- What ultrasound is
- Ultrasound discovery and history
- Ultrasound generation
- Ultrasound application
- Ultrasound effects on various substances
- Summary table of ultrasound frequencies and fields of application
WHAT ULTRASOUND IS.If any body vibrates in an elastic medium faster than the medium flows around it, the medium is either compressed, or rarefied by this motion. Layers of increased and reduced pressure run away from this vibrating body right and left, and form acoustic waves. If vibrations of the body creating the wave, follow one after another not less frequently than 16 times per second and not more frequently than18 thousand times per second, then the human ear can hear them.
Frequencies of 16 - 18,000 Hz, which can be perceived by human hearing apparatus, are commonly referred to as acoustic ones, e.g. mosquito pipe is >> 10 kHz. However, air, depths of the sea and the earth interior are full of sounds located below or above this range, which are infra sound and ultrasound. In nature, ultrasound exists as a component of numerous natural noises: wind, waterfall, rain, marine rubble rolled by the wash of the sea, or in thunderstorms. Many mammals, for instance, cats and dogs, are capable of perceiving ultrasound with a frequency of up to 100 kHz, while locating abilities of night bats, night insects and marine animals are well known to everyone. In the late 19th century, due to acoustics development, the existence of unheard sounds was discovered. At the same time, initial studies of ultrasound began, however the ground for its application was only laid in the first one-third of the 20th century.
The lower border of the ultrasonic range are elastic vibrations at a frequency of up to 18 kHz. The upper border of ultrasound is defined by the origin of elastic waves, which can only spread, if the wavelength is much greater than the molecule free run (in gases) or interatomic distance (in liquids and solids). In gases, the upper border is ~106 kHz, and in liquids and solids ~ 1,010 kHz. Generally, frequencies of up to 106 kHz are called ultrasound. Higher frequencies are commonly referred to as hypersound.
Ultrasonic waves are inherently the same as the audible range waves and obey the same natural laws. Ultrasound however exhibits some specific features, which have defined its extensive application in science and technology. Here are the main of them:
- Short wavelength. For the lowest ultrasonic range in most of media, the wavelength does not exceed several centimeters. Short wavelength provides the ray propagation of ultrasonic waves. Near the transducer, ultrasound propagates in the form of bundles of rays close by size to the transducer. Falling on inhomogeneities in the medium, ultrasonic bundle behaves as a light beam suffering reflection, refraction, and scattering, that allows formation of audible images in optically opaque media using purely optical effects (focusing, diffraction, etc.).
- Short period of vibrations that allows radiation of ultrasound in the form of pulses and perform accurate time selection of propagating signals in the medium.
- A possibility of obtaining high vibration energies at low amplitude, because energy of vibrations is proportional to square frequency. This allows formation of high-energy ultrasonic bundles and fields, hence requiring no large-sized installations.
- High acoustic currents are developed in the ultrasonic field. Therefore, the effect of ultrasound on the medium causes specific effects: physical, chemical, biological, or medical, such as cavitation, acoustocapillary effect, dispersion, emulsification, degassing, antiseptics, local heating, and many others.
- Ultrasound is unheard and makes no discomfort for the maintenance staff.
Interest in acoustics was provoked by navies of the leading states - Great Britain and France, as acoustic signal is the only sort of signals widely propagating in water. In 1826, Colladon, French scientist, measured acoustic velocity in water. Colladon's experiment is considered to be the birthday of modern hydroacoustics. Underwater bell in the Lake of Geneva was hit simultaneously with gunpowder ignition. Colladon observed the powder flash from the distance of 10 miles. He also heard bell ringing using an under water acoustic tube. Colladon measured time period between these two events and calculated the acoustic velocity - 1,435 m/s. The difference with modern calculations is just 3 m/s.
