Ultrasound

Ultrasound is a type of sound with frequencies higher than 20 kilohertz, which is the upper limit of what humans can hear. These high-frequency sounds are used in many different ways.

Ultrasound helps detect objects and measure distances. It is commonly used in medicine for imaging, such as looking inside the body without surgery. In industry, ultrasound can find hidden flaws in products and structures, or help clean and mix things more effectively.

Some animals, like bats and porpoises, naturally use ultrasound to find food and avoid obstacles. This shows how useful these high-frequency sounds can be in many situations.

History

Acoustics, the science of sound, goes back a long way — even to the 6th century BC with Pythagoras, who studied the math behind stringed instruments. Much later, in 1794, Lazzaro Spallanzani discovered that bats use special sounds to find their way and catch food, even when it’s too dark to see. Then in 1893, Francis Galton created a special whistle called the Galton whistle that could make sounds too high for humans to hear. He used it to learn about how well animals can hear.

The first article about the history of ultrasound was written in 1948. During the First World War, a Russian engineer shared an idea with the French Government for detecting submarines underwater. They asked Paul Langevin, a science leader in Paris, to check the idea. Langevin used a special material property called the piezoelectric effect to build a device that could create ultrasound beams. This helped scientists learn more about how ultrasound works on living things.

Definition

Ultrasound is sound with frequencies higher than 20 kilohertz, which is the highest frequency humans can hear. In air under normal pressure, these sound waves have wavelengths of 1.9 centimeters or less.

Ultrasound can be created at very high frequencies and is used in special chemistry experiments at frequencies up to many hundreds of kilohertz. In medical imaging, equipment uses frequencies in the megahertz range. Very high-frequency ultrasound, called UHF ultrasound, can reach up to the gigahertz range. Studying these extremely high frequencies can be difficult because the waves can become very sharp and form shock waves.

Perception

Humans

Humans can usually only hear sounds up to about 20,000 cycles per second, or 20 kHz. This limit is because of how our middle ear works. But if very strong ultrasound is sent straight into the head, it can reach the ear parts inside through the bones, letting us feel some of these high sounds.

Younger children can sometimes hear very high-pitched sounds that older people cannot. Some companies tried to make phone rings that only young people could hear, but many older people could still hear them.

Animals

Bats use high-pitched sounds to find their way and catch food. They can hear sounds higher than 100 kHz, maybe even up to 200 kHz.

Many insects, especially those active at night, can hear these high sounds. Some insects, like moths, change their flight when they hear a bat to try to escape. Others, like tiger moths, make clicking sounds that might confuse bats or warn them that the moth is poisonous.

Dogs and cats can also hear sounds that people cannot. Dogs can hear up to about 45 kHz, and cats up to 64 kHz. Their ancestors developed this to better hear small animals they hunt. A dog whistle makes these high sounds to train and call dogs.

Toothed whales, like dolphins, can hear ultrasound and use it to navigate and find food. Porpoises can hear up to around 160 kHz. Some types of fish, like shad, can detect sounds up to 180 kHz.

No birds are known to hear ultrasound.

Some companies sell devices that use ultrasound to control pests or algae, but there is no proof these work.

Detection and ranging

Non-contact sensor

An ultrasonic level or sensing system does not need to touch the target. This is helpful in many industries like medicine, pharmaceuticals, and the military because it avoids contaminating liquids or getting clogged by products.

Both continuous and pulsed systems are used. In pulsed systems, short bursts of ultrasonic energy are sent out. After each burst, the system waits for a return signal during a specific time window to process the data.

A well-known use of ultrasonic sensing was in the Polaroid SX-70 camera, which used ultrasound to focus automatically. This technology was later used in many other products.

Motion sensors and flow measurement

Ultrasound is often used in automatic door openers to detect when someone approaches. It can also be used to detect intruders from a single point. Ultrasonic sensors measure the flow of liquids in pipes or open channels. In rheology, special tools use ultrasound. Fluid mechanics also uses ultrasound to measure fluid flow.

Nondestructive testing

See also: Macrosonics and Ultrasonic testing

Ultrasonic testing is a way to check for flaws in materials and measure the thickness of objects without damaging them. It is commonly used in manufacturing. It works on metals, plastics, and aerospace composites. Lower frequency ultrasound can inspect materials like wood, concrete, and cement.

