199 lines
7.6 KiB
Plaintext
199 lines
7.6 KiB
Plaintext
|
|
|
|
|
|
|
|
(word processor parameters LM=8, RM=75, TM=2, BM=2)
|
|
Taken from KeelyNet BBS (214) 324-3501
|
|
Sponsored by Vangard Sciences
|
|
PO BOX 1031
|
|
Mesquite, TX 75150
|
|
|
|
There are ABSOLUTELY NO RESTRICTIONS
|
|
on duplicating, publishing or distributing the
|
|
files on KeelyNet!
|
|
|
|
March 7, 1991
|
|
|
|
SONIC1.ASC
|
|
--------------------------------------------------------------------
|
|
|
|
Ultrasonics is the term used to describe the study of all soundlike
|
|
waves whose frequency is above the range of normal human hearing.
|
|
|
|
Audible sound frequencies (see SOUND AND ACOUSTICS) extend from
|
|
about 30 to 20,000 hertz (1 Hz = 1 cycle per second).
|
|
|
|
The actual waves and the vibrations producing them are called
|
|
ultrasound. As late as 1900 ultrasound was still a novelty and
|
|
studied only with a few specially made whistles; by 1930 it had
|
|
become an interesting but small area of physics research.
|
|
|
|
In the 1960s and '70s, however, it became an important research tool
|
|
in physics, a far-ranging instrument for flaw detection in
|
|
engineering, a rival to the X ray in medicine, and a reliable method
|
|
of underwater sound-signaling.
|
|
|
|
The range of frequencies available has been extended to millions and
|
|
even billions of hertz (megahertz and gigahertz).
|
|
|
|
Generation of Ultrasonics
|
|
|
|
The principal modern sources of ultrasound are specially cut
|
|
crystals of materials such as quartz or ceramics such as barium
|
|
titanate and lead zirconate.
|
|
|
|
The application of an alternating electrical voltage across the
|
|
opposite faces of a plate made of such a material produces an
|
|
alternating expansion and contraction of the plate at the impressed
|
|
frequency.
|
|
|
|
This phenomenon in crystals, known as PIEZOELECTRICITY, was first
|
|
discovered in the 1880s by Paul-Jacques and Pierre Curie.
|
|
|
|
If the frequency of alternation f is such that f = c/2l, where c is
|
|
the speed of sound in the material and l is the thickness of the
|
|
plate, the size of the alternating expansions and contractions
|
|
becomes very large, and the plate is said to exhibit RESONANCE.
|
|
|
|
Similar effects are observed in ceramics. Ceramic objects have the
|
|
added advantage of being able to be cast in the form of plates,
|
|
rings, cylinders, and other special shapes that are convenient for
|
|
engineering applications.
|
|
|
|
|
|
Page 1
|
|
|
|
|
|
|
|
|
|
|
|
In addition, some materials, such as cadmium sulfide, can be
|
|
deposited in thin films on a solid medium. Such material can then
|
|
serve as a transducer.
|
|
|
|
Still other ultrasonic transducers are produced in ferromagnetic
|
|
materials by varying the magnetic-field intensity in the material.
|
|
|
|
Wave Properties.
|
|
|
|
Ultrasonic waves travel through matter with virtually the same speed
|
|
as sound waves--hundreds of meters per second in air, thousands of
|
|
meters per second in solids, and 1,500 m/sec (5,000 ft/sec) in
|
|
water.
|
|
|
|
Most of the properties of sound waves (reflection, refraction, and
|
|
so forth) are also characteristic of ultrasound.
|
|
|
|
The attenuation of sound waves increases with the frequency,
|
|
however, so that ultrasonic waves are damped far more rapidly than
|
|
those of ordinary sound.
|
|
|
|
For example, an ultrasonic wave of 1 MHz frequency passing through
|
|
water will lose half of its intensity over a distance of 20 m (66
|
|
ft) through absorption of the energy by the water; in air, the
|
|
distance over which the intensity falls by half would be a few
|
|
centimeters.
|
|
|
|
At the audio frequency of 20,000 Hz, the corresponding distances for
|
|
water and for air would be about 50 km (30 mi) and 5 m (16.5 ft),
|
|
respectively.
|
|
|
|
In addition to waves that travel through the bulk of a material, it
|
|
is also possible to send waves along the surface of a solid.
|
|
|
|
These waves, called Rayleigh waves, can be produced and detected by
|
|
minute metallic "fingers" deposited on the surface of a
|
|
piezoelectric substrate. Techniques utilizing surface waves have
|
|
been widely exploited in signal processing.
|
|
|
|
Applications.
|
|
|
|
Perhaps the most widespread use of ultrasound has been in the
|
|
detection of obstacles in materials that do not transmit light
|
|
(optically opaque materials).
|
|
|
|
Thus, ultrasound is used in underwater signaling because a low-
|
|
frequency ultrasonic beam can penetrate many kilometers of the ocean
|
|
and be reflected back from any obstacle.
|
|
|
|
This is the principle of SONAR, which can be used to identify
|
|
submarines and map the ocean bottom. Sonar can even be used to
|
|
measure the thickness of ice packs by submarines traveling under the
|
|
polar ice caps.
|
|
|
|
If a short pulse of ultrasound is sent into a metal, it will be
|
|
reflected from any cracks or minute defects such as blowholes. A
|
|
system generating such pulses is widely used in flaw detection in
|
|
solids (nondestructive testing).
|
|
|
|
|
|
Page 2
|
|
|
|
|
|
|
|
|
|
|
|
Because different solids and liquids reflect at different rates, the
|
|
reflection of an ultrasonic pulse can also be used in medical
|
|
examinations, especially of unborn fetuses, and in the detection of
|
|
brain tumors and breast cancers.
|
|
|
|
Echocardiography, the study of heart motions by ultrasonic means, is
|
|
another medical application. An important physical property of
|
|
ultrasound is the vigorous small-scale vibration of the medium that
|
|
it represents.
|
|
|
|
This property has led to many industrial applications. The
|
|
vibrations can be used to shake dirt or other deposits off metals
|
|
(ultrasonic cleaning). Such vibrations can also be used in soldering
|
|
or welding.
|
|
|
|
The ultrasonic transducer serves to remove oxide from the outer
|
|
surface of the material, making more efficient the use of heat in
|
|
the joining process. Plastic powders can be molded into small
|
|
cylinders by similar techniques.
|
|
--------------------------------------------------------------------
|
|
|
|
If you have comments or other information relating to such topics
|
|
as this paper covers, please upload to KeelyNet or send to the
|
|
Vangard Sciences address as listed on the first page.
|
|
Thank you for your consideration, interest and support.
|
|
|
|
Jerry W. Decker.........Ron Barker...........Chuck Henderson
|
|
Vangard Sciences/KeelyNet
|
|
--------------------------------------------------------------------
|
|
If we can be of service, you may contact
|
|
Jerry at (214) 324-8741 or Ron at (214) 242-9346
|
|
--------------------------------------------------------------------
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Page 3
|
|
|
|
|