How Do Opera Singers Break Glass?

Maybe you’ve seen it in person at a magic show or something, but it’s more likely you’ve seen something like this on TV or in a movie. Then you were just kind of told that this was something that could happen in real life and then you didn’t question it. But now we’ve implanted the question into your mind, who do opera singers break glass?

Further Reading: Why Do Wine Glasses Look Like That?

It Just Has to Resonate with You

Well now it’s time to pull out physics things you’ve probably never thought of in your entire life. 

Anyway, now we’re going to talk waves. Specifically, resonance. Even more specifically, we’re concerned with acoustic resonance. Resonance is broadly what happens when things vibrate. If you’ve ever seen those sinusoidal wave curves (squiggly lines!), you’re thinking in the right direction. 

These waves have their peaks and valleys, where each valley is exactly the same as each peak—just in the opposite direction. Amplitude is a measure of how high each peak is, while frequency is how quickly the wave goes from peak, to valley, back to peak again, etc. per unit of time (like seconds). When two waves with matching amplitudes and frequencies, but are moving in opposite directions, you get a standing wave. This is a wave that can oscillate between having peaks/valleys twice as large as one of the two waves (just think of them being stacked on top of each other), and being effectively a straight line (the peaks and valleys cancel out). 

Long short, when a standing wave is created you get a wave that is either the two waves combined together, or just one canceled-out wave. The most common places you’ll see this is with acoustics, like a tuning fork

These standing waves occur at what is known as a resonant frequency. It finally came all the way back around. 

So, Breaking Glass

The important thing about resonance going into the shattering of glass is the vibration. If you know a bit about how the world works, you know everything is actually vibrating all the time. The vibration pattern an object has without any kind of outside influence is called its natural frequency. This natural frequency can be exploited with sound.

Sound is just the air vibrating in a specific way, it’s why you can feel a song thumping in your chest at a concert. This means that sound can also make other objects, like glass, vibrate. If the glass vibrates just right (at the glass’ resonant frequency), it can shatter. 

But for this to happen, a lot of things have to go right. For starters, to shatter a wine glass (which is pretty thin), you’re going to need to hit above 100 decibels. This might be easy with a speaker, but it’s not so easy for humans—normal speech is at around 50 decibels. Wine glasses are exceptionally thin, and that thinness makes them comparatively easier to shatter than, say, a beer mug. 

The shattering of a wine glass also relies on the imperfections of the glass. Because the human voice is kind of at the edge of what can shatter the glass, you also have to have just the right, minute imperfections in the glass’ manufacturing to get a shatter. It’s part of why when MythBusters tested this in 2005 not every glass broke consistently. 

See if you know your opera composers here.