Lesson video

Hello, my name's Dr.

George.

This lesson is called "Ultrasound" and it's part of the unit "Waves." The outcome for the lesson is I can describe what ultrasound is and explain its properties, and also describe some uses of ultrasound.

Here are the keywords for the lesson, which I'll be introducing as we go along.

And if you need to come back anytime to remind yourself of the meanings, just return to this slide.

The lesson has three parts called "Frequency range for humans and other animals," "Properties of ultrasound," and "Some uses of ultrasound." Let's start with a reminder of what sound waves are.

Sound waves through air are made by air particles vibrating backwards and forwards, and you can see that in this animation.

You can watch one of the red particles to see what movement each particle is doing.

And this movement backwards and forwards as vibration is in addition to how particles move all the time in a gas, which is they move around in different directions at different speeds randomly.

If a sound wave reaches your ear, your eardrum is made to vibrate and moves backwards and forwards too.

The vibrations are made bigger by small bones and then detected by tiny hair cells inside a part of the ear called the cochlea.

Those send electrical signals along nerve cells to the brain, and your brain uses these signals to create the sensation of hearing sound.

There's a difference between a sound wave and a sound.

Sound is what you experience and a sound wave is vibrations of particles travelling through the air.

What moves forwards with a sound wave? Air particles, sound particles, vibrations of air particles, or vibrations of sound particles? And with short questions, I'll wait five seconds, but if you need longer, press pause and press play when you have your answer ready.

The correct answer is vibrations of air particles.

The vibrations travel along.

Particles themselves don't, they just move backwards and forwards.

And there's no such thing as sound particles.

And now a recap on different kinds of sound.

Higher pitch sounds have a higher frequency, and lower pitch sounds, like the rumble of thunder, have a lower frequency.

And frequency means number of vibrations per second.

The lowest pitch sounds that humans can hear are caused by sound waves with frequency of around 20 hertz.

That's 20 vibrations per second.

And the highest pitch sounds humans can hear are caused by sound waves with a frequency of around 20,000 hertz.

And here's a representation of that on a number line.

Below human hearing, there are sound waves called infrasound.

Their frequencies are too low for human ears to detect.

And there are frequencies that are too high for human ears to detect, and these kind of sound waves are called ultrasound.

Let's look at a few other animals and their hearing ranges.

Dogs and cats have a similar range of hearing to humans, but they can hear frequencies a little lower and a little higher.

Small birds have a much narrower range of frequency that they can hear.

Elephants can hear most of the same frequencies that we can but also some infrasound.

And mice can hear some ultrasound.

Bats can hear even higher frequencies of ultrasound, over 200,000 hertz.

And bats use ultrasound waves to sense the world around them.

They can actually create ultrasound, so they emit, they send out, short bursts of ultrasound, and that ultrasound reflects off nearby objects.

And that reflection is called an echo and it comes back to the bat who hears it and can use that to work out how far away an object is.

Here's an example of a bat detecting a moth, which would be something that it might want to eat, using what we call echolocation, finding things using echoes.

The orange lines represent the ultrasound that the bat is sending out and the green lines represent some of that sound echoing back off the moth.

Which of the following animals can hear ultrasound? Bats, birds, cats and dogs, elephants.

Out of those, bats can, and cats and dogs can hear some ultrasound as well, although not as higher frequencies as bats can.

And here are some written questions for you to answer.

Press pause while you read these and write your answers.

Press play when you're ready to check them.

I'll show you the answers.

A young person with good hearing can hear sounds that are between about 20 hertz and 20 kilohertz, or 20,000 hertz.

Frequencies that are too high for humans to hear are called ultrasound.

And dog whistles make ultrasound, which is outside the range of frequencies that humans can hear but within the range that dogs can hear.

They can be used to call your dog to come back to you.

Properties of ultrasound now.

Ultrasound waves are sound waves that are too high frequency for human ears to detect, above 20,000 hertz.

And their higher frequency actually allows 'em to travel through solids that are not very rigid.

