You’ve probably seen it in a movie or on TV: an opera singer strikes a note, holds her vibrato … and crack, there goes the wine glass. But is that really possible? Can you crack a glass with just your voice? Or is it a nice piece of Hollywood magic?
In this blog, we dive into the world of vibration, resonance and glass. Not heavy physics, but a fascinating exploration of the power of sound. And yes, you will also find out if you can break glass with your own voice.
What happens when a glass cracks?
Let’s start with the basics. When a glass is cracked by sound, it doesn’t just happen by singing “loudly.” There is a physical principle behind it called resonance.
Every object has its own frequency at which it vibrates when sound strikes it. This applies to guitar strings, bridges, buildings … and wine glasses. If you sing in exactly the right tone – the same frequency at which the glass naturally vibrates – the glass can vibrate along with it.
If that vibration gets intense enough, the material won’t hold. And then … snap. Glass shattered.
What exactly is resonance?
Resonance is the physical phenomenon in which an object starts vibrating along with it when exposed to an external vibration of exactly the same frequency. Compare it to someone sitting on a swing. If you give a push at exactly the right moment, the swing goes higher and higher. If you give pushes at the wrong times, nothing happens.
It works the same way with a wine glass. The glass chalice has a certain frequency at which it prefers to vibrate. That frequency is determined by the shape, thickness of the glass, and type of material.
If you hit that frequency exactly with your voice AND you hold it steady enough, then the glass can vibrate. And with enough energy: cracking.
How do you know what tone to hit?
Most wine glasses have a resonant frequency between 500 and 1000 Hertz. That corresponds to a tone somewhere between a low A and a high C. But it varies from glass to glass.
The only way to find out what the frequency of a glass is is to test it. Gently tap the glass with a pencil or spoon and listen to the sound it makes. That ping sound is the tone at which the glass resonates.
With a voice (or musical instrument), you can try to exactly mimic that tone. There are apps that can analyze and compare tones, such as Spectroid, Tunable or Audio Spectrum Analyzer.
Can anyone do that?
Fair is fair: no, not everyone can crack a glass with their voice. Here are a few reasons why it’s pretty tough:
- You need a very pure voice that stays exactly in tune
- You have to produce enough volume and energy to really get the glass moving
- The glass must be fragile enough (e.g., thin and made of crystal glass)
- You often need amplification, such as a microphone and speaker, to fire enough sound waves at the glass
The famous videos where people break a glass with their voice? There’s usually a microphone + speaker + clever setup behind that. So if you try it at home with your bare voice and you fail: it’s really not (only) because of you.
Myth or truth?
So it is not a myth. It is possible. But it’s tricky. American physicist and TV personality Jamie Vendera showed on the program MythBusters that it is actually possible to break a glass using only your voice – provided all the conditions are right.
But it is so difficult that in practice it almost never succeeds without the help of reinforcement or technical tricks.
What kind of glass do you need?
Want to give it a try anyway? Then choose a wine glass that meets these requirements:
- Thin glass (the thicker the glass, the harder it is to break)
- Crystal glass or fragile hand-blown glass
- No stems or ridges to dampen vibration
- Place the glass on a solid surface, preferably in a quiet room
Also, make sure there is no liquid in it – that changes the resonant frequency.
How many decibels do you need?
Experiments show that you need around 100 to 110 dB to generate enough vibration to break glass. That’s comparable to the volume of a chainsaw or rock concert. Not something you can just get out of your throat without damage to your own ears (or those of your neighbors).
That’s why professionals often use a speaker aimed at the glass, so you can still deliver enough power with less effort. The voice is then first recorded, amplified, and then played back through the speaker at exactly the right frequency.
Fun fact: opera and glass
Opera singers are more likely to do this trick. Why? They are trained to sing extremely pure and powerful in tone, often in exactly the pitches in which glass resonates. In the process, they often sing with a lot of vibrato, which can also help get the glass moving. But they also use a speaker in most cases.
So, can you do it?
Want to try it yourself? Then make sure to at least:
- A glass with a clear, clean tone
- A device or app to check your pitch
- Possibly a speaker to amplify your voice
- Lots of patience … and a broom for when it succeeds
But more importantly, never try this without precautions. A breaking glass can cause sharp shards. Wear eye protection and do it in a controlled setting. And be kind to your vocal cords – don’t force it anywhere.
And what does this say about hearing?
What this whole exercise also nicely demonstrates is how powerful sound actually is. We often underestimate how much influence sound waves can have. Not only on glass, but also on our own bodies – and especially our hearing.
You can replace a glass. Your ears can’t. So should you try this experiment at home and set a speaker to 110 dB: wear hearing protection please.
Or, even better: watch a movie in which it works. Then your vocal cords and your glasses will stay in one piece.
