Ultrasound

Ultrasound is sound with a frequency above the upper limit of human hearing (about 20,000 Hz, or 20 kHz). Even though you cannot hear it, ultrasound is just like any other sound: a wave of vibrations travelling through a material. Many animals (bats, dolphins, dogs) can hear and use ultrasound naturally. Humans have invented many uses for it too, including medical scans, sonar, industrial cleaning, parking sensors and ultrasonic welding. Ultrasound has become one of the most useful technologies of the modern world.

• What it isSound above 20 kHzToo high for human hearing
• Medical scans use1 to 20 MHzMillion vibrations per second
• Bat echolocation20 to 200 kHzFor hunting insects
• Industrial cleaners20 to 400 kHzVibrate dirt off objects
• Ultrasonic weldingAround 20 kHzJoins plastic with vibration
• Discovered1942 (medical use)Pioneered by Karl Dussik

Why we cannot hear ultrasound

Healthy young human ears can detect sounds from about 20 Hz to 20 kHz. Above 20 kHz, the vibrations are too rapid for the hair cells in our cochleas to respond to. The same waves are still travelling through the air, but our hearing machinery just cannot pick them up.

Many other animals have evolved to hear higher pitches. Dogs hear up to about 45 kHz (which is why "silent" dog whistles work). Cats hear up to about 65 kHz. Mice and rats squeak at frequencies above human hearing. Dolphins hear up to about 150 kHz. Bats can hear up to 200 kHz or more.

Medical ultrasound

The most famous use of ultrasound is in medical scans. A handheld probe emits short pulses of ultrasound (typically 1 to 20 MHz, far above human hearing) into the patients body. Most of the ultrasound passes through soft tissues, but a small fraction bounces back from any boundary between tissues of different density (skin, fat, muscle, organ walls, bone).

By analysing the timing and strength of the returning echoes, the machine builds a moving real-time image of the inside of the body. The most familiar use is scanning unborn babies in the womb, but ultrasound is used for many other things:

• Checking the structure and movement of the heart.
• Looking at the liver, kidneys, bladder and gallbladder.
• Detecting tumours and other masses.
• Guiding needles for biopsies (taking tissue samples).
• Measuring blood flow through arteries and veins (Doppler ultrasound).
• Diagnosing torn muscles, tendons and ligaments in sports injuries.

Ultrasound is safe and painless, with no harmful radiation. It is one of the cheapest and most useful imaging techniques in modern medicine.

Fact The first medical ultrasound scan was performed by Austrian neurologist Karl Theodore Dussik in 1942, who used ultrasound to try to detect brain tumours. The early machines were very crude, but the principle worked. Over the next 30 years, ultrasound was developed into the precise diagnostic tool we have today.

Sonar and underwater uses

Ships and submarines use ultrasonic sonar to navigate, find fish, map the ocean floor and detect other vessels. Submarines emit a pulse of ultrasound, then listen for the echoes from anything in the water. The Royal Navys early sonar (called ASDIC) was developed during World War One specifically to hunt German U-boats.

Modern fishing boats use sonar fish finders to locate shoals of fish before casting nets. Oceanographers use sonar arrays mounted on the bottoms of survey ships to map the deep ocean floor with metre-scale precision.

Industrial uses

• Ultrasonic cleaning: a tank of water with an ultrasound transducer creates tiny vibrating bubbles that knock dirt off jewellery, watches, machine parts, even contact lenses. Far gentler and more thorough than scrubbing.
• Ultrasonic welding: two pieces of plastic are pressed together while a tool vibrates one of them at around 20 kHz. The friction at the join melts the plastic and welds it together in a fraction of a second. Used for car parts, packaging and electronics.
• Crack detection: ultrasound is sent into a metal beam or weld. A crack inside reflects sound differently from undamaged metal, revealing the flaw without cutting the part open. Used to check the safety of aircraft components, train wheels and oil pipelines.
• Thickness measurement: a similar technique can measure the thickness of a metal sheet or pipe wall without touching the other side.
• Mixing and emulsifying: powerful ultrasound can blend liquids that would otherwise refuse to mix, like oil and water.

Did you know? Some pest control devices claim to drive away mice, rats and insects by emitting ultrasound. Scientific tests have been mixed. Some short-term effects are seen, but pests often get used to the sound after a few days and ignore it. The devices may help slightly in some situations, but they are not a guaranteed pest-control solution.

Ultrasonic parking sensors

Many modern cars have ultrasonic parking sensors built into their bumpers. Tiny transducers fire short ultrasound pulses into the air around the car. Any obstacle (a wall, kerb, post or another car) reflects the pulse back. The car measures the round-trip time and warns the driver if anything is close, usually with beeps that get faster as the distance closes.

Treating with ultrasound

High-intensity focused ultrasound (HIFU) is a newer treatment that focuses ultrasound waves to a single point deep inside the body, generating intense heat. Doctors can use this to destroy small tumours or break up kidney stones without surgery. It is non-invasive (no cut needed) and recovery is faster than traditional surgery.

Try this If you have a dog at home, look up "dog whistle test" videos online (with the sound on). The whistles produce sounds above 20 kHz that you cannot hear but most dogs can. Watch the dogs reaction: ears perking up, head tilting, sometimes wagging or running to the source. You are watching real-time evidence that dogs hear sounds completely invisible to your own ears.

Deeper dive: how a bat finds an insect in pitch darkness

Bats are the most skilled ultrasonic hunters on the planet. Most species feed on insects, finding and catching them in the air at night, often inside complex environments like dense forests. They do all this using nothing but ultrasonic echolocation.

The hunting bat emits a rapid stream of high-pitched clicks from its mouth or nose, usually at frequencies between 20 and 200 kHz. As the bat flies, the clicks travel out, bounce off anything in front of it, and return to the bats sensitive ears. Each species of bat has its own specific frequency range and click pattern, tuned for the kind of prey it hunts.

From the returning echoes, the bats brain builds a detailed real-time picture of:

• Direction: by comparing the timing of echoes at each ear, the bat can pin down direction with extraordinary precision.
• Distance: by measuring the time delay between click and echo.
• Size and shape: bigger objects return stronger echoes; complicated shapes return complex patterns.
• Speed and direction of movement: using the Doppler shift, just like a police radar gun. An insect flying towards the bat returns a slightly higher-frequency echo; one flying away returns a slightly lower one.

As the bat closes in on prey, the click rate speeds up dramatically, from about 10 clicks per second during cruise flight to 200 clicks per second in the final attack. This "feeding buzz" gives the bat the most up-to-date possible information for the split-second catch.

The whole system is so accurate that bats can catch tiny mosquitoes in mid-air, fly through the gaps in fine wire mesh, and avoid each other when foraging in groups of millions. All without using their eyes at all.

Some moths have evolved counter-measures. They can hear bats clicks and dive to the ground to avoid being caught. A few species can even emit their own ultrasonic clicks to jam the bats sonar, the worlds smallest electronic warfare.

For more, see echoes and frequency and pitch.