1) All else equal (type/quality of motor, fan screen, etc), the 230mm fan will move more air and therefore generate more noise.
2) All else is never equal.
Large 230mm fans can move enough air at lower rpm, and therefore can operate more quietly when used for that purpose. Crank them up, and its a different story.
Okay, but say there's 1x 240mm moving at low speed vs. 2x 120mm moving at high speed (moving equal amounts of air)
which one makes more noise?
what if RPM is same for every fan. what's quieter
240mm @ 1000RPM vs. 2x 120mm @ 1000 RPM
I think you need to ask the question specifically (what equipment are you actually comparing), and maybe we can give a judgement. But I have no answer to your question, and I don't know if it would actually help you if I did.
It is a rule of thumb that the larger the fan the quieter it is when it comes to computers. Reason is that they are a lower RPM to reduce noise....but not so much that they move less air. They are still fast enough to move more air than a smaller fan.
So they are designed that larger fans move more air with less noise. One thing to watch out for is odd sizes that are hard to find replacements. I remember way back a guy bought a Thermaltake case with a 92mm fan space and he had such a hard time finding 92mm fans. Nowadawys it might be a different story.
This quetion is fairly simple to answer: As lon as you nknow waht kind of CFM u want to archive, look for the fan with lowest dB.
Now to the qustion whats louder, 2 15dB fans or 1 30dB fan, dB is not added when you want to know the total output so so 15 and 15 are somewhat 15.8 dB total.
Anyway look for tacens fans, those move decent amounts of air (50-75 CFM for 120MM) and are all inaudible, while the 250MM moves around 134CFM with 16 dB.
Now answering your question, the bigger the fan the quiter?
Yes but only if consumes more power, and this is basicly explained by physics:
the bigger the fan, the easier it comes to keep it running once its at its nominal speed, since the gravitational center is more efectivly stable in the same spot, creating less vibration and therefore making less noise.
Now a bigger fan with same RPM as a smaller one will always do more noise as long as both are near perfect in their mecanical systems (AKA the engine itself is inaudible) simply cus it moves more air.
At the same rpm, I think the 230mm fan will make more noise.
The reason is that the outer part of the larger fan will be moving through the air faster than the smaller fan, creating more turbulence and noise.
A 230mm fan should be quieter if it is running at a lower rpm. A 230mm fan has 3 times the area of a 120mm fan, and can therefore deliver more cooling at a lower rpm.
-11309 mm squared (surface area of the 120mm fan) times two equals 22618 mm squared total surface area.
-41547 mm squared surface area (230mm diameter fan)
So, one would think, at the same RPM, the 230mm fan would be able to move the same amount of air with less of a pitch on the blade, creating less disturbance in the air, therefore less noise. Or, given the same blade pitch, the 230mm fan would create similar amounts of noise, with a greater CFM.