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 Post subject: Confused about CFM in undervolting conditions
PostPosted: Sat Sep 15, 2012 7:28 pm 
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Joined: Thu Aug 30, 2012 6:37 pm
Posts: 57
Location: U.S.A.
Most fans appear to require moderate undervolting to become very quiet, which reduces CFM. Some of the recommended fans I've seen (such as Nexus Basics) have low CFM to start with, so undervolted it's even lower. How would something like that do on a radiator? (A basic closed-loop system in a push or push-pull configuration).

I'm sure there a point where an undervolted fan pushes so little air that there's not enough heat dissipated off a radiator to make much of a difference. Or am I wrong about seeing such low numbers as 20-25CFM and thinking it's not enough for a radiator? (120mm fans). I want a quiet system as much as the next person here, but I'm really lost selecting good fans for a future radiator (2 or 4 120mm fans).

My prior go-to fans were Noctua, but I've read about many other possibilities, such as Noiseblockers, that now I'm not really sure what I should be looking at and which specifications to trust (since many manufacturer-listed specs for CFM and noise are shown to be exaggerated in reviews). And where one review praises a fan, another criticizes it (happened so many times - I see fans praised highly only to read elsewhere that they have many faults.)


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 Post subject: Re: Confused about CFM in undervolting conditions
PostPosted: Sat Sep 15, 2012 10:22 pm 
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Joined: Sun Aug 05, 2012 11:58 pm
Posts: 37
Is this not the same situation as fans for a heatsink or even an intake? I mean, at some point, the amount of air pulled into a case is going to be too little to effectively cool a certain system

I'm no watercool buff but Scythe GTs seem to get good reviews when it comes to static pressure. They perform really well on cpu coolers as well :)


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 Post subject: Re: Confused about CFM in undervolting conditions
PostPosted: Sun Sep 16, 2012 8:40 pm 
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Joined: Thu Jun 21, 2012 8:51 pm
Posts: 24
The ultimate goal of any fan is to cool your system with the least noise. Temperature, however, depends entirely on your specific system; its case, components, ambient conditions, internal airflow, etc. There is zero way to predict this reliably ahead of time. It may very well be that your system doesn't need a lot of CFM to keep cool enough. Or maybe you have a 3770K OC'd to 5.2 GHz in a stuffy room with no A/C down in Alabama sharing space with a bunch of vid cards in SLI/Crossfire, and you need massive CFM.

In a relative sense, temperature will be related to airflow through your heatsink or radiator. This unfortunately is not just a matter of a fan's nominal CFM, but also its static pressure and the characteristics of your rad. Why am I spelling this out so explicitly, when I'm sure you understand these points already? Because, well, you've already answered your own question in a way. Nobody can tell you exactly what will work for your system. You have to make an educated guess based on various objective and subjective reviews, prevailing wisdom, and knowledge of your own setup. Or else you have to pay the piper and experiment yourself with a selection of fans.

Just keep in mind the above, and realize that it means the following points of comparison are arranged in decreasing usefulness:

  1. Your own subjective noise perception vs objective temps on your own computer for a selection of fans.
  2. Your own objective noise measurement vs objective temps on your own computer for a selection of fans.
  3. Someone else's objective noise measurements vs objective temps on an identical computer for a selection of fans.
  4. Someone else's objective noise measurements vs objective airflow through an identical or similar radiator for a selection of fans.
  5. Someone else's objective noise measurements vs objective airflow in open air for a selection of fans.
  6. Someone else's objective noise measurements vs objective RPM in open air for a selection of fans.
  7. Manufacturer's claimed noise measurements vs airflow for a given fan.
  8. Manufacturer's claimed noise measurements vs RPM for a given fan.
  9. Someone else's subjective noise assessment vs any variable for anything (unless in conjunction with above data).
  10. Your dog or cat's reaction to your speaking the name of a given fan.
  11. Anything said by YouTube commenters.

Since points 1, 2, and 3 are usually not available, unless you have already bought some fans and tried them yourself, that leaves points 4 and 5 as usable research, with 9 being useful if and only if it is used to corroborate or detail that data. Points 6 and below are not useful on their own.

So what fan reviews would I trust?

SPCR's, for one. Both the objective official reviews as well as the prevailing opinions.
Some of the xbitlabs tests are well done.
http://martinsliquidlab.org/category/fans/ has excellent tests for fans on rads, with sound-normalized youtube vids to demonstrate so you yourself can hear the noise characteristics.
There are probably some more out there, but you have to do your own research and make your own conclusions.

Good luck!

FWIW, I recommend the Scythe Gentle Typhoons (AP-15 or AP-14), as they give ultimate CFM vs noise through a rad, but they are also known to produce a bit of a hum at higher RPMs.


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