Lesson video

Hi, my name's Mr. Norris, and today we're gonna be looking at human hearing.

This is from the Measuring Waves topic.

This is a really interesting lesson because it's all about how we sense the world around us.

And the physics of sound waves can help us answer that question of how we hear.

By the end of this lesson, the outcome should be that you can describe how the human ear detects sound, and also you should be able to explain why there are limits to human hearing.

Here are some keywords we're gonna be focusing on in today's lesson: transmit, eardrum, cochlea, infrasound, and ultrasound.

I'm gonna read out an example sentence of each word being used.

Once I've done that, you might want to pause the slide to reread and absorb as much information about each keyword as you can.

So waves are transmitted when they pass across the boundary from one material into another.

The eardrum is a membrane within the ear that's caused to vibrate by incoming sound waves.

The cochlea is a spiral-shaped structure within the ear that creates electrical signals when set vibrating.

Sound waves of frequency below 20 hertz usually cannot be heard by humans.

They're called infrasound.

And sound waves of frequency above 20,000 hertz usually also cannot be heard by humans.

They're called ultrasound.

The lesson is split into three sections.

The first section is on the transmission of sound.

The second section is on the structure of the ear and how hearing works.

And the third section puts it all together and looks at the limitations of wave transmission processes, which are also the limitations of human hearing.

So let's get started with the section on the transmission of sound.

So how does sound travel? Well, sound travels as longitudinal waves.

There's a longitudinal wave in the diagram.

And the dots represent air particles, which, of course, are oscillating forwards and backwards.

Each particle in the wave medium is oscillating forwards and backwards.

If we zoom in to have a look at some individual air particles, we can see that they're oscillating.

We look at the arrows.

And how is sound transmitted? Well, oscillated particles collide into the particles next to them and then set them oscillating, and then they'll collide into the particles next to them and then set them oscillating, and then they'll collide into the particles next to them and set them oscillating.

And then that's how energy is transferred.

So oscillating particles collide into their neighbours causing them to oscillate too.

And that's how energy is transferred forwards.

So that individual particles don't move forwards.

The individual particles are just oscillating in place and don't end up anywhere different to where they started.

Individual particles don't move forward, so they're just oscillating, but energy is being transferred forwards through the collisions between oscillating particles.

And the direction of oscillation is parallel to the direction of energy transfer.

And that's the definition of a longitudinal wave.

So that's how sound is transmitted through a medium.

But the word transmission can also mean when a wave is transmitted from one medium across a boundary into another.

So when waves reach a boundary between materials, between media, three things can happen.

The wave can be reflected, such as shown with the arrow that just appeared, The wave could be absorbed.

Now, I can't show that because if the wave's absorbed, then the wave doesn't exist anymore.

So the incoming wave kind of just disappeared because the energy of the wave is absorbed, and there are no more oscillations because the wave energy's been absorbed.

But the third thing that can happen is the wave can be transmitted.

And if there's a change of direction as well as the wave being transmitted, that's called refraction.

But we're not focusing on that today, we're just focusing on transmission, so whether there's a change of direction of the wave or not.

So waves are transmitted when they pass across a boundary from one wave medium, one material into another.

That's when waves are transmitted.

Let's take a closer look at the transmission of sound from one medium to another.

So on the left of this diagram we've got air and air particles and the arrows represent the oscillations of the air particles, longitudinal oscillations of the air particles.

And those oscillations are going to be transmitted from that gas from the air to a solid, maybe a metal, something like that.

So the process of transmission is exactly the same.

Oscillating air particles collide into particles of the solid causing them to oscillate too.

That's how the energy is transferred forwards across the boundary between the two media, between the two materials.

Zooming in for a closer look, the particles on the left are air particles.

They're quite far apart, they're oscillating, and they're gonna collide into the darker particles of the solid, of the metal, whatever the solid is, setting them oscillating as well, and that's how energy is transferred forwards across the boundary.

That's how sound is transmitted from a gas to a solid.

Okay, time for a quick check.

Which best describes how sound waves are transmitted? A sound waves transmitted by high-frequency oscillations, by refraction, by collisions between vibrating particles, or by thermal conduction.

Which best describes how sound waves are transmitted.

Make sure you've got an answer.

Well done if you put C.

Sound waves are transmitted by collisions between vibrating particles.

