How do we hear sounds?

A wobble in the air, caught by the outer ear

Every sound starts with something that wobbles, or vibrates. The wobble pushes on the air and sends out sound waves. Sound waves are vibrations that travel through the air. Your ears catch these waves and change them into signals that your brain can understand. Each ear has three parts: the outer ear, the middle ear and the inner ear.

The outer ear is the part you can see, which is called the pinna. Its shape helps to collect sound waves. The waves travel down a narrow passage called the ear canal, which leads to the eardrum.

The skin that lines the ear canal makes wax. Ear wax is a helper. It stops the skin from drying out, helps to fight infection, and carries dead skin out of the ear. Healthy ears clean themselves, like a slow conveyor belt. So never poke anything into your ears, not even a cotton bud or a finger. It can push the wax in further and hurt your ear.

The middle ear: a drum and three tiny bones

At the end of the ear canal is the eardrum, a thin, tight sheet a bit like the skin of a drum. When sound waves hit it, it vibrates.

Behind the eardrum is the middle ear, a small space filled with air. It holds three tiny bones with funny names: the malleus, the incus and the stapes. They are also called the hammer, the anvil and the stirrup, because of their shapes. The eardrum shakes the hammer, the hammer shakes the anvil, and the anvil shakes the stirrup. The three bones make the vibrations stronger. The stirrup is the smallest bone in your whole body.

The middle ear is also joined to the back of your nose by a narrow passage called the Eustachian tube. It works like a pressure valve. When you swallow or yawn, it opens, and it keeps the air pressure the same on both sides of your eardrum. That is why your ears can pop when a plane takes off or lands.

The inner ear: from a wobble to a signal

The stirrup passes the vibrations on to the cochlea. The cochlea is a tube filled with fluid, coiled up like a snail shell. The vibrations make the fluid inside it ripple, and a wave runs along a stretchy sheet inside the tube.

Sitting on the sheet are thousands of tiny hair cells. They are not like the hairs on your head. Each one has tiny hair-like tips. As the wave passes, the tips bend, and the hair cell sets off a tiny electrical signal. Different hair cells work on different pitches. The ones near the wide start of the snail pick up high sounds, such as a baby crying. The ones near the centre pick up low sounds, such as a big dog barking.

The signals travel along the auditory nerve to your brain. The brain turns them into sounds you know, such as a friend's voice or footsteps. Your inner ear has another job too. It helps you to keep your balance, using three fluid-filled loops that sense when your head turns.

Why do you have two ears? A sound from one side reaches the nearer ear a tiny bit sooner, and it sounds a little louder in that ear. Your brain notices these tiny differences and uses them to work out where the sound came from. It is harder to tell if a sound is straight in front of you or straight behind you. It reaches both ears in almost the same way.

Looking after your hearing

Loud noise is the biggest single cause of hearing loss in Australia, says the National Acoustic Laboratories. Loud sounds can harm the inner ear even when they do not hurt. The damage can build up over time, a bit like the sun slowly harming your skin.

Sound is measured in units called decibels. Sounds below 75 decibels are unlikely to harm your hearing, even after a long time. But hearing can be harmed if you hear sounds of 85 decibels or more again and again, or for a long time. That is about as loud as a vacuum cleaner. Music through earphones at full volume is about 106 decibels. Australia's Department of Health says it is safe to listen to that for less than 4 minutes.

Hearing that is damaged often cannot be fixed, so it pays to be careful. Turn the volume down. You should be able to understand someone who is talking to you from one metre away. Take listening breaks. Wear earplugs or earmuffs at loud events, and when a grown-up is using noisy tools. If you need to shout to be heard, the noise is probably too loud. If your ears hurt or ring, or you find it hard to hear, tell a grown-up. They can take you to a doctor or a hearing clinic for a check.

An Australian breakthrough: the bionic ear

Sometimes part of the inner ear does not work properly, and sound signals do not reach the brain. A cochlear implant can help some people with severe hearing loss. The multi-channel version, called the bionic ear, was developed in Australia. A sound processor with small microphones sits behind the ear. It picks up sounds and passes them to an implant that a surgeon fits in the inner ear. The implant goes around the damaged part and triggers the hearing nerve. The nerve then sends the signals to the brain.

Professor Graeme Clark, who was born in Camden in New South Wales, began the work because his father had severe hearing loss. He led the research at the University of Melbourne. In 1974, television producers who were helping to raise money for the research asked for a catchier name, and the device became known as the bionic ear. On 1 August 1978, a man named Rod Saunders became the first person to receive Professor Clark's multi-channel implant. It was fitted at the Royal Victorian Eye and Ear Hospital in Melbourne. He had lost his hearing in an accident about 18 months earlier. After the implant, he could hear sounds again, and he could understand speech.

📚 Words to know

Eardrum

A thin, tight sheet at the end of the ear canal that vibrates when sound waves hit it.

Cochlea

A coiled, fluid-filled tube in the inner ear, shaped like a snail shell, where vibrations are turned into signals.

Hair cells

Thousands of tiny cells in the cochlea. Their hair-like tips bend with the vibrations and set off electrical signals.

Auditory nerve

The nerve that carries signals from the inner ear to the brain, so the brain can turn them into sounds.

Decibel

A unit for measuring how loud a sound is. Sounds of 85 decibels or more can harm your hearing if you hear them often or for a long time.

🧪 Try this: build an eardrum

1. You will need a large bowl, a piece of plastic wrap big enough to cover the top, a rubber band, 20 to 30 grains of uncooked rice, a baking tray and a wooden spoon.
2. Stretch the plastic wrap tightly over the top of the bowl and hold it in place with the rubber band. This wrap is your model eardrum. Sprinkle the grains of rice on top of the wrap.
3. Hold the baking tray close to the bowl, but well away from anyone's ears, because it will be loud. Tap the tray with the spoon. Watch the rice jump! Sound waves make the real eardrum vibrate in a similar way.
4. Try other noisemakers, such as a bell, a tambourine or a small drum. Which one makes the rice jump the most? Then think: what do you think would happen if there was a hole in the plastic wrap?
5. Challenge: test two ears against one. Sit on a chair in a clear space, with your eyes shut, in the middle of a circle of friends. A friend jingles some keys softly from different spots, and you point to where the sound is coming from. Then cover one ear with your hand and try again. Which way is easier? Only use your hand to cover your ear. Never push anything into your ears.