Hello, my name's Dr.

George.

This lesson is called hearing sound, and it's part of the unit, waves.

You're going to be finding out about how your ears work and also testing the limits of your own hearing.

The outcome for the lesson is I can explain how sound waves cause sounds to be heard and describe the range and limits of human hearing.

Here are the key words for the lesson.

I'll introduce them as they come up, but if you need to remind yourself of the meanings anytime, you can come back to this slide.

The lesson has two parts, human hearing and the range and limits of human hearing.

Let's get started on the first part.

But first, a reminder of how sound waves work.

Sound waves are pulses that travel through a medium, a material, caused by forward and backward vibrations of its particles.

And here's an animation showing air particles vibrating as a sound travels.

The red ones are just there to make it easier to keep your eye on one particle and see how it moves.

In the human body, sound waves, of course, are detected by the ear.

And when I say the ear, I don't just mean the visible part on the outside.

The ear extends inside the head as shown in this diagram.

In a sound wave, particles aren't transported forwards.

They vibrate, move repeatedly.

And the type of wave a sound wave is is called a longitudinal wave.

So that's a wave in which the vibrations are parallel to the direction of travel.

So this wave is travelling from left to right, and the vibrations are left, right, left, right.

The pattern of vibrations creates moving pulses called compressions of the sound wave.

And now a short question for you.

When sound travels through air, what happens to the particles of the air? Are they moving away from the source of sound? Are they carried along with the sound wave, or do they vibrate back and forth and not travel? With short questions like this, I'll wait five seconds, but if you need longer, press pause and press play when you have your answer ready.

Did you get the correct answer? It's c, the particles don't all move in one direction.

They don't travel away from the thing that's making the sound.

They move back and forth.

Now let's look more closely at the ear.

The outer part that you can see is called the pinna, and that helps to funnel sound waves into the ear canal, which is a tube that's just filled with air.

Here's the ear canal.

And at the end of the ear canal is the eardrum.

It's a membrane, it's like a drum skin.

And when each sound wave pulse, each compression when the particles get close together, arrives at the eardrum, lots of air particles collide into the eardrum at the same time, and that sets it vibrating too.

Another short question for you.

Which of these three labels is the eardrum, and which is the ear canal? Well, the eardrum is c, at the end of the ear canal, and b is showing the ear canal itself.

And when there isn't a sound wave present, air particles do move, but they move with a range of speeds and directions.

You could say their movement is random.

And this animation represents that.

It's not a perfectly accurate representation.

Air is really a mixture of gases and the particles are really further apart than that relative to their size.

But what this means is air particles in the ear canal are colliding with the eardrum all the time.

In fact, air particles collide with everything they come into contact with.

But this happens on both sides of the eardrum.

There's actually air on both sides, so you don't hear anything.

Sound wave adds extra movement on top of the random motion of air particles.

In this animation, the random motion isn't shown.

To make it simpler, just the vibration from the sound wave is shown.

And when a sound wave pulse arrives at the eardrum, lots of air particles collide into it once, just from one side.

And this provides enough force to disturb the eardrum from its rest position, it moves, and it's set vibrating.

And that's the first stage in hearing a sound.

And which of the following can cause a human to hear a sound? Press pause while you read the options, and press play when you've chosen your answer.

And the correct answer is d.

What causes you to hear a sound is the motion of many air particles on one side of the eardrum, hitting the eardrum on its outside side.

Then there are three tiny bones connected to the eardrum.

And when the eardrum vibrates, these bones are set vibrating too.

They're in here, and then here's an enlargement of that.

You can see the bones labelled 1, 2, and 3.

And they act as levers to amplify the vibrations, to make vibrations bigger.

It's a bit like this effect if you need to open a lid.

The effect of the force you use is amplified by using a lever.

You can see that bone 1 is up against the eardrum, so when the eardrum vibrates, it sets bone 1 vibrating, but that's in contact with bone 2, so that sets that vibrating.

And bone 2 is in contact with the really tiny bone 3, which sets that one vibrating.

And then there's this organ, it's called the cochlea.

And the tiny bones cause a liquid inside the cochlea to vibrate.

