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Ultrasonic wave is a sound wave of cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound. The production of ultrasonic wave is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium, a property also used by animals such as bats for hunting. The most well known application of ultrasonic sound wave is its use in sonography to produce pictures of foetuses in the human womb
A disturbance or variation that transfers energy progressively from point to point in a medium and that may take the form of an elastic deformation or of a variation of pressure, electric or magnetic intensity, electric potential, or temperature.
A transverse wave is a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y-z plane.
Longitudinal waves are waves that have the same direction of oscillations or vibrations along or parallel to their direction of travel
Sound is a type of longitudinal wave (compression type)
What is a sound wave?
Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.
What is an ultrasonic wave?
Ultrasonic wave is a sound wave of cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults and thus, 20 kHz serves as a useful lower limit in describing ultrasound. The production of ultrasonic wave is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium, a property also used by animals such as bats for hunting. The most well known application of ultrasonic sound wave is its use in sonography to produce pictures of foetuses in the human womb.
Properties of ultrasonic wave
Frequency of ultrasonic wave varies from 20khz to 200mhz
Wavelength of ultrasonic wave is lower than that of sound
How ultrasonic waves are generated?
In most applications, ultrasonic waves are generated by a transducer that includes a piezoelectric crystal that converts electrical energy (electric current) to mechanical energy (sound waves). These sound waves are reflected and return to the transducer as echoes and are converted back to electrical signals by the same transducer or by a separate one. Alternately, one can generate ultrasonic waves by means of magnetostriction (from magneto, meaning magnetic, and strictio, meaning drawing together.) In this case an iron or nickel element is magnetized to change its dimensions, thereby producing ultrasonic waves. Ultrasound may also be produced by a whistle or siren-type generator. In this method, gas or liquid streams are passed through a resonant cavity or reflector with the result that ultrasonic vibrations characteristic of the particular gas or liquid are produced.
Working principle of ultrasonic wave transducer
When a specially cut piezoelectric quartz crystal is compressed, the crystal becomes electrically charged and an electric current is generated: the greater the pressure, the greater the electric current. If the crystal is suddenly stretched rather than being compressed, the direction of the current will reverse itself. Alternately compressing and stretching the crystal has the effect of producing an alternating current. It follows that by applying an alternating current that matches the natural frequency of the crystal, the crystal can be made to expand and contract with the alternating current. When such a current is applied to the crystal, ultrasonic waves are produced. Depending on which way the crystal is cut, the waves can be focused along the direction of ultrasound propagation or at right angles to the direction of propagation.
Sound field of a non focusing 4MHz ultrasonic transducer with a near field length of N=67mm in water. The plot shows the sound pressure at a logarithmic db-scale
How an ultrasonic wave is detected?
Since piezoelectric crystals generate a voltage when force is applied to them, the same crystal can be used as an ultrasonic detector. Some systems use separate transmitter and receiver components while others combine both in a single piezoelectric transceiver.
Alternative methods for creating and detecting ultrasound include magnetostriction and capacitive actuation
Application of ultrasonic waves
Medical sonography (ultrasonography) is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs, to capture their size, structure and any pathological lesions with real time tomography images.
Biomedical ultrasonic applications
Ultrasound also has therapeutic applications, which can be highly beneficial when used with dosage precautions:
Ultrasounds are useful in the detection of pelvic abnormalities and can involve techniques known as abdominal (transabdominal) ultrasound, vaginal (transvaginal or endovaginal) ultrasound in women, and also rectal (transrectal) ultrasound in men.
Focused high-energy ultrasound pulses can be used to break calculi such as kidney stones and gallstones into fragments small enough to be passed from the body without undue difficulty, a process known as lithotripsy.
Treating benign and malignant tumours and other disorders via a process known as high intensity focused ultrasound (HIFU), also called focused ultrasound surgery (FUS). In this procedure, a generally lower frequencies than medical diagnostic ultrasound is used (250-2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by magnetic resonance imaging (MRI).
Enhanced drug uptake using acoustic targeted drug delivery (ATDD).
Therapeutic ultrasound, a technique that uses more powerful ultrasound sources to generate cellular effects in soft tissue has fallen out of favour as research has shown a lack of efficacy and a lack of scientific basis for proposed biophysical effects. Ultrasound has been used in cancer treatment.
Cleaning teeth in dental hygiene.
Focused ultrasound sources may be used for cataract treatment by phacoemulsification
Ultrasonic range finding
Principle of active sonar
A common use of ultrasound is in range finding; this use is also called SONAR, (sound navigation and ranging). This works similarly to RADAR (radio detection and ranging): An ultrasonic pulse is generated in a particular direction. If there is an object in the path of this pulse, part or all of the pulse will be reflected back to the transmitter as an echo and can be detected through the receiver path. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine how far away the object is.
The measured travel time of SONAR pulses in water is strongly dependent on the temperature and the salinity of the water. Ultrasonic ranging is also applied for measurement in air and for short distances. Such method is capable for easily and rapidly measuring the layout of rooms.
Although range finding underwater is performed at both sub-audible and audible frequencies for great distances (1 to several kilometres), ultrasonic range finding is used when distances are shorter and the accuracy of the distance measurement is desired to be finer. Ultrasonic measurements may be limited through barrier layers with large salinity, temperature or vortex differentials. Ranging in water varies from about hundreds to thousands of meters, but can be performed with centimetres to meters accuracy.
A non-chemical approach to eliminating algae using ultrasound waves
Ultrasonic waves, emitted from a transducer suspended from a discreet float, literally rip the outer cell walls of algae cells inhibiting their proliferation and maintaining a more sterile environment if that is desired. By keeping algae at bay there is less build-up on piping, drains and machinery in water basins and irrigation systems can remainclogfreelonger.
