when propagating in the same medium, as the frequency increases, the energy absorbed by the medium increases. For example, the frequency is

The ratio of energy absorbed by Hz ultrasound in the air is

The sound waves of Hz are 100 times larger. For the same frequency of ultrasonic transmission due to different media. For example, when propagating in gas, liquid and solid, its absorption is the strongest, weaker and smallest respectively. So ultrasonic waves travel the shortest distance in the air.

When ultrasonic waves are propagated in a uniform medium, the acoustic intensity is weakened with the increase of distance due to the absorption of the medium, which is the attenuation of sound waves.

When the initial intensity of ultrasonic wave is J0, after a distance of x meters, its intensity is

Jx Joe - 2 ax = ""

Where a is the absorption coefficient (attenuation coefficient).

The absorption coefficient of sound waves in various media can be obtained from above.

It can be seen from this that the ultrasonic strength decreases exponentially. For example, the intensity of ultrasonic wave with a frequency of 106Hz will be reduced by half after it leaves the sound source and passes 0.5m in the air. It's traveling in water, it's going to be 500 million miles before it's going to be half as strong.

It can be seen that the distance traveled in water is 1000 times the distance traveled in air. The higher the frequency, the faster the decay. If ultrasound with a frequency of 1011Hz is transmitted through the air, it will disappear without trace in an instant when it leaves the sound source. In viscous liquids, ultrasound is absorbed faster. For example, at 200C, the intensity of the ultrasonic frequency of 300kHz is reduced to half. Only 0.4m thick air is enough

In water, it will pass through 440m. In transformer oil, it will spread about 100m. In paraffin wax, it will spread about 3m. Therefore, materials with large size (rubber, bakelite, asphalt) are good insulators for ultrasonic sound.

Big energy

Ultrasonic waves transmit much more energy than audible sounds. Because when the sound waves to reach a certain material, due to the effect of sound wave makes the molecules in a material also follow the vibration, the vibration frequency and acoustic frequency is same, so the molecular vibration frequency to determine the speed of molecular vibrations, the higher the frequency the greater the speed. Thus substance molecules by vibration and the energy, the energy besides is related to the mass of the molecules, and molecules is proportional to the square of the vibration velocity, and vibration velocity is related to molecular vibrational frequency, so the higher the frequency of the sound waves, namely material get the higher the energy of the molecules. Ultrasonic waves are much more frequent than sound waves, so they give material molecules more energy. This shows that ultrasound itself can be

To supply matter with enough energy.

The normal human ear can hear sound waves of low frequency and low energy. For example, the loud voice is about 50uW/cm2. But ultrasonic waves have much more energy than sound waves. For example, the frequency is

The ultrasonic vibration of Hz has the same energy than the amplitude and frequency

Hz waves vibrate a million times more energy because the energy of the sound waves is proportional to the square of the frequency. It can be seen that it is mainly the huge mechanical energy of ultrasonic wave

The mass point of matter produces a great acceleration.

In normal operation, the normal loudness of the loudspeaker sound intensity is

W/cm2. The gun shot loudly

W/cm2. The sound of moderate loudness makes the point of mass of water receive only a few percent of the acceleration of gravity (980cm/s2), so it will not affect water. However, if ultrasound is applied to water, the acceleration of the water point may be hundreds of thousands or even millions of times greater than that of the force, so it will be

The water point produces rapid movement. It plays an important role in ultrasonic extraction.

Cavitation phenomenon

Cavitation is a common physical phenomenon in liquids. In a liquid due to physical effect, such as eddy current and ultrasonic for some parts of the liquid form of local negative pressure zone, thus cause the fracture of liquid or a solid interface, form tiny cavity or air bubbles. The cavitation or bubbles in the liquid in the unsteady state, is born, the process of development, then quickly closed, when they quickly closed burst, creates a shock wave, make the local area has a lot of pressure. Such cavitation occurs when bubbles or bubbles form in a liquid and then close quickly.

