Can you Really Break Glass with Sound?

You’re probably familiar with the urban legend: the opera singer ascends the stage and clears his throat. His audience cheer and wave their champagne flutes in anticipation.

He opens his mouth – and a roomful of glasses smash to pieces. We have no record that this has ever actually happened, but there were rumours that the legendary tenor Enrico Caruso could quiver a glass into a million pieces.

And according to physics, it should be possible. It all comes down to a phenomenon known as resonance. When sound hits an object – such as a champagne flute – it excites the particles inside, causing them to vibrate. Each object will naturally vibrate at a particular frequency – known as its resonant frequency, and if you choose a soundwave that matches that pitch, the object will start to shake more and more vigorously.

Think of it like the act of pushing someone on a swing in the park, where you give them a little shove each time they reach you. Get the pace right and you’ll have them to screaming to slow down with very little effort. Get it wrong and your efforts will actually slow them down.

As the above clip from Dara O Briain's Science Club shows, you first need to identify the resonant frequency of the glass – the sound it makes when you tap it. Next all you have to do is recreate that note, by singing, screaming – or, as O Briain did, using speakers. 

To smash a glass, you’ll need to blast it with upwards of 100 decibels of sound, which is roughly equivalent to a lawn mower.

Alas, it won’t work with tumblers. Wine glasses and champagne flutes are especially resonant because of their hollow shape and narrow stems, which allow them to be held without dampening down the vibrations.

And as regular dinner party hosts will know, the most expensive glasses are the easiest to smash – since they are thinner and made of crystal, which is extremely brittle. Antiques are also likely to break, since they may contain more microscopic cracks to fracture under pressure. So you’d better be careful playing music around your grandparent’s finest glassware.

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