In this extract from the QI Elves' new book, they reveal how loud a soundwave has to be for it to turn into a shockwave.
When the volcano Krakatoa erupted in 1883, it destroyed an island, threw debris 17 miles into the air, at a speed of half a mile a second, and killed 36,000 people.
The noise it made was so loud that sailors 40 miles away suffered burst eardrums. Even 100 miles away, the volume was 170 decibels – loud enough to do lasting damage to those that heard it. And it could still be heard 3,000 miles away – the equivalent of a sound made in Britain being audible in the US.
At its source, the sound was so loud that it went beyond what we mean by ‘sound’.
The decibel is named after Alexander Graham Bell, but a single 'bel' is so large that scientists tend to divide it by 10, giving us a 'decibel'.
All noises are the result of molecules knocking against each other, in a chain stretching from the source of the sound to your eardrum. When you snap your fingers, the air molecules surrounding them are disturbed. They then bump into their neighbours, and those molecules then pass that ripple of disturbance on to their nearest neighbours, and so on, until the sound hits your eardrum, which passes the message on to your brain.
So while each molecule travels only a tiny distance, there’s a wave of energy that makes it all the way from the source of the sound to your ear. The wave consists of high-pressure areas with lots of molecules, followed by low-pressure areas that are relatively sparse.
Once you get to a certain level (194 decibels, to be precise), there comes a point where the low-pressure regions are completely empty – there are no molecules in there at all. The sound can’t get ‘louder’ than that, technically. If there is more energy in the noise source, the air molecules are just pushed along wholesale, rather than moving back and forth, and the soundwave has turned into a shockwave.
The shockwave from Krakatoa was so strong it circled the Earth four times.
The ‘QI Elves’ are the team of researchers and writers behind BBC Two’s smash hit comedy panel show QI. Their latest book is Funny You Should Ask (£12.99, Faber) and is out now.