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.
Brief History
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.
Cleaning
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.
Flow Metering
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.
Non-destructive Testing
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.
Machining
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.
Electronics
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.
Materials Science
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.
Sonochemistry
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.
Agriculture
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.
Oceanography
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.
Medical Applications
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.
The Future
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.
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