Ultrasonic Sound Information
Ultrasonics is the study and application of high-frequency sound waves, usually in excess of 20KHz (20,000 cycles per second).
Modern ultrasonic generators can produce frequencies of as high as several gigahertz (several billion cycles per second) by transforming alternating electric currents into mechanical oscillations, and scientists have produced ultrasound with frequencies up to about10GHz (ten billion vibrations per second). There may be an upper limit to the frequency of usable ultrasound, but it is not yet known.
Higher frequencies have shorter wavelengths, which allows them to reflect from objects more readily and to provide better information about those objects. However, extremely high frequencies are difficult to generate and to measure.
Detection and measurement of ultrasonic waves is accomplished mainly through the use of piezoelectric receivers or by optical means. The latter is possible because ultrasonic waves are rendered visible by the diffraction of light.
Ultrasound is far above the range of human hearing, which is only about 20Hz to 18KHz. However, some mammals can hear well above this. For example, bats and whales use echo location that can reach frequencies in excess of 100KHz.
The roots of ultrasonic technology can be traced back to research on the piezoelectric effect conducted by Pierre Curie around 1880. He found that asymmetrical crystals such as quartz and Rochelle salt (potassium sodium tartrate) generate an electric charge when mechanical pressure is applied. Conversely, mechanical vibrations are obtained by applying electrical oscillations to the same crystals.
One of the first
applications for ultrasonics was sonar (an acronym
for sound navigation ranging). It was employed on a large scale by the
Sonar operates by bouncing a series of high frequency, concentrated sound wave beams off a target and then recording the echo. Because the speed of sound in water is known, it is an easy matter to calculate the distance of the target.
Prior to World War II researchers were inspired by sonar to develop analogous techniques for medical diagnosis. For example, the use of ultrasonic waves in detecting metal objects was discussed beginning in 1929. In 1931 a patent was obtained for using ultrasonic waves to detect flaws in solids.
In the 1950s
researchers in the
The first ultrasonic instruments displayed their results with blips on an oscilloscope screen. That was followed by the use of two dimensional, gray scale imaging. High resolution, color, computer-enhanced images are now common,
Ultrasonics technology is now employed in a wide range of applications in research, industry and medicine.
Perhaps the most common type of applications for ultrasonics is cleaning. This includes the removal of grease, dirt, rust and paint from metal, ceramic, glass and crystal surfaces of parts used in the electronic, automotive, aircraft, and precision instruments industries.
This cleaning is accomplished through the use of the cavitation effect. Cavitation is the rapid formation and collapse of tiny, gas and vapor filled bubbles or cavities in a solution that is irradiated with ultrasound.
The repeated collapsing of these bubbles produces tiny shock waves that scrub the contaminants off of the surfaces of the parts. A variety of cleaning solutions can be used, including water, detergents and organic solvents.
Ultrasonic cleaning can be highly efficient for applications in which extreme cleanliness is required. It is also well suited for cleaning parts with very complex shapes.
Examples of specific applications are optical glass for lenses, quartz crystals, small ball bearings and dental bridges.
Ultrasonic metering of flowing liquids is based on the Doppler effect. This type of metering has the advantages that it has no effect on the flow and can be used to monitor closed systems, such as a coolant in a nuclear power plant or the flow of blood to the human heart.
Nondestructive testing has been practiced for many decades, with initial rapid developments in instrumentation spurred by the technological advances that occurred during World War II and the subsequent defense effort. Among the techniques that have been developed are eddy currents, x-rays, dye penetrants, magnetic particles and ultrasonics.
Ultrasonics is particularly attractive for non-destructive testing because it can be used with most types of materials, and it can be used to investigate both their surfaces and their interiors.
The attenuation of ultrasonic waves is very low in solids and liquids, thus allowing solids more than 20 feet in thickness to be penetrated by both continuous and pulsed waves.
Ultrasonic testing uses sound waves to detect imperfections in material and to measure material properties. The most commonly used ultrasonic testing technique is pulse-echo, wherein sound is introduced into a test object and reflections (echoes) returned to a receiver from internal imperfections or from the part's geometrical surfaces are analyzed. Defects and other internal irregularities result in changes in the echo pattern from the waves.
The shadow method is used to inspect large castings and forgings, The echo reflection method is used primarily for the inspection of welds and castings.
The primary purpose of ultrasonic testing was initially the detection of defects so that defective components could be removed from service. However, in the early 1970's the ability to detect small flaws led to the unsatisfactory situation that more and more parts had to be rejected, even though the probability of failure had not changed.
However, the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a material property, fracture toughness, were known. Other guidelines were developed to predict the rate of growth of cracks under cyclic loading (fatigue). This made it practical to accept structures containing defects if the sizes of those defects were known. This formed the basis for new philosophy of "fail safe" or "damage tolerant" design. Components having known defects could continue in service as long as it could be established that those defects would not grow to a critical, failure producing size.
Thus it became necessary to not only detect flaws but to also obtain quantitative information about flaw size in order to make predictions of remaining life.
These concerns, which were felt particularly strongly in the defense and nuclear power industries. They led to the emergence of quantitative nondestructive evaluation (QNDE) as a new discipline.
Ultrasonic testing of forgings, castings and other metal parts has become standard. Examples of applications are axles for vehicles and machine parts.
A somewhat different type of example is railroad rails which are already in use. Specially designed vehicles containing ultrasound equipment regularly travel on major railroad lines scanning the rails underneath them for cracks and other defects which are usually invisible to the human eye but which could eventually lead to a derailment.