ULTRASONIC WELDING - SONIC WELDING
Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together
All ultrasonic welding systems are composed of the same basic elements:
A press to put the 2 parts to be assembled under pressure
A nest or anvil where the parts are placed and allowing the high frequency vibration to be directed to the interfaces
An ultrasonic stack composed of a converter or piezoelectric transducer, an optional booster and a sonotrode (US: Horn). All three elements of the stack are specifically tuned to resonate at the same exact ultrasonic frequency (Typically 20, 30, 35 or 40 kHz)
Converter: Converts the electrical signal into a mechanical vibration
Booster: Modifies the amplitude of the vibration. It is also used in standard systems to clamp the stack in the press.
Sonotrode: Applies the mechanical vibration to the parts to be welded.
An electronic ultrasonic generator (US: Power supply) delivering a high power AC signal with frequency matching the resonance frequency of the stack.
A controller controlling the movement of the press and the delivery of the ultrasonic energy.
Principles of Ultrasonic Cleaning
In general, ultrasonic cleaning consists of immersing a part in a suitable liquid medium, agitating or sonicating that medium with high-frequency (18 to 120 kHz) sound for a brief interval of time (usually a few minutes), rinsing with clean solvent or water, and drying. The mechanism underlying this process is one in which microscopic bubbles in the liquid medium implode or collapse under the pressure of agitation to produce shock waves, which impinge on the surface of the part and, through a scrubbing action, displace or loosen particulate matter from that surface. The process by which these bubbles collapse or implode is known as cavitations. High intensity ultrasonic fields are known to exert powerful forces that are capable of eroding even the hardest surfaces. Quartz, silicon, and alumina, for example, can be etched by prolonged exposure to ultrasonic cavitation, and "cavitation burn" has been encountered following repeated cleaning of glass surfaces. The severity of this erosive effect has, in fact, been known to preclude the use of ultrasonic's in the cleaning of some sensitive, delicate components
Ultrasonic waves and material analysis:
Recent advances and future trends
High-resolution ultrasonic spectroscopy is a novel technique for the direct and non-destructive analysis of the intrinsic properties of materials. It is now possible to measure both ultrasonic attenuation and ultrasonic velocity to a very high resolution in a broad range of sample types and volumes, thus greatly expanding the applications for this technique
High-resolution ultrasonic spectroscopy measures certain parameters of ultrasonic waves as they propagate through a sample. This novel technique for material analysis allows direct and non-destructive measurements of the intrinsic properties of materials, with- out taking the sample apart, adding something to it, or changing its state.
Samples may be analysed in their cur- rent state or alternatively, this technique can be used to monitor chemical and structural transformations in the samples.
Non-destructive characterisation of the interior of the analysed sample normally includes measuring the characteristics of signals which have travelled through the sample. These characteristics provide information on the inter-action of the signal with the sample. Any signal is a combination of waves and up until now, only one type of wave has dominated the field of material analysis, electromagnetic waves.
The second well-distinguished feature of ultrasonic waves is their ability to propagate through most materials, including opaque samples. It is also relatively easy to change the wavelength of an ultrasonic wave. In contrast to optics, where the wave originates from a light source and therefore special care must be taken to ensure spectral purity, ultrasonic waves are synthesised electronically. Therefore a typical ultra-sonic spectrometer can cover a broad range of wavelengths (from 10 to 100 and sometimes even more). The technique could be described as probing the interior of the sample under analysis with a set of fingers which differ in their length by more than one order of magnitude. Although the analytical power of ultra- sound has been known for some time, the application of this technique to materials analysis has been limited to certain areas due to a number of factors. These include limited resolution, large sample volumes and complicated measuring procedures. These limitations have now been overcome by advances in the modern principles of ultrasonic measurements, electronics and digital processing.
The two major parameters measured in high-resolution ultrasonic spectroscopy are ultrasonic attenuation and ultrasonic velocity (speed of ultrasound). Attenuation is determined by the energy losses in the compressions and decompressions in ultrasonic waves, which includes absorption and scattering contributions. As measurements of attenuation do not require high temperature stability of the medium, they can be performed in large samples. Thus attenuation was the parameter responsible for the largest portion of the applications of ultrasound in material analysis
Other daily application of ultrasonic waves
Ultrasonic waves can be used to break up fat globules in milk, so that the fat mixes with the milk (homogenization). In addition, pasteurization, the removal of harmful bacteria and microorganisms, is sometimes done ultrasonically.
By attaching an ultrasonic impact grinder to a magnetostrictive transducer and using an abrasive liquid, holes of practically any shape can be drilled in hard, brittle materials such as tungsten carbide or precious stones. The actual cutting or drilling is done by feeding an abrasive material, frequently silicon carbide or aluminium oxide, to the cutting area.
In ultrasonic soldering, high frequency vibrations are used to produce microscopic bubbles in molten solder. This process removes the metal oxides from the joint or surface to be soldered, and eliminates the need for flux.
Conversations can be overheard without using microphones by directing ultrasonic waves at the window of the room being monitored. Sounds in the room cause the window to vibrate; the speech vibrations produce characteristic changes in the ultrasonic waves that are reflected back into the monitor. A transducer can be used to convert the reflected vibrations to electrical signals that can be reconstructed as audible sounds.
Radio talk shows routinely use ultrasonic delay lines to monitor and cut off abusive callers before their comments are aired during radio talk shows. The ultrasonic delay line bounces the voice signal back and forth between two transducers until it has been monitored, then releases it for broadcast.