How size affect fan noise?

[Assafl]

So at a given CFM the larger fan has lower noise - ALWAYS (again - I know some bearing squeal and some have shot bearing that rattle - that is OT).
And lower RPM is lower noise. But the same RPM for a larger fan means higher CFM AND higher noise (not because of the CFM - but because at the same RPM the velocity of the tip at greater radius is higher).

With fans bigger is always better. Except when it doesn't fit. Then it isn't better.

This was the quick answer I had already, apparently to quantify and get numbers it is not so easy.

True that. However - the physics does tell you that if noise annoys you: get a bigger fan or reduce the RPM. That will be the most ROI.
If you cannot get a bigger fan or cannot reduce the RPM - only then start thinking about things such as using porous blades, blade vortex fins, etc. that will help reduce the noise.

[Zucca]

BTW I don't get why people are so excited about overpriced ebm-papst. IME of seeing them in laboratory equipment working in the same equipment for many years, their reliability was much worse than of much cheaper Sunon.

Could be all the design efforts were to minimize dB(A). Reliability was not the first priority.

[wraper]

BTW I don't get why people are so excited about overpriced ebm-papst. IME of seeing them in laboratory equipment working in the same equipment for many years, their reliability was much worse than of much cheaper Sunon.

Could be all the design efforts were to minimize dB(A). Reliability was not the first priority.

They were not even silent. Those were mostly 120x38mm 230V AC fans. But they had die cast metal blades (look almost the same as plastic, except weight). They had quite a bit of both motor failures and ball bearing seizures. Same spec (except much higher efficiency) Sunon fans with plastic blades (that were used by equipment manufacturer a bit less often) developed loud bearing a few times but never failed entirely IIRC. There were some other devices with EBM-papst DC fans but I did not find them particularly good either.