That bone labelled 3 is up against the cochlea, which is filled with liquid.

So the liquid vibrates inside, and this makes tiny hair cells inside the cochlea vibrate.

It's lined with these tiny hair-like cells.

And when those vibrate, they send electrical signals along nerve cells to the brain.

And there's the nerve.

And the brain uses these signals to create the sensation of hearing sound.

So there's this chain of events that happen that lead to you hearing a sound.

Now which of the following has to happen for you to hear a sound? Press pause while you read these, and press play when you've decided your answer.

And the correct answer is many air particles have to vibrate your eardrum, causing other parts of your ear to vibrate.

This is what needs to happen for you to hear a sound.

So human hearing involves the transmission, the passing along of the vibrations of sound waves to and through different parts of the ear.

So the sound wave starts in the air, a gas, it hits the eardrum, which is a solid, which vibrates, which hits the tiny bones, which are solid, they vibrate.

And then that hits the cochlea and makes the liquid vibrate.

Now something a bit subtle here.

Sound waves are not the same thing as sounds.

Sound waves are patterns of vibrating particles.

They don't have sound or make any sound as they travel.

They just move.

Sounds are created inside your brain in response to nerve signals from your ear.

So a sound is what you experience because of the way your brain works.

So which part of the diagram below represents a sound? D is representing what the person is experiencing, and that is the sound.

Now a longer task for you.

Can you label this diagram using the labels given here? And then can you describe how a sound wave is detected by the ear and explain how this causes a sound to be heard? Press pause while you write down your answers to these questions.

Press play when you're ready to check them.

So here are the answers.

Here are the labels in the correct places on the diagram.

We have the ear canal leading to the eardrum.

Next to that are the three tiny bones.

And then this shape that looks a bit like a snail shell is the cochlea.

And then there's the nerve that takes signals to the brain.

And here's an example answer to question two.

A sound wave makes many air particles in the ear canal vibrate, and this makes the eardrum vibrate.

This makes three tiny bones connected to the eardrum vibrate.

They act as levers to increase the size of the vibrations.

The vibrating bones cause a liquid inside the cochlea to vibrate as well.

When tiny hair cells in the cochlea are made to vibrate, they send electrical signals to the brain along nerve cells.

The brain uses these signals to create the sensation of hearing a sound.

Well done if you included many of these points, and you might find it helpful to compare your answer with this one to see if you missed anything out or made any mistakes with the order.

Now we're ready for the second part of this lesson.

The range and limits of human hearing.

So reminders about kinds of sound that we hear.

Louder sounds are heard when sound waves have a greater amplitude.

And in sound waves with a greater amplitude, vibrations are larger and particles are moving back and forth over a bigger distance.

If the amplitude of a sound wave is too low, air particles won't exert enough force to vibrate an eardrum.

And if your eardrum doesn't vibrate, you don't hear anything.

So you won't hear anything when very low amplitude sound waves arrive at your ears.

If the amplitude of a sound wave is too high, air particles may exert so much force on the eardrum that they can damage it.

Many animals are more sensitive to sound waves than humans.

Their ears are made differently, their eardrum vibrates more easily, and they can hear some sounds that are too quiet for humans to hear.

How can you fill each gap with either amplitude or volume? Press pause while you do this, and press play when you're ready to check your answers.

The first missing word is amplitude.

The amplitude of a sound wave sets the volume of sound that you hear.

So volume is what you experience the loudness of the sound.

Amplitude is a property of the wave itself.

If a sound wave's amplitude is too low, then humans may not hear a sound.

What about high and low sounds? Higher pitch sounds are heard when sound waves have a greater frequency.

That's more pulses per second.

And lower pitch sounds are heard when sound waves have a lower frequency, as with a rumble of thunder, for example.

So frequency, number of vibrations of a sound wave each second.

Pitch, how high or low the sound we hear is.

So pitch pictures about what you experience.

Frequency is a property of the wave itself.

Is each of the following statements about the amplitude or the frequency of sound waves? So for each statement, decide which of those it's describing.

Let's check the answers now.

Frequency is a number of vibrations per second.

Amplitude is the size of each vibration.

