Sunda Strait · 27 August 1883 · 10:02 in the morning
You are standing on the deck of a British ship forty miles from an island called Krakatoa. In about three minutes the sound will reach you. In four hours it will reach the middle of the Indian Ocean. Over the next five days its pressure wave will sweep over Scotland seven times.
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Act One · You are on the deck
Light is fast. Sound is slow. So the first thing that happens is that the northern sky turns white — and then nothing happens at all, for more than three minutes, while sixty-four kilometres of air quietly gets on with the business of carrying the news at 340 metres a second.
Your ship is the Norham Castle. You are 40 miles from the island. The captain has already written in the log that his crew are in trouble, and he writes it in a hand that is not steady.
“So violent are the explosions that the ear-drums of over half my crew have been shattered. My last thoughts are with my dear wife. I am convinced that the Day of Judgement has come.”— the captain of the Norham Castle, ship’s log, 27 August 1883, about 64 km from Krakatoa (as quoted in later histories of the eruption)
He was wrong about the Day of Judgement and right about the eardrums. What arrived at his ship was not really a sound at all in the ordinary sense. It was a wall of squeezed air travelling at about the speed of an ordinary sound, but with so much squeeze in it that the air itself briefly stopped behaving like something you can hear and started behaving like something that hits you.
Here is the thing nobody tells you about sound: it is not a substance. Nothing travels from the volcano to your ear except a rumour. Each little parcel of air shoves the parcel in front of it, gets shoved back, and returns almost exactly to where it started. The air does not move to you. The squeeze moves to you. Sound is a shape passing through a crowd.
Instrument One · The pressure bench
In Batavia — modern Jakarta, 158 km away — the gasworks kept a barograph: a pen resting on a slowly turning paper drum, drawing the weight of the sky. At about a quarter past ten it jumped by more than two and a half inches of mercury. That is roughly 8,500 pascals of extra air pressure, and it is the single most useful number in this whole story, because from it you can work out — with arithmetic a ten-year-old can do — exactly how loud Krakatoa was. Drag the slider and watch the air get squeezed.
A sound wave you can only just hear squeezes the air by twenty millionths of a pascal. Every decibel number on Earth is measured against that one whisper.
Four hundred million times bigger. Written as a decibel: 20 × log₁₀(8500 ÷ 0.00002) = 172.5 dB, at 158 km away.
Normal air pressure. A sound can’t pull harder than empty, so around 194 dB the wave stops being a sound and becomes a shock wave — a moving wall.
Act Two · You are the wave now
Leave the ship. You are the pressure pulse now, a ring of squeezed air widening across the Indian Ocean at about 340 metres every second. The map below is drawn in a strange and useful way: Krakatoa is at the exact centre, and every millimetre outward is the same number of kilometres in every direction. So you — the wave — are a perfect expanding circle. The rim of the disc is the far side of the world.
Keep watching after the boom fades. On this map the far side of the world is the rim, so a wave that goes all the way round looks like a ring that swells to the edge, folds back, shrinks onto Krakatoa — and swells again. Every time it sweeps over Glasgow, a pen in Scotland twitches.
Tap any marker to read what people there actually reported. Most of them thought they were hearing a ship in distress, and sent boats out to look.
Act Three · You are the pen
The audible boom died out somewhere past Rodrigues. But underneath the boom was a second, slower thing — a long, low swell of pressure with a period of one to three hours. Far too slow to hear. Nothing to feel. And utterly unstoppable. It rolled outward across the whole atmosphere, converged on the far side of the Earth near Colombia, spread out again, came home, and set off again — and everywhere it passed, barograph pens twitched.
More than fifty weather stations recorded it. At Glasgow the pen twitched seven separate times over five days. Below is a working model: pick a station and the drum computes, from nothing but that city’s distance and a wave speed of 313 m/s, when each passage should arrive. Then compare it with what Glasgow’s pen actually drew in 1883.
The job · Station Keeper
The ladder
Decibels are a ladder where every 10 rungs means ten times the sound energy. Krakatoa at Batavia sits 32 rungs above a jet engine you are standing next to.
Everyday values are approximate and standard; the Krakatoa figure is calculated from the Batavia barograph reading of ~8,500 Pa at 158 km.
Instrument Four · Below hearing
Your ears are a narrow window. Above about 20,000 wobbles per second you hear nothing; below about 20 wobbles per second you hear nothing either — the wobble is still there, still real, still carrying energy, but your eardrum has stopped reporting it. Slide down through the window and find the edge of your own hearing.
The record
Six things that are true
The big idea
The examination
Do this at home
You need: a big flat wall (a school, a barn, a cliff), a friend, two blocks of wood or two saucepan lids, and a phone stopwatch. That is all Isaac Newton had, roughly, and he got it wrong by 15%.
Bonus round. Do it again on a hot afternoon and again on a freezing morning. Sound moves faster in warm air — about 0.6 m/s faster for every extra degree. You are now measuring the temperature of the atmosphere with two saucepan lids.
Right now, everywhere on Earth, waves too slow for you to hear are washing through the air around you — from thunderstorms, from ocean swell, from meteors, from rockets. Your ears filter them out. Somewhere, a machine is writing them down. What else is happening in the room you're in that nobody has built the right instrument for yet?