Ultrasound is used in many different
fields. Ultrasonic devices are used to detect objects and measure distances.
Ultrasonic imaging (sonography) is used in both veterinary medicine and human
medicine. In the nondestructive testing of products and structures, ultrasound
is used to detect invisible flaws. Industrially, ultrasound is used for
cleaning and for mixing, and to accelerate chemical processes. Organisms such
as bats and porpoises use ultrasound for locating prey and obstacles.
An ultrasonic examination in East
Germany, 1990
Ultrasonics is the application of
ultrasound. Ultrasound can be used for medical imaging, detection, measurement
and cleaning. At higher power levels, ultrasonics is useful for changing the
chemical properties of substances.
History
Acoustics, the science of sound, starts
as far back as Pythagoras in the 6th century BC, who wrote on the mathematical
properties of stringed instruments. Sir Francis Galton constructed a whistle
producing ultrasound in 1893. The first technological application of ultrasound
was an attempt to detect submarines by Paul Langevin in 1917. The piezoelectric
effect, discovered by Jacques and Pierre Curie in 1880, was useful in
transducers to generate and detect ultrasonic waves in air and water.[2]
Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he
demonstrated that bats hunted and navigated by inaudible sound and not vision.
Perception in humans and animals
Humans
The upper frequency limit in humans
(approximately 20 kHz) is due to limitations of the middle ear, which acts as a
low-pass filter. Ultrasonic hearing can occur if ultrasound is fed directly
into the human skull and reaches the cochlea through bone conduction, without
passing through the middle ear.[3]
Children can hear some high-pitched
sounds that older adults cannot hear, because in humans the upper limit pitch
of hearing tends to decrease with age.[4] An American cell phone company has
used this to create ring signals supposedly only able to be heard by younger
humans;[5] but many older people can hear the signals, which may be because of
the considerable variation of age-related deterioration in the upper hearing
threshold. The Mosquito is an electronic device that uses a high pitched
frequency to deter loitering by young people.
Some people find high-frequency sounds
and ultrasound extremely painful. This is often associated with autism and
sensory defensiveness[6] but can also be caused by hyperacusis
Animals
Many insects have good ultrasonic hearing
and most of these are nocturnal insects listening for echolocating bats. This
includes many groups of moths, beetles, praying mantids and lacewings. Upon
hearing a bat, some insects will make evasive manoeuvres to escape being
caught.[8] Ultrasonic frequencies trigger a reflex action in the noctuid moth
that cause it to drop slightly in its flight to evade attack.[9]Tiger moths
also emit clicks which may disturb bats' echolocation,[10][11] but may also in
other cases evade being eaten by advertising the fact that they are poisonous
by emitting sound.[12][13]
Dogs with normal hearing can hear
ultrasound. A dog whistle exploits this by emitting a high frequency sound to
call to a dog. Many such whistles emit sound in the upper audible range of
humans, but some, such as the silent whistle, emit ultrasound at a frequency in
the range 18–22 kHz.
Toothed whales, including dolphins, can
hear ultrasound and use such sounds in their navigational system (biosonar) to
orient and capture prey.[14] Porpoises have the highest known upper hearing
limit, at around 160 kHz.[15] Several types of fish can detect ultrasound. In
the order Clupeiformes, members of the subfamily Alosinae (shad), have been
shown to be able to detect sounds up to 180 kHz, while the other subfamilies
(e.g. herrings) can hear only up to 4 kHz.
Ultrasound generator/speaker systems are
sold as Electronic pest control devices, that claim to frighten away rodents
and insects, but there is no scientific evidence that the devices work.
Detection
and ranging
Non-contact sensor
An ultrasonic level or sensing system
requires no contact with the target. For many processes in the medical,
pharmaceutical, military and general industries this is an advantage over
inline sensors that may contaminate the liquids inside a vessel or tube or that
may be clogged by the product.
Both continuous wave and pulsed systems
are used. The principle behind a pulsed-ultrasonic technology is that the
transmit signal consists of short bursts of ultrasonic energy. After each
burst, the electronics looks for a return signal within a small window of time
corresponding to the time it takes for the energy to pass through the vessel.
Only a signal received during this window will qualify for additional signal
processing.
A popular consumer application of
ultrasonic ranging was the Polaroid SX-70 camera which included a light-weight
transducer system to focus the camera automatically. Polaroid later licenced
this ultrasound technology and it became the basis of a variety of ultrasonic
products.
Motion sensors and flow measurement
A common ultrasound application is an
automatic door opener, where an ultrasonic sensor detects a person's approach
and opens the door. Ultrasonic sensors are also used to detect intruders; the
ultrasound can cover a wide area from a single point. The flow in pipes or open
channels can be measured by ultrasonic flowmeters, which measure the average
velocity of flowing liquid. In rheology, an acoustic rheometer relies on the
principle of ultrasound. In fluid mechanics, fluid flow can be measured using
an ultrasonic flow meter.
Non-destructive testing
Ultrasound inspection of welded joints
has been an alternative to radiography for non-destructive testing since the
1960s. Ultrasonic inspection eliminates the use of ionizing radiation, with safety
and cost benefits. Ultrasound can also provide additional information such as
the depth of flaws in a welded joint. Ultrasonic inspection has progressed from
manual methods to computerized systems that automate much of the process. An
ultrasonic test of a joint can identify the existence of flaws, measure their
size, and identify their location. Not all welded materials are equally
amenable to ultrasonic inspection; some materials have a large grain size that
produces a high level of background noise in measurements.
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