Part 1: Ding Dong Merrily

Fact: You can ‘unring’ a bell.

You thought unboiling an egg was impossible? You haven’t seen anything yet. The famous phrase ‘you can’t unring a bell’ is meant to highlight the consequences of our actions to make us think more carefully about what we do before we do it. But it might be a bit less appropriate now.

In this story, scientists rang an imposing monastery bell in Myanmar. The crowds all around heard a resounding ringing, loud enough to wake the dead – but not, apparently, loud enough to wake the sleeping baby lying only metres away from the bell itself.

How could this be possible? Well, science. Ever heard of ‘every action has an equal and opposite reaction’? That’s Newton’s Third Law of motion, and it’s quite relevant here. The scientists analysed and ‘inverted’ the sound wave produced by the bell: in other words they created the ‘opposite’ of the sound that the bell produced.

By playing this inverted sound at the same time as the bell was rung, these two sound waves (which are ‘equal and opposite’) met each other in the air and cancelled each other out, in a process known as destructive interference. It’s pretty much “-1 + 1 = 0”, but with sound waves, and it looks a bit like this:

Destructive Interference: from‘You Can’t Unring A Bell’ – GE Electric

This creates a ‘quiet zone’ where the waves cancel out, letting the baby get some well-earned rest after a long night of keeping the rents up.

Destructive interference is more than just cool, it’s useful – it’s the same technology used in noise-cancelling earphones. So next time you listen to some tunes on the tube imagine the sound waves around you battling it out to keep your personal space quiet. Although that might make things a little less relaxing.

This series of posts is based on the General Electric ‘Unimpossible’ video series currently on Youtube – check it out, its awesome:

Another one of Mr Shaunak’s Little Bites of Science.
Image under creative commons license.


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