I mean, I understand that it’s difficult and has to be a very certain key at a specific volume, but I still don’t understand how that alone is capable of breaking a solid object. And why just glass? What about glass? Surely we can break other stuff too? I’m confused.
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A human voice can cause vibrations at a frequency that can create resonance in glass. The resonance magnifies the vibration until the glass breaks. This is usually done with very thin wine glasses for show. Humans can’t easily generate the frequencies to create the same kind of resonance in other materials with their voices.
TLDR: Sound is vibrations. Vibrations move things back and forth. Glass is already thin and fragile, so if you can scream the right note at it with enough power it will vibrate itself apart.
So, every object has what is called a resonate frequency. It’s the frequency of sound where the given thing shakes the hardest in response to the sound wave. If you play a note of that frequency the thing in question will attempt to vibrate in response. This is how f.i tuning forks work: strike the fork and they will vibrate at their resonate frequency, which is the note that we want to use to tune our instruments.
For glasses, you can hear the resonate frequency the same way: tap a glass and it produces a tone.
The thing about wine classes is that they tend to
* Have a resonate frequency around the higher parts of what trained humans can produce
* Are rigid, and so don’t damp vibrations very well
* Have thin, brittle walls that are easy to break.
If a trained opera singer sings the right note the sound wave will start to shake the glass. The glass will start resonating, and amplify the vibration. If the singer sings loud enough and long enough the glass will just keep absorbing that energy, vibrating faster, and faster, and with more and more power, until eventually the brittle glass gives and shatters.
You can do this with things that aren’t glass, but not a lot of things are as fragile as wine glasses are with the right resonating frequency.
It’s a two part scenario.
The first part is something called “resonance” and if you’ve ever been on a swing set you can understand it. A wave is like a person going back and forth on a swing, if you add energy to wave in the just the right way (pushing someone on their way down) you can increase the energy (make them swing higher). Compare this to pushing someone when they are swinging *up*. You’ll reduce their height. You need to apply the energy input in just the right way. In a resonant system the energy inputs are all coming together in just the right way that the wave gets bigger and bigger and bigger. It’s sort of like not just any shape can be come a saxophone. The form of a saxophone is designing in just the right way that a small input (someone buzzing on a tiny reed) becomes a massive output, a loud saxophone you can hear across the orchestra pit.
In our example, the human voice is the small buzzing reed, the glass cavity is the saxophone and the the human voice hitting “the right pitch” is like fingering a note on the saxophone keys. The three qualities all come together in just the right way to create acoustic resonance inside the glass cavity.
The acoustic energy hits the glass and makes the glass start to vibrate back and forth. With a proper resonance the pushing and pulling all lines up in a way that makes the vibration significant. When I say “vibration” what I’m saying is if you imagine a slice of a ring of the glass, that ring is getting wider and narrower back and forth. That widening is literally similar to taking your fingers on the inside of the glass and pulling them apart, creating tension in the glass material. Glass is extremely weak to tension forces and will shatter violently.
So to answer your final question, that’s why glass specifically shatters like this, glass is extremely weak to tension. Other materials like wood or metal are pretty strong in tension and won’t fail due to such small forces.
Think of swinging on a swing set. You pump your legs back and forth which generates a little bit of forward and back movement, but not more than a few inches. However, pumping your legs in time with the back and forth motion of the swing allows you to add up that little bit of motion your legs create. Over time you can get the swing moving a massive amount. The same thing is happening with the glass. The glass wants to resonate at a certain frequency, just like the swing wants to swing back and forth at a certain tempo. Singing a note into the glass at just the right frequency is like pumping your legs with the right timing. The little “push” the sound waves give the glass line up just right with the way it wants to resonate, and the vibration of the glass increases until it breaks itself apart. To compare this back to the swing example, at a certain point you’ve added so much movement into the swing set that the chain starts to go slack at the top of each swing, and pushing it further throws it into chaos.
Every object has a frequency at which it swings if stimulated once, like if you tap on a glass, you will hear a ringing of a certain frequency. If you produce a sound of the same frequency, the object will not only preserve its own swinging, but the sound waves will perfectly stimulate it each cycle, making the swinging grow bigger and bigger. A swing is a good comparison. Each time the swing swings back, you give it another push and it swings higher each time.
If the swinging keeps growing, it will eventually reach a point where the object is bent so much with a swing, that it breaks apart.
Glass is special in that it is rather inflexible compared to many other materials and simultaneously it is rather hard. Wine glasses (with which this is usually done) also have rather thin walls. This combination of properties leads to the frequency of the glass being at a pitch that a human voice can reach and to the glass breaking when it swings out beyond a certain point.
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