Ultrasound has been used to check welded joints since the 1960s, offering a safer and cheaper alternative to other methods. It can find flaws, measure their size, and locate them. Not all materials are easy to inspect with ultrasound, as some can create noise in the measurements.

Ultrasonic thickness measurement helps monitor the quality of welds.

Ultrasonic range finding

Main article: Sonar

Ultrasound is commonly used underwater for measuring distances, known as sonar. A pulse of ultrasound is sent out, and if it hits an object, part of the pulse bounces back as an echo. By measuring the time between sending the pulse and receiving the echo, the distance can be determined.

The time it takes for sonar pulses to travel in water depends on temperature and salt content. Ultrasound is also used on land for short, accurate distance measurements, such as measuring room sizes.

Ultrasound Identification (USID)

Ultrasound Identification (USID) is a technology used to track and find the location of objects in real time. It uses small, inexpensive tags attached to objects that send out ultrasound signals to sensors, which then determine their location.

Imaging

Ultrasound can be used to create images of objects. In 1939, a scientist named Sergei Sokolov saw that sound waves could make images as clear as optical ones, but the technology wasn’t good enough yet. Today, ultrasound uses sound waves of 2 megahertz and higher to show small details inside structures and tissues. It’s used in many ways, like checking products for flaws and looking inside the human body.

In medicine, ultrasound helps doctors see muscles, tendons, and organs. It’s been used for over 50 years and is safe because it doesn’t use harmful radiation. Doctors also use it to check on babies during pregnancy. Ultrasound is also helpful in emergencies and for checking animals like horses and cattle. It’s a useful and portable tool that helps doctors and scientists see inside things without surgery.

Processing and power

High-power uses of ultrasound often work best between 20 kilohertz and a few hundred kilohertz. The strength of these sound waves can be very strong; above 10 watts per square centimeter, it can create special effects in liquids and even change how tiny living things act.

Ultrasound has been used since the 1940s to help heal injuries in ligaments, tendons, and other tissues. It can help with problems like sprains, strains, and joint swelling. Strong ultrasound can also break up hard bits in the body, help medicines work better in certain spots, and even help scientists study tiny pieces of materials.

Ultrasound can also change how metals behave, making them stronger and more reliable. This is done by using special sound waves that match how the metal naturally moves. By choosing the right sound frequencies and strength, engineers can improve metals used in many tools and machines.

Ultrasound is also useful in mixing liquids and changing how tiny particles act. The strong sound waves create tiny bubbles that pop and mix things together very well. This helps make smoother mixtures and can even help make new materials or medicines.

Ultrasound can also clean things by making tiny bubbles pop near dirty surfaces, washing away dust and germs. It can also break apart tiny living cells, which helps scientists study them or clean water.

Some humidifiers use ultrasound to create a cool mist by shaking a metal plate very fast. This mist can also carry tiny bits from the water, so it’s important to keep these humidifiers clean.

Ultrasound can also join pieces of plastic together by shaking them fast enough to create heat through friction, making strong bonds.

Powerful ultrasound helps speed up chemical reactions by creating extreme temperatures and pressures in tiny spots, breaking apart solids, and helping more of the material react. This makes reactions happen faster and more efficiently.

Other uses

When ultrasound is used in special ways, it can create tiny flashes of light in a process called sonoluminescence.

Ultrasound helps scientists study tiny particles by using methods like ultrasound attenuation spectroscopy, watching electroacoustic phenomena, or using transcranial pulsed ultrasound.

Wireless communication

We can send sound signals through the air using modulated ultrasound.

In the past, some TV remote controls used ultrasound. These remotes, made by Zenith in the 1950s, worked without batteries and did not interfere with other TVs like radio waves might. Later, these were replaced by infrared remotes in the 1980s.

In 2015, researchers from the University of California, Berkeley studied using a special material called graphene with diaphragms for ultrasound communication. Because graphene is very thin, light, and strong, it could be useful for sending messages underwater, where radio waves do not work well.

Ultrasonic signals have also been used to follow people’s activities across different devices on the Internet, called cross-device tracking.

Safety

Very loud ultrasound can hurt our ears and bodies. If ultrasound is louder than 120 dB, it might cause hearing loss. If it’s even louder, above 155 dB, it can heat up body tissues and cause harm. Very high levels, above 180 dB, could be dangerous.

Special rules help keep people safe when using ultrasound for medical checks. These rules stop certain harmful effects from happening.