Okay, let's do a task on this.

So a vacuum is an area of space where no matter is present.

So not a vacuum cleaner.

That works by creating a vacuum, which is an area of space where no matter is present, and then matter like nearby gases, nearby air rushes into fill the space sucking up into the vacuum cleaner.

So a vacuum cleaner works by creating a vacuum, an area of space where no matter is present.

So explain why sound waves cannot be transmitted through a vacuum.

Why can't sound waves be transmitted through a vacuum? So think about everything we just talked about how sound is transmitted.

And then think about a vacuum, which is an area of space where there's no matter, so no solid, no liquid, no gas.

And think about why sound couldn't be transmitted through that.

What needs to be there for sound to be transmitted? What needs to happen for sound to be transmitted, and why would that not happen in a vacuum? So try and write a 2 mark answer for that question.

Pause the video now and have a go.

Okay, let's see how you got on.

So first thing that you could have included would be to explain what is needed for sound to be transmitted.

So sound is transmitted when vibrating particles collide.

That might get you a first mark, but in a vacuum, no matter is present.

So there are no particles present, no solid, no liquid, no gas.

That's what a vacuum is, an area of space where no matter is present.

So there are no particles present to collide, therefore, that process by which sound travels, vibrating particles colliding, that process by which sound travels cannot occur for the second mark.

So very well done if you've got both of those.

So we're now ready to look at the structure of the human ear, which involves wave transmission processes.

So let's start that now.

Okay, so the first thing to say is that sound waves are not the same thing as the sounds you actually hear.

So that's a sound wave.

A sound wave is something physical, a longitudinal wave caused by oscillations of particles.

Whereas the sounds you hear are not physical, they're sensations experience kind of in your mind.

So on one hand you've got something physical, and on the other hand you've got an experience, something non-physical, a sensation.

So the question of how we hear is how does the physics, the sound waves, longitudinal waves, how does that create the sensations of the sounds you actually hear? And, of course, it's the ear that's responsible for that.

The ears is the organs that sense and respond to sound waves to create those sensations of hearing.

So let's look in detail at the ear now.

So here's a diagram of the external part of the ear.

Of course, the ear is an organ which actually extends into your head, but this is the part you'll be most familiar with as an ear.

This part is called the pinna, and its job is to act like a funnel to direct sound waves into the ear.

Extending into the ear now we have the ear canal.

Now the pinna and the ear canal make up the outer ear.

So the ear is actually divided into three parts.

You've got the outer ear, the middle ear, and the inner ear extending the furthest into the head.

The ear canal is an air-filled tube, so plenty of air particles there to oscillate and collide into each other and transmit the sound waves down towards the middle ear, which comes next.

So the eardrum is a membrane that separates the outer ear from the middle ear.

The eardrum is set vibrating by the incoming sound waves.

A bit like how the skin of a drum is a bit like a membrane that sets vibrating when you hit the drum.

Of course, when a drum skin is set vibrating, that creates sound.

Whereas when the eardrum, that's set vibrating by sound.

So there is a slight difference there.

The oscillations are then transmitted to the ossicles, which are three tiny bones within the ear.

We'll just zoom in on that square.

So we can see the three tiny bones, the ossicles more clearly, one, two, and three.

That zoomed in portion of the image.

They have Latin names.

The first one is called the malleus.

That's Latin for hammer.

The second one is called the incus.

That's Latin for anvil.

And the third one is called the stapes.

That is Latin for stirrup.

So a hammer and an anvil are tools used in metalworking.

You pop a sheet of metal on the anvil, which is very, very strong, solid metal.

And then you can hammer out your metal sheet because it's on top of the anvil to support it.

So that kind of explains what the malleus and the incus bones do in the ear.

So the malleus is attached to the eardrums. So when the eardrum vibrates, the oscillations are transmitted to the malleus.

The malleus then strikes the incus, the hammer strikes the anvil just like in a metalworking environment.

The oscillations are passed on by those collisions.

And then finally the oscillations are transmitted to the stapes, the stirrup.

Now, this bone is named after what it looks like.

It looks like a stirrup that's used in horse riding.

The shape of it is similar.

So the oscillations are transmitted through each ossicle in turn because they're all connected.

So the obstacles are set vibrating by the eardrum, and the vibrations are transmitted through each one in turn.

