Article: Ultrasound

For other meanings of "ultrasound" or "ultrasonic", see ultrasound (disambiguation).

A baby in its mother's womb, viewed in a sonogram (brightness scan)

A baby, aged 29 weeks, in a "3D ultrasound"

Ultrasound is sound with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz).

Ability to hear ultrasound

Some animals, such as dogs, dolphins, bats, and mice have an upper limit that is greater than that of the human ear and thus can hear ultrasound. Children can hear some high-pitched sounds that older adults cannot hear, as in humans the upper limit pitch of hearing gets lower with age. This frequency limit is caused by the middle ear that acts as a low-pass filter. If ultrasound is fed directly into the skull bone and reaches the cochlea without passing through the middle ear, much higher frequencies (up to about 200 kHz) can be heard. This effect (sometimes called ultrasonic hearing) was first discovered by divers exposed to a high-frequency (ca. 50 kHz) sonar signal.

Sonogram of a fetus at 14 weeks (Profile)

Medical sonography

Main article: Medical Sonography

Medical sonography (ultrasonography) is a useful ultrasound-based diagnostic medical imaging technique used to visualize the fetus, muscles, tendons, and many internal organs, their size, structure and any pathological lesions. Ultrasound scans are performed by medical health care professionals called sonographers. Obstetric sonography is commonly used during pregnancy.

Other medical uses

Ultrasound also has therapeutic applications:-

• Treating benign and malignant tumors and other disorders, via a process known as Focused Ultrasound Surgery (FUS) or HIFU, High Intensity Focused Ultrasound. These procedures generally use lower frequencies than medical diagnostic ultrasound (from 250 kHz to 2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by MRI, as in Magnetic Resonance guided Focused Ultrasound.
• More powerful ultrasound sources may be used to clean teeth in dental hygiene or generate local heating in biological tissue, e.g. in occupational therapy, physical therapy and cancer treatment.
• Extracorporeal shock wave lithotripsy uses a powerful focused ultrasound source to break up kidney stones.
• Focused ultrasound sources may be used for cataract treatment by phacoemulsification.
• Additional physiological effects of low-intensity ultrasound have recently been discovered, e.g. the ability to stimulate bone-growth and its potential to disrupt the blood-brain barrier for drug delivery.
• Ultrasound is used in UAL (= ultrasound-assisted lipectomy), or liposuction.
• Doppler ultrasound is being tested for use in aiding tissue plasminogen activator treatment in stroke sufferers. This procedure is called Ultrasound-Enhanced Systemic Thrombolysis.
• Low intensity pulsed ultrasound is used for therapeutic tooth and bone regeneration.

Industrial ultrasound

Ultrasound is also used in industry to find flaws in materials. 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.

Ultrasound flow meter

Main article: Ultrasound flow meter

Ultrasonic cleaning

Ultrasonic cleaners, sometimes mistakenly called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. An ultrasonic cleaner works mostly by energy released from collapse of millions of microscopic cavitations near the dirty surface. The bubbles formed by cavitation collapse forming tiny jets directed at the surface. Home ultrasonic cleaners are available and cost about US $60 or more.

Ultrasound and animals

Rodents

Ultrasound generator/speaker systems are sold with claims that they frighten away rodents and insects, but there is no scientific evidence that the devices work; controlled tests have shown that rodents quickly learn that the speakers are harmless. However, the pitch used is well within the range that most children can hear, and can cause headaches.

Insects

There is evidence that ultrasound in the range emitted by bats causes flying moths to make evasion maneuvers, because bats eat moths.

Dogs

The dog whistle is used to call to a dog. It makes ultrasound at a frequency that dogs can hear.

Sonochemistry

Main article: Sonochemistry

Power ultrasound in the 20-100 kHz range is used in chemistry. The ultrasound does not interact directly with molecules to induce the chemical change, as its typical wavelength (in the millimeter range) is too long compared to the molecules. Instead:-

• It causes cavitation which causes local extremes of temperature and pressure in the liquid where the reaction happens.
• It breaks up solids and removes passivating layers of inert material to give a larger surface area for the reaction to occur over.

Both of these make the reaction faster.

Ultrasonic disintegration

Some sorts of ultrasound can disintegrate biological cells including bacteria. This has uses in biological science and in killing bacteria in sewage. See for example:-

• http://www.sanyogallenkamp.com/proddetail.asp?prodid=236
• http://www.iwaponline.com/wst/04209/wst042090073.htm

Ultrasound as a weapon

Main article: Sonic weaponry

Other uses

Ultrasound when applied in specific configurations can produce exotic phenomena such as sonoluminescence. These phenomena are being investigated partly because of the possibility of bubble fusion (a nuclear fusion reaction hypothesized to occur during sonoluminescence).

Ultrasonic range finding

A common use of ultrasound is in range finding; this use is also called sonar. This works simiarly to radar: 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 sender as an echo and can be detected. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine how far away the object is.

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey.

See also

• Infrasound (sound at extremely low frequencies)
• Light
• Physics