Ultrasound

Last updated: March 17, 2008.

You often hear people using the phrase "as blind as a bat"—but if bats could talk, would they criticise us for being "as deaf as a human"? We may think we're good at hearing things but our ears can detect only a relatively narrow band of frequencies (sounds of different pitch) centered on the human voice—the sound we most need to hear. Bats, moths, dolphins, and various other creatures can hear much higher frequencies of sound beyond the range of human hearing, which is known as ultrasound. Since scientists discovered ultrasound, they have found all kinds of important uses for it, from medical diagnosis to materials testing and scientific research. Let's take a closer look at ultrasound and how it works.

Photo: An ultrasound (echocardiograph) image of a beating human heart. NASA (the US space agency) uses this equipment to study the long-term effects of space travel on astronauts. Photo courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

What is sound?

Before we can understand ultrasound, we need to know a little bit more about sound itself.

Sound isn't just something you hear. It's energy that travels through the air by making the gas molecules in the air vibrate (move back and forth). When someone bangs a drum across the room from you, the drum skin vibrates, making the air immediately around it vibrate too. These vibrations travel through the room and soon reach your ear, and you hear them when they make your eardrums (two thin membranes, just like tiny drum skins, one inside each of your ears) vibrate as well.

Sound travels from where it's made to your ears in the form of waves. Sound waves are a bit different from the waves that romp across the sea. Sea waves travel up and down, with the water moving at right angles to the direction the wave is travelling in, with noticeable peaks and troughs, so they are called transverse waves. Sound energy travels in a different way. The waves that carry it are invisible and they move back and forth (in the same direction the wave is travelling). In other words, as sound travels through a room, different zones of the air are constantly being squeezed in and out, a bit like an accordion. Some parts of the air are squeezed tightly together while others are stretched further apart. The squeezed-together bits are called compressions, while the stretched-apart bits are known as rarefactions. This is why sound waves are called compression waves or longitudinal waves. There's a good animation showing how a sound wave travels, by compressing and stretching the air, on the Wikipedia page about Longitudinal waves.

Photo: Energy travels across the surface of the sea in waves. It's obvious that there could be no waves on the sea if there were no water, because sea waves need the water to travel over. In just the same way, sound waves need something (called a medium) to travel through. Take away the air, and there's no way sound could travel. That's why you can't hear things in space.

Different sounds travel through air at the same speed—at what we call the speed of sound, which is typically around 1230 km/h (770 mph). But the waves that carry them differ in important ways. High-pitched sounds (like mice squeaking or the top notes on a piano) have high frequencies, which means the sound waves that carry them vibrate very quickly. Low-pitched sounds (like dogs barking or the low notes on a piano) have lower frequencies and the sound waves that carry them vibrate more slowly.

What is ultrasound?

Human ears can hear sound waves that vibrate in the range from about 20 times a second (a deep rumbling noise) to about 20,000 times a second (a high-pitched whistling). (Children can generally hear higher-pitched sounds than their parents, because our ability to hear high frequencies gets worse as we get older.) Speaking more scientifically, we could say that the sounds we can perceive have a frequency ranging from 20-20,000 hertz (Hz). A hertz is a measurement of how often something vibrates and 1 Hz is equal to one vibration each second. The human voice makes sounds ranging from a few hundred hertz to a few thousand hertz.

Suppose you could somehow hit a drum-skin so often that it vibrated more than 20,000 times per second. You might be able to see the skin vibrating (just), but you certainly couldn't hear it. No matter how hard you hit the drum, you wouldn't hear a sound. The drum would still be transmitting sound waves, but your ears wouldn't be able to recognize them. Bats, dogs, dolphins, and moths might well hear them, however. Sounds this like, with frequencies beyond the range of human hearing, are examples of ultrasound.

(Interestingly, just as there is sound that is too high-pitched for us to hear, so there are also sounds that are too low-pitched for our ears. These are called infrasound. The seismic waves that we know as earthquakes are examples. We need special detectors to know when earthquakes have happened, because we can't always hear or feel them.)

Photo: Bats like this "see" with sound instead of light. They navigate by sending out beams of ultrasound and listening for the echoes—a technique called echolocation. Photo of big brown bat (Eptesicus fuscus) by Don Pfitzer, courtesy of US Fish & Wildlife Service.