But why are they there? It's because they act as levers to amplify the vibrations.

Now, a lever is anything that acts a bit like a seesaw to produce greater movement.

So you will have all, I'm sure, used a lever or seen a lever in action to lever off, for example, a painting lid.

You can push down on one end of the lever, and the other end will apply a greater force upwards because it kind of magnifies the movement, magnifies the vibrations.

So the ossicles act as levers to amplify the vibrations, make the vibrations kind of large enough amplitude that the next parts of the ear, which is gonna be the inner ear, can then sense and respond to that sound, to those oscillations.

So those amplified vibrations are then transmitted to the next part of the ear, which is called the cochlea.

The word cochlea is Latin for snail shell.

And you can see the cochlea's got that spiral shape, which is why it's named after the Latin for snail shell.

The cochlea is part of the inner ear now.

So we've reached kind of the furthest in to the head.

The cochlea is filled with fluid, and it's lined with tiny hairs called cilia.

So when the cochlea is set vibrating by the ossicles, then all of the fluid is vibrating, and that sets all of the tiny hairs vibrating.

And it's the cells of the cochlea, which then creates electrical signals, which are what's gonna be carried to the brain, which actually then causes that sensation of sound.

So it's the cells of the cochlea that actually they produce electrical signals, but only when only when they're vibrating with large enough amplitude.

So this takes us to the last part of the ear which actually connects all the way to the brain.

It's the auditory nerve.

Anything begins with audi generally has to do with sound, you know, audio equipment, it's sound equipment.

So audi generally refers to sound.

So the auditory nerve is the nerve cell which carries the signals for the sensation which become the sensations of sound when they arrive at the brain.

So the auditory nerve carries the electrical signals that were created by the cochlea.

The cells of the cochlea created the electrical signals, and the auditory nerve transmits them to the brain.

And when they arrive at the brain, that's what creates the sensation of sound.

So the structure of the ear is quite complex, there's lots of parts.

So let's do plenty of checks to make sure we understand the names of the different structures and the functions of each one.

So which part of the ear funnel sound into the ear canal? Which part of the ear funnels sound into the ear canal? Is that the cochlea, the pinna, the eardrum, or the ear funnel? Make a decision now.

Hopefully that should have been fairly straightforward.

It is the pinna that is the parts of the ear that funnels sound into the ear canal.

That's the external part of the ear that you can see on the outside of people's heads.

Next question, which part of the ear is set vibrating directly by the incoming sound waves? Is it the tiny bones, is it the cochlea, is it the eardrum, is it the auditory nerve? Which part of the ear is set vibrating by the incoming sound waves? Did you get an answer? It is the eardrum.

The eardrum is set vibrating directly by the sound waves.

Which of the following best describes the eardrum? Is it a flap, a tube, is it fluid filled, or is it a membrane? Make a decision.

Okay, well done if you chose a membrane.

The eardrum is best described as a membrane that oscillates, set oscillating by the incoming sound waves.

What's the name of this structure? Now, look carefully at that diagram because before we were looking at a right ear, but this is someone's left ear, okay? If you were that person, that ear would be on your left.

However, the structure is exactly the same.

So you should be able to recognise the structures, whether it's a left ear or a right ear.

So what is the name of this structure? Is it an eardrum, is it the ear canal, is it the cochlea, is it an ossicle? You should have committed to an answer.

Well done, it is the cochlea, which is Latin for snail shell.

It's that spiral structure.

Starting from the outermost part, sort these parts of the ear into the order that sound waves are transmitted through the ear.

Cochlea, eardrum, and ossicles.

That's the wrong order.

Sort those into the correct order that sound waves are transmitted through them.

Okay, hopefully you have the correct order now.

The correct order is sound waves are transmitted to the eardrum first, then to ossicles the tiny bones, and finally to the cochlea.

Well done if you've got that.

Next, quick check.

On the four lines, identify which state of matter, solid, liquid or gas, sound waves are being transmitted through for each stage of hearing.

Okay, make sure you've got an answer on all four lines.

Okay, when sound waves are in the air, that is a gas.

When the sound waves are transmitted to the eardrum, the eardrum is a membrane, so that's a solid sheet.

The oscillations are then transmitted to the ossicles, which are tiny bones, so they're solids, vibrating solids again.