About the basic process of cavitation and the difference between the cavitation and boiling briefly as follows: when the liquid at constant pressure heating or constant temperature by static or dynamic method under reduced pressure, can achieve 茌 liquid vapor cavity or cavity filled with gas (or holes) began to appear and development, and then closed. If this state is caused by temperature rise, it is called "boiling". If the temperature is basically constant and the local pressure drops, it is called "cavitation".

It can be seen from the basic process of overhead cavitation that cavitation has the following characteristics: cavitation is a phenomenon occurring in liquid, which will not occur in any normal environment. Cavitation is the result of liquid decompression, so the cavitation can be controlled by controlling the degree of decompression. Cavitation is a dynamic phenomenon which involves the development and closure of cavitation.

Ultrasonic cavitation is strong ultrasonic propagation in the liquid, caused by a kind of peculiar physical phenomena, also is the production of hollow liquid cavity caused, grew up, compression, closed, bounce fast repetitive movement of the peculiar physical process. Local high pressure generated in the bubble collapse when closed, high temperature, because of the sound field of frequency, sound intensity and liquid surface tension, viscosity, and the surrounding environment of temperature and pressure effects, such as liquid particles of gas nucleus in the sound field under the action of response may be moderate, can also be strong. Therefore, sound cavitation is divided into steady state and transient cavitation.

Steady cavitation refers to the dynamic behavior of cavitation bubbles containing gases and vapors. This cavitation process is usually produced when the sound intensity is less than 1W/cm2. Cavitation bubbles vibrate for a long time and last for several sound waves. The vibrating air bubbles in the sound field, due to the expansion of bubble surface area than the compression of the big, spread the expands to the gas inside the bubble spread to the outside of the bubble, more than when the compression and make bubbles in the process of vibration increases. When the vibration amplitude is large enough, the bubble will change from stable state to transient cavitation and then collapse.

Transient cavitation generally refers to the cavitation bubbles generated when the sound intensity is greater than 1W/cm2, and the vibration is only completed within one sound period. When the sound intensity is high enough and the sound pressure is negative for half a week, the liquid is subjected to great tension. The bubble core expands rapidly and can reach several times its original size. Then, when the sound pressure is half a week, the bubbles are compressed and burst into many small bubbles to form new cavitation nuclei. When the bubble contracts rapidly, the gas or steam in the bubble is compressed, and within a very short time of cavitation bubble collapse, the bubble generates a high temperature of about 5000K, similar to the temperature on the surface of the sun. Local pressure of about 500 atmospheres, equivalent to the pressure of the deep ocean floor; The temperature change rate is as high as 109K/s. Accompanied by a strong shock wave and a 400km/h jet, luminescence phenomenon, can also be heard small bursts. It can be seen that the energy provided by cavitation makes the local flow of high pressure, high temperature and high gradient, and provides a new way to extract the difficult components of medicinal materials.

Study of ultrasonic cavitation, began in the 1930 s, found in Monnesco and Frenzel sonoluminescence (SL), caused by recourse glow causes the study of ultrasonic cavitation bubbles movement and a survey of the basic effect. They used ultrasonic cavitation group bubble measurement in liquid to study the "cavitation of multiple bubbles". To cheng-hao wang, de-jun zhang of the Chinese academy of sciences in 1960 s should worship under the guidance of the academician, power type is used to study the method of complete movement process of a single cavitation bubble, and the experiment proved that the cavitation radiation and electromagnetic radiation in bubble closing time, they also studied the cavitation

Emulsifying and mechanical effects. In the 1980 s the United States Gaitan and Crum using acoustic levitation technology will be a single bubble "imprisoned" in container standing wave field wave abdomen place, with plus ultrasonic field synchronous cyclical process of cavitation and measured. These results provide a theoretical basis for the application of ultrasound in industry, agriculture, medicine and other fields, and also provide conditions for the measurement of ultrasonic cavitation.