In 1838, in the USA, sound was first used to determine the seafloor profile for the purpose of laying a telegraph cable. The acoustic source, as in Colladon's experiment, was underwater bell, and receivers were acoustic tubes, submerged from the ship board. Test results were disappointing. Bell ringing (as well as exploding of gunpowder charges in water) produced a too weak echo, almost unheard among other sounds of the sea. Shifting to the zone of higher frequencies allowing formation of directional sonic bundles was required.
In 1883, Francis Galton, English scientist, invented the first ultrasonic generator. Ultrasound was generated like whistling on the knife edge, if you blow on it. In Galton's whistle, the role of such edge was played by sharp edge of a cylinder. Air or another gas pressurized through a circular nozzle of the same diameter, as the cylinder edge, ran up the edge and produced high-frequency vibrations. Blowing hydrogen through the whistle produced vibrations of up to 170 kHz frequency.
In 1880, Pierre and Jacques Curie made a crucial discovery in the ultrasonic technology. Curie brothers noticed that if quartz crystals are compressed, they generate electric charge directly proportional to the force applied. This phenomenon was called "piezoelectricity" from Greek word that means "press". Moreover, they demonstrated the reverse piezo-effect occurred when quickly variable electric potential was applied to the crystal making it to vibrate. Henceforth, there appeared technical capability to make small-sized ultrasonic transducers and receivers.
Titanic’s wreckage from collision with an iceberg, the necessity of antisubmarine (the novel weapon) warfare demanded fast development of ultrasonic hydroacoustics. In 1914, Paul Langevin, a French physicist, in cooperation with Konstantin Vasilievich Shilovsky, a talented Russian scientist-expatriate, designed a hydrolocator based on piezo effect and consisted of an ultrasonic generator and a hydrophone, ultrasonic wave receiver. Langevin-Shilovsky hydrolocator was the first ultrasonic device used in actual practice. At the same time, Russian scientist S.Ya. Sokolov developed a basis for ultrasonic testing in industry. In 1937, German psychiatrist Karl Dussick and his brother Fridrich, the physicist, for the first time applied ultrasound in detection of brain tumors; however, the results obtained were unreliable. In medical practice, ultrasound only started to be applied since 1950's in the USA.
Ultrasonic transducers can be subdivided into two large groups:
1) Vibrations are produced by obstacles on the way of gas or liquid flow or by gas or liquid flow termination. These are used with limitations, mainly for obtaining a strong ultrasound in gas.
2) Mechanical vibrations are obtained by conversion of current or voltage oscillations. In most ultrasonic devices, transducers of this group are used, which are piezoelectric and magnetostriction transducers.
Besides converters based on the piezo effect, a powerful ultrasonic bundle is obtained using magnetostriction transducers. Magnetostriction is variation in dimensions of bodies with the change of their magnetic state. A core from magnetostrictive material placed inside a suitable winding changes its length with respect to the shape of current signal passing through the winding. In 1842, this phenomenon was discovered by James Joule and was found typical of ferromagnetic materials and ferrite magnets. Most useful magnetostriction materials are alloys based on nickel, cobalt, iron, and aluminum. The highest intensity of ultrasonic radiation is obtained from permendur alloy (Co – 49 %, V – 2 % V, and the rest is Fe), which is used in powerful ultrasonic transducers, in acoustic antiscale devices «Acoustic-T», manufactured by our Company, in particular.
Various ultrasound applications may be conditionally subdivided into three directions:
- Obtaining information about a substance
- Affecting a substance
- Signal processing and transmission
The dependence of acoustic waves propagation velocity and attenuation rate on substance properties and processes occurring in them is used in the following types of studies:
- investigation of molecular processes in gases, liquids and polymers
- studying structure of crystals and other solids
- control for chemical reactions, phase transitions, polymerization etc.
