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Old January 16, 2008, 09:32 PM
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Default dB-to-CFM ratios

ok, every time i see them in reviews, i can't help but cringe. how can one make a ratio out of those numbers? one is linear (CFM), and the other is logarithmic.

i mean, look at this:

40 CFM / 20dB = ratio of 2
80 CFM / 30dB = ratio of 2.66

twice the airflow, and twice as loud - but the "ratio" indicates otherwise (it seems to indicate that the 80 CFM is a way better deal)

here's a kicker:
two fans of 40 CFM / 20dB = 80 CFM / 23dB, and the ratio is 3.47 (even though it should still be 2 as per above)

why is it even used???
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Old January 16, 2008, 09:51 PM
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I think it's a way for a review site to warn people that they're ignorant enough to divide a linear measurement by a logarithmic measurement, and treat the answer linearly. That way, you don't go assuming anything else about the review was done properly either.
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Old January 16, 2008, 09:57 PM
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Quote:
Originally Posted by solaris10 View Post
ok, every time i see them in reviews, i can't help but cringe. how can one make a ratio out of those numbers? one is linear (CFM), and the other is logarithmic.

i mean, look at this:

40 CFM / 20dB = ratio of 2
80 CFM / 30dB = ratio of 2.66

twice the airflow, and twice as loud - but the "ratio" indicates otherwise (it seems to indicate that the 80 CFM is a way better deal)

here's a kicker:
two fans of 40 CFM / 20dB = 80 CFM / 23dB, and the ratio is 3.47 (even though it should still be 2 as per above)

why is it even used???
Static Pressure > CFM
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Old January 16, 2008, 10:04 PM
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Quote:
Originally Posted by enaberif View Post
Static Pressure > CFM

chicken > mouse


i'm not sure i'm understanding what you meant...
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Old January 16, 2008, 10:10 PM
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On a heatsink/rad, sure. Open case-fans, not so definite. Taken literally, I'd be hooking up my 12V air compressor.

For that matter, I thought I read somewhere that static pressure tended to increase exponentially with fan rpm...
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Old January 16, 2008, 10:10 PM
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Quote:
Originally Posted by solaris10 View Post
chicken > mouse


i'm not sure i'm understanding what you meant...
You can have all the CFM you want but it doesn't mean its gonna move a lot of air effectively.

What really matters about a fan is the amount of static pressure it has and this is basically kinda like a torque rating of a fan; Put a fan with a low static pressure up against something like the Ultra 120, Tuniq, Noctua and it won't work well because it can't push the air through the small fins.

Now take a fan that has a high static pressure and use those coolers and it will work awesome because it has enough "torque" to push air through the small spaced fins.

CFM is really only good if you need to push air "across" your case but even then you gotta watch for obstructions.
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Old January 16, 2008, 10:23 PM
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ah, ok, yes that part i understand (and agree with).

though, the whole aspect of a review talking about dB/CFM ratios bugs the crapola out of me. and, um, the most recent case was in that Thermaltake TMG A1 review that's linked from another thread on here...


quick question on the topic of static pressure, though:

if i have a fan that's supposed to PUSH through an obstruction (like a rad or a hard drive cage), i can understand it needing to be a high-pressure-capable fan
but what about if the fan is attempting to PULL air through that same obstruction? does it still need to be a high-pressure-capable fan, or not as much?
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Old January 16, 2008, 10:32 PM
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I'm not positive, but I'm fairly certain that the effective static pressure applies to both sides of the fan. The behaviour of the air affected by that pressure will differ, especially at very close range to the blades, which is why performance can still vary depending on whether a push or pull config is used. In general, I believe 'pull' configurations will tend to pull air from a wider field, whereas the 'push' effect is initially more focussed.
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Old January 16, 2008, 10:41 PM
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yeah, they do (re.: airflow - it's like a wide funnel on the intake side, and a straight column on the exhaust side), and because of that i'm wondering if a high-pressure-capable fan working in a PULL config would actually exhibit better performance than a standard fan in the same PULL config. in my case (an Ultra Aluminus), i'm mounting two scythe ultra kaze 2000rpm fans on the inside to pull air through the lower hard drive cages (i'll have to seal them to the actual cage), but since there is going to be around 3/8" of space between the blades and the cage, i wonder if the HP fans are even worth it.
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Old January 16, 2008, 10:59 PM
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I wouldn't think 3/8" would be significant. If the blades are too close, they'll get even noisier because of the turbulence created.

Haven't really seen any glowing reviews on those Ultra fans, either. The 1000rpm version got spanked in a recent review at this site.
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