Measurement of cavitation intensity

According to a report in the current, the intensity of ultrasonic cavitation is not an absolute measurement method, but the application of ultrasound in the actual effect is in some ways has a direct relationship to the intensity of cavitation, so look for ways to measure cavitation strength has important significance in practical application. And the intensity of cavitation and cavitation bubble is not only closed when the pressure from the size, the number of cavitation bubble in unit volume, also related to the various types of cavitation bubble, so can only measure the relative intensity. At present, it is mainly studied from the perspective of ultrasonic cleaning, so as to directly measure the effect of ultrasonic cleaning, and the methods are as follows:

Corrosion method: will be about 20 um thickness of aluminum, tin or lead foil in the sound field in a certain distance, the cavitation corrosion, in a certain period of time, according to the corrosion, the weight of the sample to measure relative cavitation intensity, this method is called pseudo corrosion method. This method can measure the relative cavitation intensity from the liquid surface to different depths. The method of measurement is to ask metal sample surface finish is consistent, carry out several measurements, in order to find out the average value.

Chemical method: when sodium iodide is placed in carbon tetrachloride, the relative cavitation intensity is measured by the amount of iodine released under acoustic cavitation. This method is called chemical method. This method is to use spectrophotometer or radioactive tracer method for the quantitative determination of iodine release. Because in ultrasonic intensity 5-30 W/cm2, the amount of iodine released increased with the increase of sound intensity after 1 min treatment, the cavitation intensity was measured with the size of the amount released.

Scavenge method: clean with radioactive contamination artifacts as a sample, use after ultrasonic cleaning, quantitative measuring the amount of dirt removed, in order to measure the effects of ultrasonic cleaning or relative cavitation intensity, this method is called to remove dirt. In the practical application, there are also measurement methods of cavitation noise, which are not described here.

The negative effect and application of ultrasonic cavitation

Because of the nonlinear vibration of bubbles caused by acoustic cavitation and the blasting pressure when they burst, many physical and chemical effects can be produced with cavitation. These effects have negative effects, but they also have applications in engineering technology. For example, the surface of high-speed rotating propeller blades used by ships is often hit by cavitation pressure, and "corrodes" into some marks. When cavitation is serious, the presence of a large number of air bubbles will affect the thrust of the propeller. In civil industry, cavitation "corrosion" can damage pipes and devices. However, the use of cavitation shock waves or the local high temperature of the closed bubbles can be beneficial in the industry. For example, ultrasonic cleaning refers to the complex construction of abnormal channels by sound waves, and the cleaning of machine parts and microcomputer parts placed in detergent by ultrasonic cavitation. Ultrasonic descaling and descaling can also be carried out in the boiler. The emulsifying process of pharmaceutical production can also be achieved by cavitation. Emulsions of mixed solutions such as oil and water can be prepared in industry. Ultrasonic welding (breaking the oxide layer of metal surface and facilitating metal welding); Ultrasonic cavitation is used to promote some chemical reaction processes. Breaking down the fine wall of plants, promoting the dissolution of chemical components into solvents, and improving the rate of chemical composition. [2]