And then the oscillations are transmitted to the cochlea.

Now the cochlea is fluid filled, so the oscillations are transmitted to a fluid or a liquid.

So that's what you should have on that line.

So very well done if you've got all four.

Next question.

Match up each part of the ear to it's function.

So I would pause the video, read through each one carefully and try and match up each to the correct function.

Pause the video now.

Okay, you should have had a go at doing the matching.

So the eardrum is what's set vibrating by the incoming sound waves.

Ossicles are small bones.

There's three of those in one ear.

They amplify the vibrations by acting as levers.

And the cochlea, that is, the cells of the cochlea are what translate the vibrations into electrical nerve impulses.

It's the cells of the cochlea that do that.

Very well done if you've got all three.

Now time for a task.

First task is to label the diagram of the ear with all of the correct names for the different parts of the ear.

So you should pause the video now and give that a go.

Okay, let's go through those labels for part one of this task.

The outermost part of the ear is called the pinna.

Then the oscillations are transmitted through the air-filled tube.

That's the ear canal.

Then the oscillations are transmitted to the eardrum.

Then the oscillations are transmitted to the ossicles, the three tiny bones.

Well done if you put the names of each one, malleus, incus, stapes, or hammer, anvil and stirrup.

That would be extra.

The oscillations are then transmitted to the cochlea.

And finally, that last part is the auditory nerve, which carries the electrical impulses to the brain to cause the sensation of sound.

Very, very well done if you've got all of those parts.

Part two of this task.

I'd like you now to write four detailed bullet points that describe how the human ear detects sound.

You should include short descriptions of the key structures within the ear and what happens to each one.

Now, in a minute, I'm going to put up a possible writing frame which you could use.

But if you are confident doing that already, you should pause the video now and make a start.

If you'd like to use the writing frame, then keep watching.

So here is a structure you could use to help structure your answer.

So you should pause the video now and have a go at this task.

Okay, let's see how you got on.

So I'm going to show you a really detailed example answer.

So you would want to say something first about how incoming sound waves cause the eardrum, which is a thin membrane to vibrate.

Look how the extra detail's been added in brackets there.

That's a really nice way of adding extra details succinctly about what the eardrum is, a thin membrane.

Then at the vibrations are transmitted to the ossicles, which are three tiny bones which act as levers to amplified vibrations.

So that's what the ossicles do.

Then the vibrations are transmitted to the cochlea, which is a spiral-shaped fluid-filled structure.

Look at that description that's been added there.

Fluid-filled structure lined with tiny hairs called cilia, which are also set vibrating.

And then finally that triggers the cells of the cochlea to create an electrical signal, which is a translation of the original sound, the translation of the original vibration.

And then the auditory nerve carries that electrical signal to the brain where it causes the sensation of hearing.

So that was an example of a really detailed, well sequenced answer.

But what I also wanna show you now is like a slightly simplified answer that doesn't contain as much detail, but really shows you what the key points are for the parts of the ear and the functions.

What's the minimum I'd want you to include? So incoming sound waves cause the eardrum to vibrate.

The vibrations are then transmitted to the ossicles, the three tiny bones.

The vibrations are then transmitted to the cochlea, and the cells of the cochlea are what create the electrical signal which is carried to the brain.

So they're the key points I was really looking for, but I wanted to show you the detailed, really nice answer first.

So we've looked at how hearing works, and now it's time to put it all together and look at the limitations of wave transmission, which are also the limitations of human hearing.

So we've said that human hearing involves the transmission of sound waves from different materials from the air, which is a gas, to the eardrum, which is a solid membrane, to the ossicles, which tiny bones, that's a solid, to the cochlea, that's a liquid.

So human hearing involves the transmission of sound waves, but there are two factors that limit transmission processes.

So also limit human hearing.

The first factor is the wave amplitude, the loudness of the sound.

If amplitude is too low, so if sounds are too quiet, then air particles won't oscillate far enough to set the particles of a solid oscillating as well.

Okay, so if those air particles are on the left, if they're not oscillating far enough, if the sound is too quiet, they're not going to set the particles of the solid oscillating as well when they collide into them because they're just not oscillating far enough.

That's why you can't hear very quiet sounds.

