Lesson video

Hello, my name's Dr.

George, and this lesson is part of the unit, Sound, Light and Vision.

The lesson is called Sound Vibrations, and I'm going to help you understand how sounds are actually made.

Here's the outcome for the lesson.

By the end, I'm going to help you be able to describe how sounds are caused and explain what sets the volume and pitch of a sound.

Here are the key words for this lesson.

You don't need to know and understand them now because I'm going to explain them as we go along, but this slide is here in case you want to come back later and look at the meanings.

The lesson has three parts.

The first part is about describing sounds, using the words, volume and pitch.

The second part is learning about how sounds are caused by vibrations.

And the third part is about describing those vibrations, using the words amplitude and frequency.

So let's start with describing sounds.

Sound is anything you can hear, whether it's people talking, music, traffic, dogs barking, and we use the word noise to mean sounds that are not wanted.

Now here's the first key word, volume.

The volume of a sound is how loud it is.

You may have used the word in everyday life.

We sometimes say turn up the volume if we want some music to be louder.

So a loud sound has a high volume and a quiet sound has a low volume.

Now, some sounds are low and rumbling, deep, while other sounds are high, perhaps squeaky.

And the next key word, pitch, describes how high or how low a sound is.

A squeaky sound has a high pitch and a deep sound, like thunder, has a low pitch.

So a sound has a pitch, but also a volume.

A sound could have a low volume, and at the same time a high pitch, a bit like this.

(high pitched sound) Or a sound could have a low volume and a low pitch, a bit like this.

(low pitched sound) High volume and high pitch.

(loud high pitched sound) Or high volume and low pitch.

(loud low pitched sound) Now a question for you.

Which of these statements do you think best describes the sound of a lion's roar? I'll give you five seconds to think, but if you need longer, just pause the video and press play when you're ready.

And the answer is D, low pitch and high volume.

Low pitch, because the sound of a lion's roar is quite deep and high volume, because it's usually loud.

And now a task for you.

Here are some sentences about sounds, and I want you to fill in each gap, but you can only use the word volume or pitch for each.

You'll need to use each of them more than once.

Take as long as you need for this task by pressing pause and then pressing play when you're ready with your answers.

So here are the answers.

A piano has some notes that are deep and rumbling.

These have a low pitch.

These are on the left side of the keyboard.

But if you press keys more gently, you get sounds with a lower volume.

That's about how loud the sound is.

So with a piano, you can make sounds with different pitches by pressing different keys and different volumes by pressing them softly or harder.

Now a police siren.

A police siren has two notes, a little bit like this.

(imitating police siren sound) Each of these two notes has a different pitch.

One is a bit higher than the other.

A siren has a very high volume so it can be heard.

It's loud.

You've learned about how to describe sounds, now, using the words volume and pitch, now let's look at how sounds are actually made.

Here's a guitar and someone is plucking one of the strings.

And you can see it must be moving.

It looks like a blur.

The string is doing what we call vibrating.

This is another key word today.

And when the string is vibrating, it's moving back and forth very quickly.

And if you put your finger on the string to stop it from vibrating, the sound stops too.

Another instrument, a drum, makes a sound when you hit it.

And when you do that, the skin on top of the drum vibrates.

But these vibrations are hard to see.

You can feel them though if you lightly touch the skin of the drum when it's been hit, you'll feel that it's moving.

Or if you sprinkle powder onto the drum skin, it bounces when the vibrations happen.

A question for you.

Which of the following are sounds caused by? And there's one correct answer here.

If you need more than five seconds, press pause and then press play when you're ready.

And let's check the answer.

It's vibrations.

Whenever something vibrates, it makes sound, and whenever there's a sound, it's made by something vibrating.

If you thought strings, well, yes, strings can make sounds, but they're certainly not the only thing that make sounds and they only do it if they're vibrating.

Another kind of instrument that we make sound with is wind instruments and brass instruments.

And these make the air inside a tube vibrate, and that makes sound.

There are different ways to make the air vibrate.

A lot of instruments we blow into, such as a recorder or a flute or a trumpet, but there are other instruments that have different ways to make the air vibrate.

And how do we make sound using our own voice? Well, something must be vibrating, and it's the air inside your throat, mouth, and even your nose that vibrates to make a sound.

And the thing that starts that air vibrating is your voice box or larynx, here, at the front of your neck and that controls how the air vibrates so that you can make different kinds of sounds with your voice.

And another question.

Which of the following vibrate to make a sound? There might be more than one correct answer here.

If you need more than five seconds, press pause and then play when you're ready.

And let's check the answer.

