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[–] goatsandbros 0 points 6 points (+6|-0) ago  (edited ago)

The metal of the hinges, I'd figure. Sort of like rubbing your finger on wet glass.

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[–] Philosopher_King 0 points 5 points (+5|-0) ago 

Great question!

This has to do with a stick and slip motion. The main cause of this is because of friction, where we have static friction and kinetic friction. Static friction is the friction on stationary objects and kinetic friction is the friction of a moving object. Static friction has a stronger force and it's harder to move objects that are stationary than to move objects in motion.

So, with a squeaky door, it's based around the hinges which tends to rust and deteriorate over time. What happens is hinges are locked in a stationary position, then the hinge bends a bit, and as you keep applying more force to open the door, the force becomes strong enough which makes the hinge jump forward a bit and stops. This continues to repeat during the entire process (stick and slip motion) of while you're opening the door.

The jumping that's caused creates a vibration, or pitch, and makes the sound that you hear which is a creepy creaking sound.

If you're interested, check out hysteresis, which is a topic discussed in many dynamic courses in engineering.

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[–] SIuuNEeFzkfasdPmqtAv 0 points 0 points (+0|-0) ago 

I think it's the same phenomena that makes chalk jumps on board given enough pressure and the right speed.

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[–] killer7 [S] 0 points 0 points (+0|-0) ago 

Thanks! Best answer here so far.

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[–] dkyuss 0 points 1 points (+1|-0) ago 

Friction (likely caused by door hinges not being aligned perfectly) causes parts of the hinge/pin assembly to stick and release rapidly, exciting a critical frequency of the door and hinge combination. As the door and hinge is designed to be stiff, the frequency it wants to resonate at is very high.

Think of a bass string on a guitar VS a metal tuning fork, the stiffness (and geometry) of the vibrating component will dictate the critical frequency, thus the pitch emanated.

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[–] Scruffy_Nerfherder 1 points 1 points (+2|-1) ago 

Your mom.

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[–] pushthis 0 points 0 points (+0|-0) ago 

Metal hinge parts.

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[–] lord_nougat 0 points 0 points (+0|-0) ago 

The action of opening or closing a door can be so exciting for the door that it's testicles vibrate excitedly!

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[–] Rellik88 0 points 0 points (+0|-0) ago 

It's the metal hinge squeaking.

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[–] killer7 [S] 0 points 0 points (+0|-0) ago 

But why it's vibrating so fast?

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[–] Hall_of_Cost 0 points 0 points (+0|-0) ago 

Probably similar to the strings of a violin bow and strings of the instrument itself. When rubbed together they vibrate and create sound.