But if the amplitude is too great, so look at those larger arrows now showing particles oscillating further, then particles of the solid might be forced to oscillate so far that they're gonna overcome the forces holding together so the solid might break.

And this is why loud sounds are dangerous because the air particles are oscillating too far with too great an amplitude which will make the particles of a solid like the eardrum oscillate too far, and that might break the solid and say tear the eardrum or something like that.

So that's why loud sounds can be dangerous.

Time for a quick check on that.

Match up the reasons why amplitude can limit the transmission of sound waves to solid.

Make sure you've committed to a matching.

Well done if you got that if sound wave amplitude is too low, that means air particles won't oscillate far enough to set the particles of a solid oscillating as well.

Whereas if sound wave amplitude is too high, then particles may be forced to oscillate too far so far that the particles holding them together may be overcome, that would break the solid.

So in fact these were already matched up.

So very well done if you've got that.

We now need to look at the second factor that limits wave transmission processes.

So the first factor was the wave amplitude, but the second factor is the wave frequency, which is the pitch of the sound.

If frequency is too low, then vibrating air particles won't exert enough force when they collide to set the particles of a solid oscillating.

So if frequency is too low, particles are only taking a long time to oscillate forwards and back.

So that means the speeds will be lower, so a collision at lower speed won't exert enough force to set the particles of a solid oscillating.

So that's why some sounds are actually too low pitched, too low frequency to actually hear.

And those sounds are called infrasound.

That's too low pitched, too low frequency to actually hear.

The word infra is actually Latin for below.

So infrasound are sounds that are below the frequency that we can hear.

But what if frequency is too high? Well, if frequency is too high, then vibrating air particles, they're vibrating really quickly.

So every oscillation takes very, very little time.

And that means a collision between a vibrating air particle and a particle of a solid, that vibrating air particle, because it's oscillating so quickly forwards and backwards, when it actually collides, it won't exert force on the particle of a solid for enough time to actually then have an effect on the particles of a solid and displace them and set them oscillating too.

So if a frequency is too high, vibrating air particles won't exert force for enough time to displace the particles of a solid to have an effect on the particles of the solid.

So this is why you can't hear sounds that are too high frequency or too high pitch.

They're called ultrasounds, because in Latin ultra means beyond.

So ultrasounds are beyond the highest frequency that the human ear can detect.

So we could present this diagram or this scale showing frequencies that humans can and cannot hear.

Now the first thing I should say is this is a logarithmic scale.

So equal spaces on the scale above actually represent factors of 10, not equal amounts, but equal factors, a factor of 10 every time.

So between each scale marking is a factor of 10, 2 to 20 is times 10, 20 to 200 is times 10, 200 to 2000 is times 10.

That's called a logarithmic scale.

So humans are able to hear sounds in this range of frequencies.

That's from 20 hertz to 20,000 hertz, or 20 kilohertz.

Anything below 20 hertz in frequency we can't hear.

It's too low pitched.

The frequency's too low to hear, and it's called an infrasound.

Anything above 20,000 hertz is too high frequency to hear, and it's called an ultrasound.

So infrasound is sound waves that are too low frequency because infra means below the range of human hearing.

And ultrasound sounds that are too high frequency to hear, they're beyond the range of human hearing.

Of course, it's not the same for all animals.

Dogs are very well known to be able to hear into the ultrasound.

So sounds which are too high pitched for humans to hear, dogs can hear.

For example, a dog training whistle.

It's just like kind of a normal referee's whistle that you blow it when you're training a dog, but it doesn't make a sound that we can hear.

It makes an ultrasound that dogs can hear to help kind of train them.

Dogs can also hear into the infrasound and so can cats.

And actually cats can hear even further into the ultrasound than dogs can.

Small birds have a much smaller range of frequencies that they can hear.

Elephants can hear really quite far into the infrasound, so lower frequencies than we can hear.

They can't hear into the ultrasound.

And then mice and bats, they're obviously much smaller animals, they can hear really very far into the ultrasound.

So much higher pitch sounds than we can hear and much higher pitch sounds than even dogs and cats can hear.

And, of course, bats use their ultrasonic hearing to navigate.

So you might be wondering about if there's the hint of a pattern here as elephants are the largest and they can hear the lowest pitch sounds, and mice and bats are smaller and they can hear higher pitch sound.

