What causes CPU fan noise?

[2timer]

Is it more related to the mechanical part of the fan, ie the sleeve or bearings, or is it more related to the acoustics of air pressure?

For example: I have a netbook with an Intel Atom processor. It makes noise, not a great amount, but it's not silent. I'm wondering if the fan noise is caused by the mechanical pieces or is it due to the acoustics of the air within the chamber? If I were to take this fan out of the case and out of it's plastic sleeve, and then run it in open air, would it still make noise?

 

[Bill Brasky]

Very good question. Unfortunately, the answer varies a TON depending on the type of fan and the speed it's spinning at. For a high rpm notebook fan, it's likely the majority of noise comes from the bearings within the fan. Bearing fans are louder, but have a much long lifespan. It makes sense to use them in places that are difficult to access.

On the other hand, case fans for a desktop computer usually have sleeve bearings which are more quiet and have a shorter lifespan. Usually their noise comes from the air turbulence generated by the propellers.

So yes, if you took the notebook fan out of the chassis, it would probably be about as loud.

 

[ThatsABigOne]

It is usually the acoustics in all of my fans.

 

[Concillian]

A good large fan will be primarily air movement making the sound.

A BAD fan it can be anything. Imbalance and such can cause motor and bearing noises like ticking type noises.

As speed increases and motor size decreases (notebooks) the motor noise whine starts getting predominant.

Part of noise is the signature of the noise, and not just how loud it is. equal Db from low, medium and high frequency noises will be perceived different by people, with generally the lower frequencies being less annoying.

 

[Piroko]

I'd say if it's a 'swooshy' sound like this http://www.youtube.com/watch?v=Q64fyM_JpEA&feature=player_detailpage#t=165s from 2:45 to 2:50, then you're hearing the acoustics of moving air for the most part. (also, ignore the even deeper sound starting at ~2:50, that's wind blowing into his camera mic)

If there is any alternating or higher pitched 'whine' in that noise then that's the fan/its bearing in 9 out of 10 cases, or coil whine in 1 out of 10 cases.
Here are some examples of higher pitched sounds: http://www.youtube.com/watch?v=iiXNYrY-FzY especially the second and fourth one have bearing noise mixed into the wind noise.

Taking it out of its plastic sleeve will probably not lower or remove the bearing noise, except if the fan is grinding on something.

 

[Ajay]

It's just the sound of the impeller blades moving through the air causing allot of turbulent flow. That, plus the sound of the fan induced air flow impinging on the heatsink cooling fins/plates. Notebooks use blower fans which are generally noisier than 'case' fans. The motors on PC fans are very simple and generally do not create much noise. Unless the bearings on a fan are bad, they won't make much noise either.

 

[veri745]

Mechanical movement of the fan creates small oscillating variations in air pressure.

 

[2timer]

Very good question. Unfortunately, the answer varies a TON depending on the type of fan and the speed it's spinning at. For a high rpm notebook fan, it's likely the majority of noise comes from the bearings within the fan. Bearing fans are louder, but have a much long lifespan. It makes sense to use them in places that are difficult to access.

On the other hand, case fans for a desktop computer usually have sleeve bearings which are more quiet and have a shorter lifespan. Usually their noise comes from the air turbulence generated by the propellers.

So yes, if you took the notebook fan out of the chassis, it would probably be about as loud.

Thanks, that's excellent. I didn't even realize that my tiny netbook fan would be a bearing type. But I suppose that makes sense, given that the cost difference is only marginal and fan failure could be catastrophic. So it makes sense for the laptop OEMs to use bearing types, even if it's a little louder.