Have you ever heard someone say about a particular singer “She’s got a voice that can break glass”? Or that if you scream in a high-pitched voice long enough in front of a mirror, it will crack?
According an explanation by BBC Future, this is actually possible because of a phenomenon known as resonance. When sound hits an object – glass, for example – it excites the particles that make up the glass, causing them to vibrate. Each object will naturally vibrate at its own unique frequency – known as its resonant frequency – and if you choose a sound wave that matches that pitch, the object will start to shake more and more vigorously.
To help you imagine how it works, think of it like the act of pushing someone on a swing, where you give them a little push each time they reach you. If you get the pace right, with very little effort, you can push them to even higher heights. But if you get the pace wrong, your effort will well slow them down.
Similarly, to identify the resonant frequency of the glass, you would want to tap it to get the unique sound it makes. Next, all you have to do is recreate that note by singing, screaming, or using speakers – steadily increasing the frequency all the while. To smash a glass, you will need to blast it with upwards of 100 decibels of sound, roughly equivalent to the sound of an airplane taking off at close range to you.
But you have to choose your glass carefully, because 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 reducing the strength of the vibrations.
Further, the most expensive glasses are the easiest to smash – since they are thinner and made of crystal, which is extremely brittle. Also, much older glasses are also likely to break, since they may contain more microscopic cracks that will cause them to fracture under pressure.
So, it is possible to smash glass using just sound, but from the explanation above, it’s not exactly as some people think.