- determination of solution concentrations
- determination of strength properties and composition of materials
- determination of admixtures
- determination of liquid and gas flow rate
Information about molecular structure of a substance can be obtained from measurements of sound velocity and absorption factor in it. This allows measurement of solution concentrations and suspensions in pulps and fluids, monitoring of extraction, polymerization, aging, and chemical reaction kinetics. Determination of substance composition and presence of admixtures using ultrasound is highly accurate up to percent fractures.
Measurement of sound velocity in solids allows determination of elastic and strength parameters of structural materials. This indirect method of strength determination is suitable due to its simplicity and availability in-situ.
Ultrasonic gas detectors monitor processes of hazardous admixture accumulation. Temperature dependence of ultrasound velocity is used for noncontact gas and liquid temperature measurements.
The principle of ultrasonic flowmeters applying the Doppler effect is based on measuring acoustic velocity in flowing liquids and gases, including heterogeneous ones (emulsions, suspensions or pulps). Similar installations are used for blood velocity and flow determination in clinics.
A large group of measurement methods is based on reflection and scattering of ultrasonic waves on interfaces. These methods allow precise determination of foreign bodies location in the medium and are used in the following branches:
- nondestructive control and testing
- medical diagnostics
- level sensing of liquids and granular materials in closed bulks
- determination of article dimensions
- sound field visualization — acoustic imaging and sonic holography
Reflection, refraction and ability to focus ultrasound is used in ultrasonic testing, acoustic microscopes, medical diagnostics, and for studying macroheterogeneities in substances. The presence of heterogeneities and their coordinates are determined by reflected signals or their shade structure.
Methods of measurement based on resonance system parameter dependence on properties of the medium loading it (impedance) are applied to continuous measurement of viscosity and density of liquids, thickness of parts only accessible from a single side. The same principle forms the basis of ultrasonic hardness gauges, level sensors and switches. The advantages of ultrasonic control methods are: short time of measurement, possibility of controlling explosive, aggressive and toxic media, and no direct effect of the tool on the controlled media and processes.
Ultrasound effect on a substance causing its irreversible changes is widely used in industry. Meanwhile, mechanisms of ultrasound action are different for different media. In gases, the main acting factor is acoustic flows that accelerate heat-and-mass exchange processes. Hence, the efficiency of ultrasonic mixing is much higher than hydrodynamic one, as the boundary layer is thinner and, consequently, possesses higher temperature and concentrations gradients. This effect is used in such processes as:
- ultrasonic drying
- burning in ultrasonic field
- aerosol coagulation
In ultrasonic treatment of liquids, cavitation is the basic acting factor. The following technological procedures are based on the cavitation effect:
- prevention of scale formation
- ultrasonic cleaning
- metal plating and soldering
- acoustocapillary effect — penetration of liquids into micropores and microcracks. This is used for impregnating porous materials and takes place at any ultrasonic treatments of solids in liquids.
- dispersion of solids in liquids
- degassing (de-aeration) of liquids
- stimulation of electrochemical processes
- aerosol production
- micro-organism killing and ultrasonic sterilization of instruments
Acoustic flows represent one of the main mechanisms of ultrasonic effect on a substance. It consists in ultrasonic energy absorption by the substance and the boundary layer. Contrary to hydrodynamic flows, acoustic flows feature low thickness of the boundary layer and ability to make it thinner as vibration frequency increases. This makes temperature and concentration boundary layer thinner and increases temperature and concentration gradients that determine heat and mass transfer rate. This provides acceleration of combustion, drying, mixing, refining, diffusion, extraction, impregnation, sorption, crystallization, dissolution, liquids and melts degassing processes. In a high-energy flow, acoustic waves act due to energy of the flow itself, by changing its turbulence. In this case, acoustic energy may give a percent fracture of the flow energy.
