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question about fan noise

H

HRslammR

2[H]4U
Joined
Jun 20, 2003
Messages
3,438
but it semi pertains to cooling, so i thought this to be the best place for it.

anywho, lookin to replace two 80mm fans with one 120mm. my question is, would two fans at 28dba be louder than one fan at 30dba?

how this all relates to cooling: i currently have 5 fans, 3 exhaust and 2 intake, 4 of the fans are 32cfm/28dba and the 5th i'm not so sure on (but it'll be replaced with one at 32/28 soon enough). and i'm looking at replacing my intake fans with one 120mm at 94/30.

so, i'm just wondering if my overall noise would decrease (along with my airflow balancing a bit)
 
The dB-ratings of fans become quite meaningless from the moment you install the fan into the case, as those ratings are under optimal conditions, i.e., with unobstructed airflow.

As 120 mm fans usually spin slower than 80 mm fans, the former should be less affected by turbulent airflow and thus stay closer to its dB-rating.

In short, yes, the 120 mm fan should be quieter than even a single of those 80 mm fans.
 
yup great point. Most folk dont take into consideration that anything in the airs way will create noise... along with the case resonating any vibration in the fan.

I try to go around this by using a few more fans at 7 or 5 volts.


As for decibles. every 2, sounds like doubling the sound.
 
RedShred said:
As for decibles. every 2, sounds like doubling the sound.
Click to expand...
Be careful with such statements :)

Technically every 3 dB added means a doubling in soundpressure, but to the human ear a doubling of the sound's volume requires around 9 - 10 dB.
 
damnit... I knew I might have been wrong, but was pretty confident in my answer.

thanks for clearing that up though ;)
 
now for the greatest trick of all... modding a pc-61 to take a 120mm fan on the intake ;)
 
take pics of the process, i'd like to do this also, just been too damn lazy
 
MaMMa said:
take pics of the process, i'd like to do this also, just been too damn lazy
Click to expand...


we did it on my buddy's lian-li, went pretty easy. took about an hour and a half, we'll do it to mine on friday and i'll take pics.
 
Technically two fans at 28dba would produce a level of noise that would be pretty close to a single bigger fan at 30dba... I say technically because the way the dba scale works (logarithm) having two sources of the same noise level would make noise go up by about 2-3dba.

The kink is that this is all hypothetical like others said... And the ratings are hardly set in stone. First off they change dramatically once the fans are actuall installed anywhere like Elledan said, and second they may not be entirely accurate even if both ratings come from the same manufacturer.

Heck the ratings might even be accurate but the noise characteristics between the 80 and 120mm fans may be entirely different, a lower sound source might be more annoying if it's a tone that rattles your attention more versus a more wooshing/soothing type noise.

In general 120mm fans tend to be less whiny, they may produce more air noise but that's usually more easily masked by the case itself (particulary if it's going behind a front-bezzel grill and not a more open wire grill). Also take into account the resulting CFM, CFM ratings are even less reliable than the dba ratings even amongst fans from the same manuf.
 
HRslammR said:
we did it on my buddy's lian-li, went pretty easy. took about an hour and a half, we'll do it to mine on friday and i'll take pics.
Click to expand...


Sweet, I'm gonna hold you to it
 
Actually twice the sound pressure level (SPL or amplitude) is 6db, for 2 signals adding in phase, but because fan noise is random in nature the actual SPL is on average 3dB higher.

Explaination...
At some point along the time axis the signal may be cancelled on others it may be doubled. To look at the average level of a random signal as percived by the ear RMS, or power must be considered. The result is that when looking at the RMS value (or the power of the waveform) it is doubled after calculating the the addition of 2 random signals but a doubling in power is only 3dB higher in actual SPL.

FYI
For every doubling in sound power level you get an increase of 3dB in SPL, what another poster said about 9dB for a perceived doubling in sound is right that means to perceive an amplifier that is 2 times louder then another amplifier it needs more then 8 times the power, that is why a 20W amp can actually sound pretty loud. So when your deciding on weather to get the high quality 100W amp or the lower quality 150W amp you know which one I would choose.
 
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