Now, there is a general principle in physics that the size of a receiver does affect the frequencies that are detectable, but ears are such complex biological structures that it's not as simple a pattern as that.

Although there is the hint of that pattern still here in that the largest ears here can hear the deepest sounds and some of the smallest ears can hear the highest pitch sounds.

But it's not as simple as that for ears in reality.

Let's do a quick check, so true or false.

Sounds with frequencies above 20,000 hertz are called infrasound.

So decide firstly is that true or is that false? Make sure you've committed to an answer.

And before I tell you the answer, I would like you to justify your answer.

So choose either A or B, justify your answer.

Okay, so you should have chosen either true or false, and then either A or B as well.

So the correct answer is false.

Sounds with frequencies above 20 hertz are not called infrasound.

They're called ultrasound for above 20,000 hertz.

And that is because infra means below.

So sounds with frequencies above 20,000 hertz are not gonna be called below sound, infrasound, they're gonna be called ultrasound, which is beyond.

Next one, match up the reasons why frequency can limit the transmission of sound waves to solid.

So if sound wave frequency is too low, and if sound wave frequency is too high, which is the correct reason why the sound waves might not be transmitted? Pause the video to make sure you commit to an answer.

Okay, let's see how you got on.

If sound wave frequency is too low, that means vibrating air particles won't exert enough force on a solid.

Whereas if sound wave frequency is too high, that means vibrating air particles won't exert force on a solid for enough time because the oscillation is so fast then there's not enough time of the collision to actually affect the particles of the solid, so the transmission doesn't work if the sound wave frequency is too high.

So they were actually correctly matched up to start with as well.

So well done if you've got both of those.

The final thing that we need to look at is the effects of age because age also limits wave transmission and limits human hearing.

So the first effect of age is that it can be more difficult to hear quieter sounds.

And a second effect is with age, the highest frequency that can be heard actually reduces.

And older people might need to use hearing aids to help with these.

Both of these happen because the cells of the cochlea and auditory nerve can become damaged just over time with use.

The cilia may become stiffer, and the cells may become less responsive, so it may not produce electrical signals at all the frequencies and all the amplitudes of vibration that they did before.

Let's do a quick check of that.

Fill in the missing words.

With age, the highest frequency you can hear decreases because the something of the something and something can become damaged over time.

What of the what and the what become damaged over time with age? Pause the video, have a go at filling those in.

Let's see how you got on.

With age the highest frequency you can hear decreases because the cells of the cochlea and auditory nerve can become damaged over time.

Well done if you've got all three missing words.

So to finish the lesson, let's pull it all together and do this task.

So it's another written task.

I would like you to explain why humans can't hear ultrasound.

And in your answer you should include what ultrasound is, a description of the process of hearing, and then what is different about the process of hearing for ultrasound, which then explains why you can't hear it.

And you should be aiming to write about four to six clear bullet points for this answer.

So pause the video now and have a good go at this.

Off you go.

Okay, let's see how you got on with this task.

I'm gonna show you an example answer, so yours might not look exactly like this, but hopefully, it's gonna cover lots of similar points.

Okay, so firstly, what's ultrasound? Well, ultrasound waves are high-frequency sound waves.

They're above 20,000 hertz.

So it is a good thing to say what ultrasound is before you explain why humans can't hear it.

So to hear a sound waves must be transmitted from the air to the eardrum and three tiny bones which are solid, and then to the cochlea, which is fluid-filled, and that triggers an electrical signal to be sent to the brain.

So that's how you'd hear normal sounds.

So how is it different for ultrasound? That's what we should say next.

However, the process of transmission across such boundaries is limited.

At frequencies that are too high, for example, ultrasound, which is above 20,000 hertz, we said earlier, then vibrating particles won't exert force on a solid for enough time to set that solid vibrating as well.

So it's just what we said earlier.

So if you need to pause the video here and compare your answer carefully to this one and add anything that improves your answer.

Okay, so we'll just run through a summary of the lesson.

Human hearing involves the transmission of sound waves through different parts of the ear as shown in the diagram.

Wave transmission to a new medium can be limited by the wave amplitude or the wave frequency.

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

Frequencies above and below this are called ultrasound for above and infrasound for below.

And the highest frequency you can hear reduces with age as the cells of the cochlea and auditory nerve can become damaged over time.

Very well done for completing this lesson.