As high-power acoustic waves pass through a liquid, the so-called acoustic cavitation occurs. In a powerful acoustic wave, during rarefaction half-cycles, cavitation bubbles are formed, which then collapse rapidly when transiting to the increased pressure zone. In the cavitation zone, powerful hydrodynamic perturbations occur looking like micro-shock waves and micro-flows. Moreover, bubble collapsing is accompanied by high local heating of the substance and gas release. Such action destroys substances even as strong as steel or quartz. This effect is used for dispersion of solids, obtaining fine dispersion emulsions of immiscible liquids, excitation and acceleration of chemical reactions, micro-organisms killing, extraction of enzymes from animal and plant cells. Cavitation also determines low glowing of liquid affected by ultrasound - sonoluminescence, and anomalously deep penetration of liquids into capillaries — acoustocapillary effect.
Acoustic antiscale devices are based on cavitation dispersion of calcium carbonate crystals (scale). Particles present in the liquid split affected by ultrasound, and their average size reduces from 10 to 1 micron, and their quantity and surface area increase. This makes scale to form directly in the liquid, rather than on the heat exchange surface. Ultrasound causes similar effect on the scale layer already formed, forming microcracks in it that provide pieces of scale chipping off the heat exchange surface.
Ultrasonic cleaning units that use cavitation and subsequently produced microflows, are applied to remove contamination both rigidly bound to the surface, such as cinder, scale or burrs, and soft pollutants like fatty films, dirt, etc. The same effect is used for electrolytic processes stimulation.
Ultrasound also causes such an interesting effect like acoustic coagulation, i.e. closing up and enlargement of particles suspended in a liquid or gas. Physical principle of this phenomenon is not definitely clear yet. Acoustic coagulation is applied to precipitate industrial dusts, fumes and mists at a frequency of up to 20 kHz, rather low for ultrasound.
Machining of solids with use of ultrasound is based on the following effects:
- friction decrease between surfaces in case of ultrasonic vibrations of one of them
- yield strength decrease or elastic deformation under ultrasound effect
- strengthening and decrease of residual stresses in metals under impact action of a tool with ultrasonic frequency
- Combined action of static compression and ultrasonic vibrations is used in ultrasonic welding
Four types of machining making use of ultrasound are distinguished:
- sizing of parts from hard and fragile materials
- cutting of hard-to-cut materials by applying ultrasound to the cutting tool
- burr removal in an ultrasonic bath
- grinding of viscous materials applying ultrasound to clean the grinding wheel
Ultrasound impact on biological objects causes various effects and responses in organism's tissues, which is widely used in ultrasonic therapy and surgery. Ultrasound plays the role of a catalyst that accelerates establishing the balanced, in terms of physiology condition of the organism, i.e. the healthy condition. Ultrasound has much higher effect on unhealthy tissues rather than on healthy ones. Ultrasonic spraying of medicines for inhalation is also used. Ultrasonic surgery is based on the following actions: destruction of tissues directly by focused ultrasound and application of ultrasonic vibrations on cutting surgical instrument.
Ultrasonic devices are applied in conversion and analog processing of electronic signals and in controlling light signals in optics and optoelectronics. Low velocity of ultrasound is used in delay circuits. Optical signals control is based on diffraction of light by ultrasound. One of the kinds of such diffraction is the so-called Bragg's diffraction, which depends on ultrasound wavelength, which provides separation of a short frequency range from the light broad spectrum, i.e. filtering light.
Ultrasound is utterly interesting phenomenon, and one may suggest that many of its practical abilities are not known to people yet. We like and understand ultrasound, and will be glad to discuss any ideas associated with its application.
Our Company Koltso-energo, LLC is dealing with manufacture and installation of antiscale acoustic devices «Acoustic-T». Our devices feature exclusively high level of ultrasonic signal, which allows them to operate in boilers without water treatment and water-steam boilers with artesian water. However, prevention of scale formation is just a minor part of all ultrasound capabilities. This wonderful natural instrument has vast opportunities we want to tell you about. Our personnel have been working at the leading Russian enterprises dealing with ultrasound for years. We know much about this phenomenon. In addition, if it should be necessary to apply ultrasound to your technology, we will be glad to help you.
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