Ultrasound waves have higher frequencies than normal sound waves, but they also have shorter wavelengths. In other words, the distance between one ultrasound wave travelling through the air and the one following on behind it is much shorter than in a normal sound wave. This has an important practical effect: ultrasound waves reflect back from things much better than ordinary sound waves, and that makes them very useful indeed.

How is ultrasound made?

It's impossible for us to make ultrasound the same way we make normal sounds—by hitting and blowing things, as we do, for example, in musical instruments. That's because we can't hit and blow things fast enough. But we can make ultrasound using electrical equipment that vibrates with an extremely high frequency. Crystals of some materials (such as quartz) vibrate very fast when you pass electricity through them—an effect called piezoelectricity. As they vibrate, they push and pull the air around them, producing ultrasound waves. Devices that produce ultrasound waves using piezoelectricity are known as piezoelectric transducers. Piezoelectric crystals also work in the reverse way: if ultrasound waves travelling through the air happen to collide with a piezoelectric crystal, they squeeze its surface very slightly, causing a brief burst of electricity to flow through it. So, if you wire up a piezoelectric crystal to an electric meter, you get an instant ultrasound detector.

Ultrasound waves can be produced using magnetism instead of electricity. Just as piezoelectric crystals produce ultrasound waves in response to electricity, so there are other crystals that make ultrasound in response to magnetism. These are called magnetostrictive crystals and the transducers that use them are called magnetostrictive transducers. (The magnetive effect is known as magnetostriction.)

What is ultrasound used for?

Using ultrasound for practical purposes is sometimes called ultrasonics—and it's used for everything from industrial welding and drilling to producing homogenized milk and photographic film.

Probably the best known example of ultrasonics is medical testing. To save having to open up your body to detect an illness, doctors can simply run an ultrasound scanner over your skin to see inside. The scanner looks a bit like a computer mouse. It has a built in transducer that beams harmless, ultrasound waves down into your body. As the waves travel through the different bones and tissues, they reflect back up again. The same transducer (or a separate one alongside) receives the reflected waves and a computer attached to the scanner uses them to draw a detailed picture of what's happening inside you on a screen. Scans of fetuses (unborn babies developing in the womb) are made this way.

Photo: This pregnant woman is watching an ultrasound scan of the baby developing in her womb. Note the ultrasound scanner (bottom right) being moved slowly across her abdomen, and the monitor (left) showing the picture of her child. Photo by Scherrie K. Gates courtesy of Defense Visual Information Center (DVIC).

Similar equipment is used to test for flaws in machines such as airplane jet engines. If there's a crack deep inside a metal, inspecting it from the inside won't reveal the problem. But if you run an ultrasound scanner over the outside of the metal, the crack inside will disturb and reflect back some of the ultrasound waves—so the defect will show up on your testing monitor. Inspecting materials this way is sometimes known as non-destructive testing, because you don't have to damage or take things apart to check them out.

Photo: Examining an airplane engine using ultrasonic, non-destructive testing. The inspector is moving an ultrasound probe over an airplane component with her right hand. She is adjusting the ultrasound beam with her left hand at the same time. Photo by Michelle Michaud courtesy of Defense Visual Information Center (DVIC).

Relatively low-strength ultrasound waves are used for medical scans and non-destructive testing. Much stronger ultrasound waves have very different uses. If you have a painful kidney stone, firing powerful ultrasound waves from outside your body can make the stone vibrate and break apart. Strong ultrasound waves are sometimes also used to destroy cancer tumours and brain lesions (damaged regions of the brain). In a similar way, ultrasound waves can be used to clean jewellery, watches, false teeth, and a wide range of machine parts that may be too difficult (or inaccessible) to clean in other ways.

Another popular use for ultrasonics is in ship navigation. Sound travels faster through water than through air, which is very helpful because light hardly travels through water at all. Most people know that whales can use sound to communicate across entire oceans. Submarines use a similar trick with a type of navigation called sonar (sound navigation and ranging), which is a bit like an underwater equivalent of radar. When a submarine is deep beneath the surface, it can find its way by sending out bleeps of sound and listening for the echoes—just like a bat using echolocation. By timing how long it takes for the echoes to come back, a submarine's navigator can figure out if there are any other ships, submarines, or other obstacles nearby. It's also possible to calculate how deep the sea is (or draw a map of the seabed) by firing sound beams straight downward. This technique is known as echo sounding.