A louder sound is heard when amplitude of the wave is greater.

A higher pitch sound is heard when the frequency of the wave is greater.

And frequency is the one that's measured in hertz.

And one hertz means one vibration every second.

So one kilohertz would mean 1,000 hertz, 1,000 vibrations every second.

The kilo there means the same thing as kilo in kilometre.

Kilometre is 1,000 metres.

The lowest pitch sounds that humans can hear are caused by sound waves with a frequency of about 20 hertz, about 20 vibrations per second.

Sound waves with lower frequencies than that can't be detected by the human ear.

Those are called infrasound.

Infra means below.

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

That's 20,000 vibrations per second.

Sound waves that have higher frequencies than that can't be detected by the human ear.

And those are called ultrasound.

Ultra means beyond.

The reason your ear can't detect ultrasound is there are parts of areas that can't vibrate as quickly as the particles in ultrasound waves do because of the size of these parts of the air and other properties.

So a very high frequency wave could hit the eardrum, but that vibration won't be passed along.

But the hearing organs of many animals can detect some sound waves that humans cannot because of their different sizes and structures.

So, for example, bats can hear some ultrasound that we can't.

Can you fill in the gaps in the sentences now? Press pause while you're thinking about it.

The missing frequencies are 20 hertz and 20,000 hertz, or you could say 20 kilohertz, which means the same thing.

So we can detect sound waves with frequencies in that range.

Sound waves with higher frequencies than that are called ultrasound, and sound waves are lower frequencies in that range are called infrasound.

Well done if you remembered those.

Now the frequency limits of human hearing can vary from person to person, especially at the higher frequencies.

A younger person with good hearing can usually detect sound waves between 20 hertz and 20,000 hertz.

And we often quote those values for the range of human hearing.

But the upper limit decreases as a person ages.

The upper limit in adults is often between 15,000 hertz and 17,000 hertz.

The frequencies humans can detect most easily are between 2,000 hertz and 5,000 hertz.

And that's around the frequency of the sounds we make when we talk.

A signal generator can be connected to a speaker to create sound waves of different frequencies.

Here's a diagram showing that.

And these are heard as sounds of different pitch.

The signal generator has a frequency control and also a volume control, so it can be used to investigate the different sound waves that you can hear.

Before we do an investigation, can you match each range of frequencies to the correct values? Here are the answers.

The standard frequency range of human hearing is 20 to 20,000 hertz.

A likely frequency range of an older adult is more like 20 hertz to 16,000 hertz.

And the frequency range that humans hear best, 2,000 to 5,000 hertz.

Well done if you match those correctly.

Now for a mini-investigation.

I'd like you to use a signal generator and loud speaker to investigate your hearing and describe what you find.

You're going to fill in the table here, so try these different frequencies and then decrease the volume until you just hear the sound and record the value of the volume that you read on the signal generator's dial.

And also, I'd like you to experiment with lowest and highest frequencies of sound that you can detect and record those too.

So press pause while you're doing this.

And when you've recorded your results, press play.

Here's a set of sample results.

Yours won't be exactly the same.

Everyone's different.

This person recorded a lowest frequency of 22 hertz and a highest frequency of 22,650.

And they've also recorded the volumes on the dial of their signal generator for these different frequencies.

And as you can see, they found that they can detect really quite low volumes for those frequencies in the thousands, 2,000, 5,000, and they need higher volumes to detect frequencies outside of that range.

And now we've reached the end of the lesson, I'll give you a summary.

Sound waves cause our eardrums, tiny bones, and liquid in the cochlea to vibrate.

Tiny hair cells in the cochlea sense these vibrations and send electrical signals to the brain along nerve cells.

The brain transforms these signals into the sensation of hearing a sound.

Human ears can detect sound waves with frequencies between 20 hertz and 20 kilohertz.

They're most sensitive to frequencies between two kilohertz and five kilohertz.

Sound waves below 20 hertz are called infrasound and above 20 kilohertz are called ultrasound.

These sound waves are not detected by human ears.

So well done for working through this lesson.

I hope you enjoyed it and learned some things that you didn't know about human hearing.

And I hope to see you again in a future lesson.

Bye for now.