# What is an object’s natural resonant frequency and why does singing/playing it make the object break?

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I’ve heard this term a lot as a musician. I’m aware that by finding an objects resonant frequency and playing that frequency it will break or smash, for example when singers break wine glasses with their voice, or the whole Janet Jackson’s Rhythm Nation thing. What is an object’s natural resonant frequency, how is it determined, and what happens to an object when you play it that makes things smash or break? Thanks!!

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The resonant frequency is the frequency an object will tend to vibrate at after being elastically deformed. Think of it like a swing, you push on one side and the swing will first swing to that side until it returns and then swings back and forth at a certain frequency. If you push against the moving swing while its moving towards you, you’ll brake the swing and remove energy from the system. If you push while the swing is moving away from you, you waste energy chasing the movement of the swing. But if you time your pushing so that you always push when the swing is at the turning point and momentarily not moving, you’ll be putting energy into the system most efficiently, making the swing go higher and higher. It works the same with a glass. You sing and the air pressure pushes against the glass, causing it to vibrate. If the frequency of the note you’re singing matches the frequency the glass is vibrating at, the peaks in air pressure will always push against the vibrating glass at the right time and put more and more energy into it, making the amplitude of the vibration larger and possibly causing it to eventually shatter.

The resonance frequency is the frequency that an object will resonate at based on its own structure, what material it’s made out of, how much stress it’s under, etc.

If we take a glass and tap it, it makes a ringing noise. That ringing is the same frequency as the resonance frequency.

If we take the glass ans tap is repeatedly faster than the resonance frequency, the natural resonance will be fighting us most of the time we add energy by tapping it because it will be vibrating the opposite way we are hitting it.

The same goes for if we hit it repeatedly below its resonance, most of the time we hit it, we will be fighting the resonance.

If we hit it repeatedly at the same speed as it’s resonance, we will always be hitting it at the same place in the oscillation cycle, so we will always be adding energy to the system. As energy grows, so does amplitude (how far it oscillates) until it exceeds its own material strength, and breaks.

Simple experiment: take a glass and flick it with your fingernail, tap it with a utensil, several times. No matter how hard you hit it or what you hit it with, it always makes a sound with the same pitch (musical note). This is the resonance frequency of the glass. When you hit it, it causes the glass to vibrate and the vibration through it has this frequency. A tuning fork is just built to a specific frequency by the manufacturer, and to hold the vibration for a long time once you hit it.

Hitting your glass with sound should also make it vibrate. If you hit it with its own vibration frequency, you can get some amplification effects into the glass as the air and glass vibrate at the same frequency, building up the intensity in the glass until it shatters.

I’ve never done this myself, and I would not recommend attempting this at home. Broken glass is dangerous. Any more details on the technical aspects of how you pull this off, google/youtube for yourself.

A swing is a good analogy. The wine glass would be the swing and the sound you play would be you pushing the swing.
If you time your pushes just right, then the swing will always keep going higher and higher. Eventually you will go over the top of the swing.
The timing of your pushes which will make the swing always go higher and higher would be the resonant frequency. With the wine glass, the sound at the resonant frequency makes the glass vibrate too much (over the top) so it cracks or shatters.

You can probably vibrate jello at it’s natural frequency and it won’t break apart. Glass breaks because it’s not flexible.

There’s another important aspect to the resonance phenomenon. Different types of objects “slow down” when vibrated, at different rates. A tuning fork keeps vibrating for a long time, at a well-defined frequency. A wooden fork quits vibrating rapidly, and has a poorly defined frequency. Mathematically, this property is called “Q”.

A high Q oscillator like a fine wine glass, can absorb incoming energy at a specific resonant frequency, while not losing energy (to various forms of friction). This makes it possible, if difficult, to break by singing at it.

A low Q oscillator, like a wood goblet, dissipates vibrational energy so fast (due to internal friction) that it is impossible to break by singing.