How much louder is 2 80 MM fans

[Reather]

About how much louder is 2 80 MM fans in the back vs. just 1 120 MM fan in the back?

[crossedup]

Quite a bit louder, they run at a higher RPM and you will have double the number.

My case has 2 80mm's in the side and a 120mm in the back. You can hear the 80's but you cant hear the 120. Next time I think I will replace the 2 80's with a 120 and cut down on the noise a little bit.

[elroy]

Depends on the individual fans. They all have individual decibal ratings. Noise will depend on the bearings, rpm speed, fan blades etc.
Look here and you can see al the variation in noise for the same size fan.
http://www.nexfan.com/cafaalad.html

[pj@usa.net]

The main reason some fans, of the same size, are noiser or quieter,
is that the noisier ones are set to run faster, and push more air.
The quieter ones are set to run slower, and push less air.
Details of construction, bearing type and such are secondary.

Cooling depends on air volume moved; for a given fan size,
air volume depends on air velocity, air velocity depends on fan
rotation speed, fan blade tip velocity depends on fan rotation speed,
and noise depends on fan blade tip velocity.

The main way to get less fan noise, for a given amount of cooling,
is to have a greater area of fan. The noise basically depends on
how fast the air is being pushed, and the bigger the area, the
slower the air needs to be pushed, to get the same volume of
air moved, and hence the same cooling.

The cross sectional area of one 120 mm fan is: 453 cm*cm.
The cross sectional area of two 80 mm fans is: 402 cm*cm.

So the single 120 mm fan has 12 % more area, or 12 % less air
velocity, for the same amount of cooling (same volume of air moved).

I don't know how to estimate the change in noise, given a change in
air velocity. But I'd guess that two 80 mm fans were 2db to 4 db nosier
than the single 120 mm fan, all else equal.