That's solids that are soft, don't exactly hold the same shape all the time, such as soft tissue in the human body.

The body contains many soft tissues such as skin and muscle.

And skin and muscle tissue transmit ultrasound frequencies well, they let ultrasound pass through them, but they don't easily transmit sound waves that we can hear.

Now is this true or false? An ultrasound wave is just a sound wave with a higher frequency.

And when you've decided, I'd also like you to say why.

How do you know your answer? And the correct answer is true, because any difference between how ultrasound waves and sound waves behave is purely a result of their higher frequency.

So there's nothing fundamentally different about sound waves and ultrasound.

We just categorise them that way because we're interested in sound waves that we can detect and we label them differently from sound waves that we can't detect.

Like all waves, sound waves and ultrasound waves transfer energy, and they transfer it parallel to the direction their particles are vibrating in, so we call them longitudinal waves.

In this picture, the direction of energy transfer is left to right, the same as the direction of the wave travel, and the vibrations are parallel to that, right-left, left-right.

If the particles have a bigger forwards and backwards vibration, we say that the wave has a bigger amplitude and the particles transfer more energy.

And that's because when the vibrating air particles hit an object, they affect the object more strongly, they make particles in the object vibrate more.

And in fact, if a sound wave has a higher amplitude, they affect your eardrum more strongly and you hear a louder sound.

Another way that a sound wave can have more energy is if it has a higher frequency.

If the particles have a faster backwards and forwards vibration, there'll be more vibrations every second, and that means more energy will be transferred by the wave.

Since they have a higher frequency, ultrasound waves transfer more energy per second than sound waves with the same amplitude.

So a quick recap.

What can cause a sound wave to transfer more energy every second? Greater amplitude, lower amplitude, higher frequency, or lower frequency.

There are two correct answers here.

Greater amplitude, larger vibrations of the particles, and higher frequency, more vibrations per second.

Both cause a higher rate of energy transfer.

Now some pupils are discussing sound waves that have the same amplitude but different frequencies.

Here we have wave A at 5,000 hertz, wave B, 19,999 hertz, and wave C at 20,001 hertz, all with the same amplitude.

Andeep says wave C is an ultrasound wave and A and B are sound waves.

Alex says waves A and C are different types of wave.

Izzy says wave C transfers the most energy each second.

And Laura says waves B and C behave very differently to each other.

I'd like you to think about which pupils are correct and then rewrite the incorrect statements to make them correct.

So press pause while you're thinking about that and writing down your answers, and press play when you're finished.

Let's take a look at who's correct.

Andeep and Izzy are both correct.

We would class C as an ultrasound wave and A and B as sound waves because the cutoff is at 20,000 hertz.

And wave C does transfer the most energy each second.

Although these waves have the same amplitude, C has the highest frequency.

Now what about Alex, can we correct what he said? A and C are both longitudinal waves in air.

Their different frequencies may cause them to behave differently in some situations.

So they're not really different types of waves, they may just affect our ears slightly differently.

And Laura's statement, we can correct it by saying waves B and C are almost identical and behave in very similar ways in all situations.

There isn't a sudden change at 20,000 hertz.

So it's an artificial divide that we make at 20,000 hertz where we say below that it's sound and above that it's ultrasound, but there is no sudden change in the properties of the waves.

Now let's take a look at some uses of ultrasound.

Quite a well-known one is that ultrasound can be used for making scans of soft tissue inside our body.

It's often used in prenatal scanning, so looking at the foetus inside a pregnant person to check how it's doing and make measurements of it.

Ultrasound doesn't harm us and it doesn't harm the foetus in the womb.

It's a very safe way of imaging.

How does it work? When ultrasound waves reach a boundary between one type of tissue and another, some of the waves are transmitted through the new tissue, they carry on through, and some are reflected back.

So when these ultrasound waves from the emitter go into the body, some of them reflect off each boundary.

There might be a boundary between muscle and some other type of tissue.

There's a boundary between the muscle and the fluid that surrounds the foetus and then between the fluid and the foetus itself, so there'll be reflections back from all of these boundaries.

