ultrasound ultrasound ultrasound is sound with frequencies greater than 20 kilohertz. this frequency is the approximate upper audible limit of human hearing in healthy young adults. the physical principles of acoustic waves apply to any frequency range, including ultrasound. ultrasonic devices operate with frequencies from 20 khz up to several gigahertz. for the application to medicine, see medical ultrasound. for other uses, see ultrasound (disambiguation). ultrasound image (sonogram) of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional scan) an ultrasonic examination fetal ultrasound ultrasound is used in many different fields. ultrasonic devices are used to detect objects and measure distances. ultrasound imaging or sonography is often used in medicine. in the nondestructive testing of products and structures, ultrasound is used to detect invisible flaws. industrially, ultrasound is used for cleaning, mixing, and accelerating chemical processes. animals such as bats and porpoises use ultrasound for locating prey and obstacles. history galton whistle, one of the first devices to produce ultrasound 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. echolocation in bats was discovered by lazzaro spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound, not vision. francis galton in 1893 invented the galton whistle, an adjustable whistle that produced ultrasound, which he used to measure the hearing range of humans and other animals, demonstrating that many animals could hear sounds above the hearing range of humans. the first article on the history of ultrasound was witten in 1948. according to its author, during the first world war, a russian engineer named chilowski submitted an idea for submarine detection to the french government. the latter invited paul langevin, then director of the school of physics and chemistry in paris, to evaluate it. chilowski's proposal was to excite a cylindrical, mica condenser by a high-frequency poulsen arc at approximately 100 khz and thus to generate an ultrasound beam for detecting submerged objects. the idea of locating underwater obstacles had been suggested prior by l. f. richardson, following the titanic disaster. richardson had proposed to position a high-frequency hydraulic whistle at the focus of a mirror and use the beam for locating submerged navigational hazards. a prototype was built by sir charles parsons, the inventor of the vapour turbine, but the device was found not to be suitable for this purpose. langevin's device made use of the piezoelectric effect, which he had been acquainted with whilst a student at the laboratory of jacques and pierre curie. langevin calculated and built an ultrasound transducer comprising a thin sheet of quartz sandwiched between two steel plates. langevin was the first to report cavitation-related bioeffects from ultrasound. definition approximate frequency ranges corresponding to ultrasound, with rough guide of some applications ultrasound is defined by the american national standards institute as "sound at frequencies greater than 20 khz". in air at atmospheric pressure, ultrasonic waves have wavelengths of 1.9 cm or less. perception a medical ultrasound result on a piece of paper humans the upper frequency limit in humans (approximately 20 khz) is due to limitations of the middle ear. auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear. 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. an american cell phone company has used this to create ring signals that supposedly are only audible to younger humans, 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. animals bats use ultrasounds to navigate in the darkness. a dog whistle, which emits sound in the ultrasonic range, used to train dogs and other animals bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. they can detect frequencies beyond 100 khz, possibly up to 200 khz. many insects have good ultrasonic hearing, and most of these are nocturnal insects listening for echolocating bats. these include many groups of moths, beetles, praying mantises and lacewings. upon hearing a bat, some insects will make evasive manoeuvres to escape being caught. ultrasonic frequencies trigger a reflex action in the noctuid moth that causes it to drop slightly in its flight to evade attack. tiger moths also emit clicks which may disturb bats' echolocation, and in other cases may advertise the fact that they are poisonous by emitting sound. dogs and cats' hearing range extends into the ultrasound; the top end of a dog's hearing range is about 45 khz, while a cat's is 64 khz. the wild ancestors of cats and dogs evolved this higher hearing range to hear high-frequency sounds made by their preferred prey, small rodents. a dog whistle is a whistle that emits ultrasound, used for training and calling dogs. the frequency of most dog whistles is within the range of 23 to 54 khz. toothed whales, including dolphins, can hear ultrasound and use such sounds in their navigational system (biosonar) to orient and to capture prey. porpoises have the highest known upper hearing limit at around 160 khz. 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. not a single bird species has been reported to be sensitive to ultrasound. commercial ultrasonic systems have been sold for supposed indoors electronic pest control and outdoors ultrasonic algae control. however, no scientific evidence exists on the success of such devices for these purposes. 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 lightweight transducer system to focus the camera automatically. polaroid later licensed 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. nondestructive testing principle of flaw detection with ultrasound. a void in the solid material reflects some energy back to the transducer, which is detected and displayed. ultrasonic testing is a type of nondestructive testing commonly used to find flaws in materials and to measure the thickness of objects. frequencies of 2 to 10 mhz are common, but for special purposes other frequencies are used. inspection may be manual or automated and is an essential part of modern manufacturing processes. most metals can be inspected as well as plastics and aerospace composites. lower frequency ultrasound (50–500 khz) can also be used to inspect less dense materials such as wood, concrete and cement. ultrasound inspection of welded joints has been an alternative to radiography for nondestructive 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. non-destructive testing of a swing shaft showing spline cracking ultrasonic thickness measurement is one technique used to monitor quality of welds. ultrasonic range finding principle of an active sonar a common use of ultrasound is in underwater range finding; this use is also called sonar. 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 the distance. 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. for example, hand-held ultrasonic measuring tools can rapidly measure the layout of rooms. although range finding underwater is performed at both sub-audible and audible frequencies for great distances (1 to several kilometers), 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 centimeters to meters accuracy ultrasound identification (usid) ultrasound identification (usid) is a real-time locating system (rtls) or indoor positioning system (ips) technology used to automatically track and identify the location of objects in real time using simple, inexpensive nodes (badges/tags) attached to or embedded in objects and devices, which then transmit an ultrasound signal to communicate their location to microphone sensors. imaging main article: medical ultrasound sonogram of a fetus at 14 weeks (profile) head of a fetus, aged 29 weeks, in a "3d ultrasound" the potential for ultrasonic imaging of objects, with a 3 ghz sound wave producing resolution comparable to an optical image, was recognized by sokolov in 1939, but techniques of the time produced relatively low-contrast images with poor sensitivity. ultrasonic imaging uses frequencies of 2 megahertz and higher; the shorter wavelength allows resolution of small internal details in structures and tissues. the power density is generally less than 1 watt per square centimetre to avoid heating and cavitation effects in the object under examination. high and ultra high ultrasound waves are used in acoustic microscopy, with frequencies up to 4 gigahertz. ultrasonic imaging applications include industrial nondestructive testing, quality control and medical uses. acoustic microscopy acoustic microscopy is the technique of using sound waves to visualize structures too small to be resolved by the human eye. frequencies up to several gigahertz are used in acoustic microscopes. the reflection and diffraction of sound waves from microscopic structures can yield information not available with light. human medicine