Pluck a guitar string and it sings. Bang a drum and it booms. Speak, and words tumble out of your mouth. A bee zips past and buzzes. These all seem so different from one another — but every single one of them plays the very same hidden trick.
Whenever there is a sound, something somewhere is vibrating: wobbling to-and-fro, backwards and forwards, faster than your eye can follow. That is the one big secret of this whole page — every sound is made by something shaking.
Once you know the secret, you start to spot the wobble everywhere. Pluck a guitar string and it turns into a blur — that blur is the string shivering from side to side, far too fast to see clearly. Tap a drum and the skin jiggles up and down. Twang a ruler on the edge of a desk and you watch the free end flap like a bird's wing.
The same thing is happening in places you can't see. Deep in your throat is a little voice box that flutters when you talk or sing. A speaker cone in a radio or a phone pushes in and out, in and out, hundreds of times every second. All of them are doing exactly one job: shaking to-and-fro to make a sound.
You don't have to take anyone's word for this — you can feel the shaking. Rest two fingers gently on the front of your throat and hum a long, low mmmmmmm. Feel that tickly buzz under your fingers? That is your voice box vibrating. Stop humming and the buzz vanishes at once.
Try it on other things too. Lay a hand flat on a speaker while music plays and you'll feel it trembling. Touch a big bell just after it has been struck and your fingers will tingle. Every time, the buzzing you feel is the object shaking — the very shaking that is making the sound.
Here is a lovely way to see a wobble that is normally invisible. Stretch a bit of cling film tightly over a bowl to make a little drum skin, and scatter a few grains of rice or sugar on top. Now hold a metal pan close by and give it a good bang with a spoon.
The rice grains suddenly leap and dance about, hopping up and down as if the bowl were alive! Nobody touched them — the banging pan set the air shaking, the air shook the film, and the trembling film flung the rice into the air. You are watching sound made visible.
Here is a guitar string stretched between two pegs. Choose Pluck and drag the slider for how hard you pull it — the harder you pluck, the wider the string swings, and the louder its sound. Now switch to Mute, just like pressing your finger flat on a real string: the wobble stops dead, and the sound stops with it.
No wobble, no sound. That little rule is the heart of everything on this page — so it is worth pinning down carefully.
Not everything shakes at the same speed. A big, floppy thing wobbles slowly; a small, tight thing wobbles quickly. Twang a ruler with lots of it hanging off the desk and it flaps slowly with a low bwooong. Slide it back so only a short bit sticks out, twang again, and it buzzes fast with a much higher note. Same ruler — a faster wobble makes a higher sound.
The wobbles are almost always far too fast to count. Your voice box can flutter more than a hundred times in a single second, and a mosquito's wings beat so fast that all you hear is an annoying high eeeeee. Fast or slow, big or small, the rule never breaks: if you can hear it, something is shaking.
A tuning fork is a little metal fork that musicians tap to sound one clear note. When you strike it and hold it up, it hums away — but its two prongs are wobbling so fast that they just look like a still, silvery blur. So how can we be sure they are really moving?
Dip the humming prongs into a cup of water. The water leaps and splutters, flicking up a little spray of droplets! The blur really was shaking to-and-fro all along — the water simply gave the invisible wobble a way to show itself.