Well, you've already seen that guitar strings can vibrate to make sound and so can the skin on a drum, but if a horn makes sound, well, that's a brass instrument and the metal of the horn itself actually vibrates and so does the air inside.

Anything that vibrates will make a sound.

Musical instruments are good at vibrating.

They're designed that way to make plenty of sound, but not everything is good at vibrating.

For example, if you hit a cloth with a drumstick, that doesn't make much sound.

The cloth is soft and soft objects don't vibrate very much.

Now, if you tap a glass gently with a drumstick, that makes a sound, but we can't see any vibration, but we know by now that if there's sound, it must be vibrating.

So what we can do is put a straw inside the glass, and then when we hit the glass, although the glass doesn't look like it's vibrating, we can see the straw move and the straw's moving back and forth a bit.

It's vibrating because the glass is hitting it.

So we know that this glass that's making a sound really is vibrating.

Speakers, of course, make sounds and they do this because part of them is vibrating and the vibrations of that part are usually too small and too fast to see.

But here's an animation of the vibrating part called the cone, and it's been slowed down and the vibrations have been made larger than real life so that you can see what that vibration is like.

If you want to actually see that a loud speaker cone is vibrating, you can put small pieces of paper or even table tennis balls onto that cone and they'll jump about as it vibrates.

And a question for you.

Two small stones are knocked together.

Which of the following correctly explains why a sound is made? If you need more than five seconds, pause the video and then press play when you're ready.

And the answer is B, the knock causes the stones to vibrate.

We know by now that vibrations are what make sound.

You might not be able to see the stones vibrate, but they do, although not for very long, which is why the sound doesn't last very long.

And now a longer task for you.

It's about a tuning fork.

Now, a tuning fork is a shaped piece of metal.

It looks a little bit like a fork you eat with, but you don't eat with this one.

It has two prongs, and if you hit the end of it against a table, for example, you can hear a sound.

It makes a note that musicians can use to help them tune their instrument such as guitar or violin.

Now, what you're going to do is predict what you think you'll see when a tuning fork, while it's making sound, is touched against a table tennis ball hanging from a thread as shown by the pictures on the right.

So you'll predict what you think you'll see, then explain why you think this will happen, and then watch this as a demonstration.

If you can't see it in real life, you could watch a video of this happening, and then think about whether you were right, and if if not, you can change what you've written, write a better explanation.

So press pause and when you're ready, press play again.

Now let's look at the answers that one student wrote.

For their prediction they said, as soon as the table tennis ball touches the tuning fork, the ball is powerfully flicked sideways by the tuning fork.

So if you had the idea that the tuning fork was going to make the ball move, then you had the right idea, and I hope that's what you saw.

And this student's explanation says, when the tuning fork is making a sound, it is vibrating, but the vibrations are too small to see.

When the tuning fork touches the ball, it will knock the ball sideways because it is vibrating from side to side.

And that's correct.

Don't worry if your answer isn't exactly the same as that or isn't as long as that.

The most important thing is that you mentioned that the tuning fork is vibrating and that's why it pushes the ball sideways.

We know it's vibrating because it's making a sound.

And now onto the final part of this lesson, which is about how we describe the vibrations themselves.

Look at the animation.

We can see a ruler being twanged so that it vibrates.

So what we're going to look at now is how we describe the vibration.

By the way, the reason the ruler keeps moving back and forth is because when it bends upwards, that stretches it, and there's a force called tension that pulls it back down.

And when it bends down, it stretches again and the tension force pulls it back up.

And this keeps happening for a while.

Now, two animations here showing vibrations with different amplitudes, the next key word.

Amplitude is how far an object moves when it vibrates and I hope you can see that the loudspeaker cone on the right was moving further each time it vibrated than the one on the left.

So the one on the left had small vibrations, low amplitude, and when vibrations have a low aptitude, that makes quiet sound.

And the one on the right was vibrating a greater distance.

So we could say those are bigger vibrations, they have a larger altitude and it would've made a louder sound.

How do we make sounds with greater amplitude? Well, if we have a, for example, percussion instrument, the type of instrument you hit, which could be a drum or a xylophone or a triangle, for example, you simply hit the instrument harder.

I'm sure you already knew that, but now you know the reason that if we hit the instrument harder, we make larger vibrations, greater amplitude vibrations, so we make a louder sound.

With a string instrument, what we need to do is make the strings vibrate a greater distance, move further backwards and forwards.

Now, depending on the instrument, we might hit the strings harder or pluck them a larger distance, pull them further with our finger, or if we're using a bow, as with a violin, we press harder with the bow.

We just need to do something to make the strings vibrate a greater distance back and forth and the sound will be louder.

With wind instruments, instruments where there's air vibrating in a tube, we need to make those vibrations have a bigger amplitude, the air moving more back and forth.