The scanner detects all the reflections.

So this device that is used, it both emits ultrasound, sends it out, and detects what comes back.

So it's a bit like what a bat uses when it uses echolocation, it receives the reflected ultrasound and uses that to build up a picture of what's out there.

We use a computer to build up an image of the inside of the body, and we can use that not only to see a still image but also to see video.

Which of the following statements about ultrasound are correct? Press pause when you read these and press play when you've chosen your answers.

There are three correct answers here.

Ultrasound can pass through soft body tissue, it reflects off bone, and it can reflect off the surface of an organ, because when it hits an organ like a kidney or the heart, that's a different kind of tissue from what it was passing through before, and it can reflect at that boundary.

Ultrasound images are not a type of X-ray.

Ultrasound and X-ray are both ways of making images of what's inside the body, but they work in very different ways.

X-rays are good for looking at bones, whereas ultrasound is good for looking at soft tissues.

Another use of ultrasound is ultrasonic cleaning.

It uses ultrasound to vibrate a liquid inside a tank.

So in this photo the tank is on the left.

It's full of a kind of liquid and you place the items you want to clean inside the tank in the liquid, and the vibrating liquid knocks dirt off the objects.

Ultrasonic cleaning is very good for small and delicate objects such as jewellery because it can clean them without causing damage.

Another use of ultrasound, in physiotherapy.

Physiotherapy is medical treatment that tries to help restore movement and function if someone's affected by injury, illness, or a disability.

If you focus ultrasound on parts of the body, it causes tissue particles to vibrate more than usual, but without damaging them.

And by doing that, the tissue gets warmed up, and that increases the blood flow to it, and that can be a good thing.

Increased blood flow speeds up repair of damaged tissue and can also help reduce swelling and pain.

Now, which of the following properties of ultrasound enable it to clean dirt off fragile objects in a tank of cleaning liquid? Press pause while you read these and press play when you're ready.

The correct answer is B.

It vibrates particles of the liquid with a very high frequency.

Ultrasound is a high frequency wave, and that high frequency means a lot of energy is transferred and that's able to knock dirt off fragile objects but without damaging the object itself.

And now a longer written task for you.

Can you use what you've learned to explain why ultrasound could be used in each of the following applications? An ultrasound scan of a foetus, and warming muscles in a person's leg to help with recovery from an injury.

Take as long as you need, press pause while you're writing your answers, and press play when you're finished and I'll show you some example answers.

Here are example answers.

So for an ultrasound scan of a foetus, ultrasound can pass through soft living tissue without damaging it.

It reflects from boundaries between tissues and an image can be built up from measurements of the reflected ultrasound.

A computer converts these to a real time picture of the foetus.

And what about warming muscles in a person's leg to help recovery from an injury? Ultrasound causes particles in the muscles to vibrate without causing damage.

This warms up the muscle, which helps to increase blood flow.

An increased blood flow speeds up repair of the injury.

So have a look at your answers and compare, and see which points you included correctly and make a note of anything that you missed.

And now we've reached the end of this lesson about ultrasound and I'll give you a summary.

The range of human hearing is about 20 hertz to 20 kilohertz.

Sound waves with a frequency over 20 kilohertz cannot be detected by humans and are called ultrasound.

Some animals can detect ultrasound.

Ultrasound waves transfer energy by vibrating particles forwards and backwards along the direction the wave travels.

And so do sound waves, by the way, but ultrasound waves transfer energy at a higher rate.

They do not cause harm to human tissue, but when concentrated on one spot vibrate particles in tissue, warming up the tissue to aid healing.

Ultrasound scans reflect ultrasound off boundaries between different types of tissue inside the body, using a computer to convert echoes into images.

Ultrasound cleaning uses ultrasound to agitate particles in a liquid in order to clean dirt off objects.

So well done for working through this lesson.

I hope you learned a few things that you didn't know about ultrasound and I hope to see you again in a future lesson.

Bye for now.