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Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Tuesday, 6 March 2012

A Voice That Could Break Glass

My sister. No, she doesn’t have a voice that could break glass. But she does have an inquiring mind combined with an idea that Physics is probably just a conspiracy. I’ve no idea why she suddenly decided to text me early on Sunday morning asking if you could break glass with sound, but she did. She was so suspicious of my texted reply that I needed to elaborate - so here is my answer.

Let’s start with sound waves. We know that of the two types of waves, transverse and longitudinal, that sound waves are longitudinal. That means that the sound source vibrates backwards and forwards and moves the air molecules accordingly.


Under normal circumstances though, these sound vibrations can’t really move much apart from your ear drum, this being how you hear the sounds. There are a number of things you can measure that tells you more about the waves – the amplitude, the wavelength and the most important one here – the frequency. The frequency means how many waves per second there are and this is measured in Hertz. The higher the frequency, the higher the pitch of the noise that you hear.

So, we know that for most frequencies, sound cannot break glass. We know this from our everyday lives – the glass we drink out of, the glass in the windows, they don’t just spontaneously shatter on a regular basis, not even when someone really annoying talks right by them.

Ok, a small aside. You know how when you’re swinging on a swing, if you get your legs helping you just right then you’ll keep getting higher and higher, but if you get your legs wrong it’s impossible? That’s all about working with the frequency that the swing naturally wants to swing at and not trying to make it go faster or slower.

The same is true with sound waves and breaking glass. Once you get the right frequency, what starts off as teeny tiny vibrations of the air next to the glass and then the glass itself, if you can get the waves pushing at just the right time, the glass will start to vibrate, then ripple more and more and more. This is called the resonating frequency.

Obviously, glass is a brittle material, but when you get it at its resonating frequency it almost looks like it’s acting like water. Well, that’s until it smashes! After all, it’s still glass.

Fancy checking this out? Search youtube for "resonating sound breaking glass" and there are some great examples!

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