Another industrial application for ultrasonics technology is the machining of materials. Ultrasonic machining has the advantage over conventional, mechanical machining techniques that it is well suited for processing unusual or complex shapes because no rotary tool is required.
This technique can be used for very hard and highly abrasive materials because the actual cutting is done by an abrasive material in a liquid carrier rather than a bit or blade which is subject to abrasion. Among the materials that can be so processed are soft steel, ceramics, glass and tungsten carbide.
Soldering and Welding
Ultrasound has also proved to be very useful for joining materials. It can be used for both soldering and welding.
In the case off soldering, the cavitation produced by high intensity ultrasonic waves destroys the oxide layer on aluminum, thus permitting parts to be joined with tin soldering materials without the use of flux.
In ultrasonic welding, pressure and heat generated by the intense vibratory action of the material to be welded and an ultrasonic welding head allows a thin sheet of metal to be joined to a much thicker section. Ultrasonic techniques can likewise be used to weld pieces of similar or dissimilar plastic to each other.
Ultrasonics is intimately related to the electronics industry. One reason is, of course, because ultrasonic waves are generated, detected and interpreted by electronic devices.
Also, ultrasonics technology is used extensively for the testing, cleaning and soldering of electronic components.
In addition, SAW (surface acoustic wave) filters are a type of electronic component which operates at ultrasonic frequencies. They are important for a growing range of electronics applications, including cellular phones and high performance TV receivers.
Applications in materials science include the determination of such properties of solids as compressibility, specific heat ratios and elasticity. Ultrasound can be used to produce an "acoustic microscope," which is able to visualize detail down to the one micron level.
Goals can range from the determination of fundamental microstructural characteristics such as grain size, porosity and texture (preferred grain orientation) to material properties related to such failure mechanisms as fatigue, creep, and fracture toughness, applications which are sometimes quite challenging due to the existence of competing effects.
Most applications of ultrasound use low power waves which pass through materials without affecting their physical or chemical structure.
However, very high intensity ultrasound can be used to cause chemical and physical changes in materials. This is accomplished by violent cavitation which results from the waves, creating stress and intensely heating a localized area.
Among the chemical processes which can be produced are acceleration of chemical reactions, oxidation, hydrolysis, polymerization, depolymerization and the production of emulsions.
The recent development of high intensity ultrasound generators suitable for large volume materials processing are making such sonochemistry more economical for commercial application.
Ultrasound has been used to measure the thickness of fat layers on pigs and cows as part of livestock management. It has also been used in improve the quality of homogenized milk. A related application is pest control, including killing insects.
In addition to the tracking of submarines, oceanographic applications include mapping the contours of the sea bottom, discovering sunken ships and searching for schools of fish.
One of the most rapidly advancing areas of application is medicine. Ultrasound is used for imaging the human body and as a means of heating tissues to treat various ailments. It is also used to sterilize surgical instruments.
Generally, the higher frequencies are used for medical imaging. The lower frequencies, 1 MHz or less, have longer wavelengths and greater amplitude for a given input energy, thus producing greater disruption of the medium.
Among the many important advances in recent years have been higher resolution, real-time monitoring and color images.
Ultrasonic scanning has the big advantage over x-rays that there are apparently no adverse health effects. For this reason, it has come into widespread use for monitoring the condition of the fetus as it grows in the womb. The increasingly high precision of such monitoring has made it possible to detect defects even at the very early stages of pregnancy.
Ultrasonic scanning has also become extremely useful for obtaining information about the flow of blood through the heart and about the condition of the heart valves. Other important diagnostic applications are the detection of kidney stones, gallstones and tumors.
An example of medical treatment applications is brain surgery, for which a sharply focused, high intensity beam can destroy diseased tissue with high precision. Ultrasound has also been used in the therapeutic treatment of arthritis, bursitis, contusions, lumbago and neuroma.
There is still considerable controversy about the mechanism of such therapy. However, there is little doubt that it can be effective. One theory is that the benefits arise from the heating and possibly a "micromassage" resulting from the ultrasound.
Generation of Ultrasound
Ultrasonic waves can be generated using mechanical, electromagnetic and thermal energy sources. They can be produced in gasses (including air), liquids and solids.
Magnetostrictive transducers use the inverse magnetostrictive effect to convert magnetic energy into ultrasonic energy. This is accomplished by applying a strong alternating magnetic field to certain metals, alloys and ferrites.
Piezoelectric transducers employ the inverse piezoelectric effect using natural or synthetic single crystals (such as quartz) or ceramics (such as barium titanate) which have strong piezoelectric behavior. Ceramics have the advantage over crystals in that it is easy to shape them by casting, pressing and extruding.
The piezoelectric effect was first studied by Pierre Curie around 1880. He found that asymmetrical crystals such as quartz and Rochelle salt (potassium sodium tartrate) generate an electric charge when mechanical pressure is applied. Conversely, mechanical vibrations are obtained by applying electrical oscillations.
There is widespread agreement among researchers and scientists that ultrasonics is still in its infancy. This is evidenced by the fact that there is a great deal which is still not known about the field and the continued rapid rate of progress on nearly all aspects of it.
Among the keys to further progress will be advances in materials (particularly piezoelectric materials for the transducers), in electronics and in computers (for interpreting and enhancing the results).
Improvements in performance will be accompanied by further reductions in cost and increased diversity in the applications, likely including the development of some completely new uses.