The principle of ultrasonic cleaning is the high-frequency oscillating electrical signal produced by the generator. The high frequency mechanical vibration is converted into high frequency by the transducer, which is transmitted to the cleaning liquid, and the workpiece is cleaned efficiently. Its working mechanism is to use cavitation effect to double or more than ten sales to improve the cleaning effect. When the liquid is put into the cleaning machine and ultrasonic wave is applied, the ultrasonic wave in the cleaning liquid is a kind of high frequency wave with dense phase and radiation transmission, which makes the liquid vibrate back and forth at high speed. In the negative pressure area of the vibration due to the surrounding liquid to supplement, countless small vacuum bubble formation, and in positive pressure area, tiny air bubbles suddenly closed, under pressure in the process of closing due to collision between liquid have a powerful shock waves formed up to thousands of atmospheres of instantaneous high pressure, effect on the cleaning of the workpiece. The greasy and impurities adsorbed on the workpiece are rapidly separated from the workpiece under continuous instantaneous high pressure. So as to achieve the goal of cleaning. Two main parameters of ultrasonic wave: frequency: F > 20KHz; Power density: p= transmitting power (W)/ transmitting area (cm2); Usually p acuity 0.3 w/cm2; In a liquid for the spread of ultrasonic cleaning of dirt on the surface of the object, and its principle can be used to explain the phenomenon of cavitation that ultrasonic vibration propagation in a liquid sonic pressure reaches a atmospheric pressure, the power density is 0.35 w/cm2, then the ultrasonic sound wave can achieve vacuum or negative pressure, the pressure peak but, in fact, there has no negative pressure, so produce a lot of pressure in the liquid, the liquid molecular nuclear ripping into empty shelves. The cavity is very close to a vacuum, and it ruptures when the ultrasonic pressure reaches its maximum when the ultrasonic pressure is reversed. The phenomenon of shock waves caused by the rupture of numerous small cavitation bubbles is called cavitation. Too little sound can't produce cavitation. Ultrasonic cleaning machine is composed of three main parts: (1) the load of the cleaning fluid cleaning stainless steel cylinder (2) (3) ultrasonic transducer ultrasonic cleaning machine ultrasonic generator with high cleanliness, machine the advantages of low noise and long life of equipment. And can be more complex geometric shape, such as a variety of blind holes, micro holes, deep holes, etc. with other cleaning methods difficult to clean parts for efficient cleaning. As a result of the above unique performance, more and more people recognize and accept. Second, the characteristics of the equipment when the ultrasonic cleaning machine filled with water, after turning on the power supply circuit converts the alternating current (ac) of 50 hz into ultrasonic frequency alternating current, generate oscillation, the formation of the oscillation is composed by inductance and capacitance transducer resonant circuit, and the oscillation signal through to the constant feedback to proceed. The transistor amplifies and then sends it to the series resonant circuit. This resonant frequency is precisely adjusted on the natural resonant frequency of the transducer before the machine leaves the factory to give the best effect to the transducer. Transducer is through the stud and strong adhesive bonding on the stainless steel cleaning tank bottom, the transducer ultrasonic mechanical energy through the bottom of the channel to pass to the liquid in the tank, and then applied to liquid of artifacts to be cleaned, so as to realize the function of ultrasonic cleaning. The high power transistor works at switch saturation, so its output waveform is square. When the square wave enters the resonant circuit and is filtered by inductance and capacitance, it becomes sine wave. Therefore, the current waveform acting on the transducer has become sine wave. There are two kinds of ultrasonic power generator of ultrasonic cleaning machine, one is self-excited circuit, the other is separately excited circuit. The self-excited circuit is simple, practical and economical. Other excited circuits have high power, with frequency tracking and current limiting, heating and other kinds of protection. The two circuits are suitable for enterprises at different levels and more customers. 1. Connect the generator to the cable in the cleaning slot. 2. Inject the selected cleaning solution into the tank. 3. Connect the generator to 220V plus or minus 10% 50hz ac power supply. 4. Turn on the generator power switch, and the power indicator light is on (at this point, the liquid in the tank starts to vibrate and cavitation). 1. In order to extend the service life, it is recommended to place the equipment in a ventilated and dry area, and the fan hole on the back side of the generator should be cleaned regularly. The generator has air vents on all sides to keep the air flowing unimpeded. 2. (1) cleaning tank must be put into the liquid to boot, the lowest water level > 100 mm (bottom) of co-vibrating type and horizontal, transducer in the side, for the tank cleaning trough along the 100 mm, as in the air condition open a chance to damage the machine. (2) when the cleaning cylinder body temperature is normal temperature, do not directly inject the high-temperature liquid into the cylinder, so as to avoid loosening the transducer and affecting the normal use of the machine. (3) when the cleaning solution need to be replaced due to pollution, not to the cryogenic liquid directly into the high temperature inside the cylinder, it can also lead to transducer, should close the heater switch at the same time, in order to avoid the heater damaged by slot without liquid. (4) check the transducer regularly to avoid dampness and impact, so as to avoid unnecessary loss. 3. After use, the main power should be turned off. 4. Do not restart the machine immediately after power off, the clearance time should be more than one minute.

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