Is larger fan (20cm) much more quiet than smaller fan?

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netmask254
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Is larger fan (20cm) much more quiet than smaller fan?

Post by netmask254 » Mon Aug 17, 2009 2:42 am

It's easy to understand larger fan can be more quiet than smaller fan. Given the same CFM, it runs slower and results less rotation noise .

But I'm very interested that which is the dominant portion (or weight) of fan noise, rotation noise (related with rotation speed) or airflow caused noise (related with CFM)? If the latter is more important, maybe the very huge fans (18~23cm) are actually not very helpful.

pcy
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Post by pcy » Mon Aug 17, 2009 3:17 am

Hi,

The factors to look at are Rotation Noice and Air Velocity (not CFM).

CFM is Ait=r Velicity x Cross Sectional Area, and provided you have a sifficiently large Cross Sectional Area you can achieve almost any CFM while retaining low Air Velocity and hence low Noise.


The bigger fan helps with both Rotation Noise and Air Noise, so you win either way.



Peter

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Post by thejamppa » Mon Aug 17, 2009 4:57 am

... Problem is that many very large fans have only average motor and bearing sounds. But basicly yes, just avoid clear plastic ones.

netmask254
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Post by netmask254 » Mon Aug 17, 2009 6:42 pm

pcy wrote: CFM is Ait=r Velicity x Cross Sectional Area, and provided you have a sifficiently large Cross Sectional Area you can achieve almost any CFM while retaining low Air Velocity and hence low Noise.
Peter
Thanks a lot for your reply! But I still don't quite understand HOW higher Air Velocity generates more noise. There is few data regarding 18/20cm fan testing (e.g. when testing Siverstone Raven case), seems like with the same rotation speed, 18cm fan generates significantly larger noise than 12cm fan, but it is still more effective CFM/noise.

18cm fan (http://www.silentpcreview.com/article942-page7.html)
12V 18dBA 700RPM
9V 14dBA 560RPM
7V 12dBA 450RPM

I find a webpage discussing various fan noise sources. Most of its points are coincide with our experience, except the so called fan law:
dB1 = dB2 + 50 log10 (RPM1 / RPM2)
It means when increasing fan speed from 800rpm to 1200rpm, the noise (caused by speed) increases by 8.8dB. It's too high comparing to SPCR's test result. Has any body see some similar laws and can provide an explanation on it?

http://www.comairrotron.com/acoustic_noise.shtml

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