We just blow harder through the tube.

Two questions for you to think about.

This is about what happens when we hit a drum.

I'll give you five seconds, but press pause if you need longer and then press play.

And the answer to question one, of course, the sound is louder when we hit a drum harder, but now why? Because it vibrates with larger vibrations.

Vibrations have larger amplitude.

Now, have another look at this guitar string.

As you can see, it's vibrating so fast that it becomes a blur.

Let's think about another way that we describe vibrations, and that's using the word frequency.

Frequency means how often something happens.

For example, for a bus service, you might say the frequency is once every 10 minutes, if a bus comes every 10 minutes, or you could say six times per hour.

The frequency of a vibration tells you how many vibrations happen every second.

And the more vibrations happen every second, the higher the frequency.

With that guitar string, there was quite a lot of vibrations per second.

Now, these two animations show different frequency vibrations, lower frequency on the left, higher frequency on the right.

The one on the right's vibrating more times per second.

And when the frequency of vibrations is higher, it makes a higher pitch sound.

That's how you make higher or lower sounds, by having different frequency vibrations.

So the speaker on the left wasn't vibrating many times per second, so its vibrations were low frequency and that makes a sound with a low pitch.

The one on the right, more vibrations per second, higher frequency vibration, and that makes a sound with a higher pitch.

Going back to musical instruments now, how do we get higher frequencies from them? Well, if you're using a percussion instrument like a drum, the only way to make higher frequency vibrations is to change the instrument in some way.

Either swap it for a smaller drum, because it turns out smaller objects vibrate at higher frequencies, or you can make the drum skin tighter over the top.

That actually will make a higher sound with higher frequency vibrations.

If you're blowing into a wind or brass instrument, then you need to make the air in the tube vibrate with a higher frequency.

And to do that, you actually need the tube to be shorter.

So air vibrating in a shorter tube will vibrate more times per second, and different instruments have different ways of doing that.

Some actually have a slider, like a trombone, that changes the tube length.

With others, you press keys, such as on a clarinet.

It may be less obvious with a recorder.

You press your finger against holes on the tube.

It turns out that by doing that, you actually change the amount of air, the length of air that vibrates inside the tube.

Or of course you could swap the instrument.

If you swap a recorder for a smaller, shorter one, you get a higher sound.

If you use a large long recorder, you get lower sounds.

And a question about that.

So a loud speaker is vibrating with low amplitude and high frequency.

Can you describe what that sound will be like? Five seconds, but pause if you need longer, and press play when you're ready.

And the answer is D, high pitch and quiet.

High pitch because the vibrations have a high frequency and quiet because the vibrations have a low amplitude.

Now, the loudspeaker is vibrating more times every second than it was before, but the size of the vibration is the same.

What has changed? Five seconds but pause if you need longer.

And the thing that changes is the frequency of the vibration more times every second, that means higher frequency.

But something else, the sound itself will change.

The sound will have a different pitch and it'll be higher because the frequency is higher.

And a task for you.

So here, there are two categories and six statements, and I want you to sort the statements into the two categories.

Some of them are about the frequency of vibrations, and some of them are to do with the amplitude of vibrations.

So pause the video while you think about which statement goes where, and press play when you're ready.

And here is where each statement belongs.

So A, C and F are all about frequency.

So A describes frequency of vibration.

It's a measure of how quickly the object moves back and forth, how many times per second, and C, the number of vibrations occurring every second.

So these are ways of describing frequency.

And F describes the effect of frequency.

The frequency of vibrations set the pitch of the sound.

B, D and E, they're all about amplitude.

B and D are different ways of describing what amplitude of vibrations means.

B, a measure of the distance or size of the vibration back and forth, and D, the furthest distance reached away from the rest position.

And E describes the effect of aptitude.

It sets the volume of the sound that's made.

So well done for doing that.

And now let's look at a summary of the whole lesson.

So we've seen that volume of a sound means how loud or how quiet it is, and pitch of a sound means how high or how low it is.

So volume and pitch describes sounds.

You've also learned that all sounds are caused by vibrations, and some vibrations are too small or too fast for us to see.

You learned to describe vibrations.

So the amplitude of a vibration measures its size, how far the object vibrates, and larger vibrations have bigger aptitude and they make louder sounds.

The frequency of a vibration is a number of vibrations that occur every second.

The more vibrations happen every second, the higher the frequency and the higher pitch of the sound that's made.

If you can remember these key terms, you'll be able to use them whenever you're describing sounds and how they're made.

So thank you for working through this lesson.

I hope you've enjoyed it.

I hope you've learned some things about sound, and I hope to see you again soon.