Everything that sounds is shaking

Put two fingers lightly on the front of your throat and hum a long, low mmmmmmm.

Feel that? Your throat is buzzing. Stop humming and the buzz stops at exactly the same instant as the sound. Hum higher and the buzz gets faster and finer. Hum louder and the buzz gets stronger.

That is the whole secret of this topic, and you just found it with your own fingers: every sound in the world is made by something shaking. A drum skin, a guitar string, a car engine, a mosquito's wings, a slamming door, your own voice box. Nothing shakes, nothing sounds.

In this lesson you will meet the shaking, follow it through the air to your ear, find out why space is completely silent, and learn why a thin string sounds high and a big drum sounds low.

Walk around your home and hunt for a sound you can see being made.

A tabla is easy: bang it and the tight skin dips and springs back, over and over, so fast it blurs. Sprinkle a few grains of rice on the skin first and they will jump.

A rubber band stretched between your thumbs is easier still. Pluck it and you see a fuzzy band instead of a sharp line, because the band is in two places at once as far as your eyes can tell. Press it with a finger and both the blur and the sound stop together.

A steel tumbler tapped with a spoon does not look like it is moving at all. But hold it gently against your lip after tapping and you feel the tingle. Dip the ringing rim into a bowl of water and the water will shiver and splash outwards. The tumbler is shaking too; just by a tiny amount, very fast.

Your ear is nowhere near the drum. So how does the news get across the room?

Think about what a drum skin does to the air touching it. When the skin bulges outwards, it shoves the air right next to it. That air gets squashed and crowded. When the skin snaps back, it leaves a little extra room, and the air there becomes thinner and stretched out.

Squash, stretch, squash, stretch. Each squashed patch shoves the next patch of air, which shoves the next, and so on across the room. What travels is not the air itself, but the pattern of squashed and stretched air, racing outwards in every direction.

When that pattern of pushes reaches your ear, it pushes on your eardrum, and the whole story starts again in reverse.

A mosquito whines. A lorry rumbles. Both are sounds, but they sit at opposite ends of something musicians call pitch.

Pitch is decided by one thing only: how many times the source vibrates each second. We count those in hertz, written Hz. One hertz means one complete back-and-forth every second.

• A mosquito's wings beat roughly 600 times a second: about 600 Hz, a thin whine.
• A big drum's skin might flick about 60 times a second: 60 Hz, a deep boom.

More shakes per second means a higher pitch. Fewer shakes means a lower pitch. Nothing else in the sound changes: the mosquito is not louder than the lorry, just higher.

Now hit the same tabla twice: once gently, once hard. The pitch does not change; it is the same drum, the same tension, the same skin. What changes is loudness.

Hit it harder and the skin swings further from its resting place on each flick. A bigger swing shoves more air, squashes it more strongly, and delivers a stronger push to your eardrum.

The size of that swing has a name: amplitude. Big amplitude means loud. Small amplitude means soft. It is completely separate from how often the skin shakes.

So every sound carries two independent pieces of news: how fast it shakes (pitch) and how far it shakes (loudness).

Sound in air travels 343 metres every second. That is fast: about 1,235 kilometres per hour, faster than most aeroplanes.

But light travels 299,792,458 metres every second, which is about 874,000 times faster. For any distance you can see across, light arrives effectively instantly, while sound plods along behind.

You already know this without being told. At a cricket match far from the boundary, you see the bat hit the ball and the thock arrives a moment later. On Diwali, the flash of a cracker comes first and the bang follows. During a storm, lightning always beats thunder.

And that gap is not a nuisance. It is a free measuring tape.

Shout into a well, or clap in an empty hall, or call out across a valley, and your own voice comes back to you a moment later. That returning sound is an echo.

Sound bounces. When the travelling pattern of pushes meets a hard, flat surface such as a wall, a cliff or the water at the bottom of a well, most of it turns around and comes back, exactly as a ball bounces off a wall.

Soft things behave differently. A curtain, a mattress, a heap of clothes or a crowd of people soak the sound up instead of bouncing it. That is why a bare empty room echoes and the same room full of furniture and people does not.

The visible part of your ear is only the funnel. The real machinery is inside your skull, and it is astonishing: it can detect a movement of your eardrum smaller than the width of an atom, and it does it in both ears at once so that you know which direction the sound came from.

Follow the path in. Sound is collected by the outer flap, travels down a short tube, and pushes on a tight little skin. Three tiny bones carry that push across an air-filled gap. The last bone presses on a coiled tube full of liquid, and inside that coil, thousands of microscopic hairs sway. Each sway becomes a nerve signal, and the nerve carries it to your brain.

The whole journey takes a few thousandths of a second.

What makes the sound of a bansuri pleasant and the sound of a scooter horn unpleasant? Both are vibrations in air, both reach the ear the same way.

The difference is pattern. A musical sound repeats: the same shape of vibration, over and over, at a steady rate. Your ear finds a clear pitch in it, and your brain enjoys the order.

A noise does not repeat. It is a jumble of many frequencies arriving in no particular arrangement, so there is no pitch to latch on to. A door slamming, gravel being poured, a horn blaring in traffic.

That is not the whole story, because a drum is musical and a tabla stroke is a wonderful sound with a definite pitch. But it is the right place to start: music is ordered vibration, noise is disordered vibration.

India is a loud country, and the noise is not harmless. Constant traffic, horns, construction, loudspeakers and generators raise blood pressure, break sleep, make it harder for children to learn, and, at the top end, damage hearing permanently.

The law recognises this. India's noise rules set limits for different zones: quietest near hospitals and schools, stricter at night than in the day, with firecrackers limited to 125 decibels measured 4 metres away.

You cannot fix a city on your own. But you can do four things today: do not use a horn unless it prevents an accident, stand well back from crackers and speakers, keep your headphones at about half volume, and give your ears quiet time after a loud event.