Does SIZE really matter???

[Snafu]

Have you ever wondered? Tired of the hearing the usual "no dear that doesn't matter to me"? Boy are you in for a surprise!!! Ole Snafu here won't lie to ya'.

Most cases worth considering for an air cooled system come with either:
Two 80mm fans as intakes on the front and two 80mm fans out back as exhausts
or
One 120mm fan up front as an intake and one 120mm fan as an exhaust in the back
or
Combination of two 80mm fans and one 120mm fan
After playing with my 80mm fans and goofing with airflows I swore my next case would have 120mm fans. Why? I heard they ran quieter and moved the same amount of air as two 80mm fans. I have always wondered about this 'cause two 80mm fans have about the same area (~100cm2) as a 120mm fan (~113cm2) so why would they make a difference?

Well today the curiosity bug got the better of me and decided to search out some fans and run some calculation to see if there really is a big deal about the size. Man, was I surprised!
:yikes:

Attached is a table of some fans and their specs just for comparison. the "cCFM" and "cDBA" are "corrected" CFM and "corrected" DBA when using two 80mm fans (basically a fancy word for doubling the single fan values). There is no science here so don't start telling me about sound waves cancelling each other or whatever. Just a warning: if you make me pull out my physics text books I will just gonna have to throw it at you - HARD!

Now if the MFR specs listed here are correct then these values prove that
Size Does Matter!!! :yup:

Panaflo fans were about the only ones worth comparing (toss the SilenX fans out unless you mod them to run like a real fan). Two of thier U1A (ultra) 80mm fans can push out 94CFM at 76DBA. Its bigger brother the U1A 120mm fan pushes out more air (115CFM) and does so much quieter (45.5DBA). Take a look at all the other specs. You will come to the same conclusion and if if that doesn't prove it then I don't know what would (maybe a physics book ;) ).

About the only 80mm fan that cleaned up were the Delta fans but running at ~100DBA it makes you kinda wonder if all that noise is really worth it. For those with Delta fans I SAID IT MAKES YOU KINDA WONDER IF ALL... oh your turned down your fans :o . Sadly the retailler did not carry 120mm versions so I could not include them for comparison.

What is even more interesting is the price difference taken from the retailler where I found the specs. The price of two Panaflos 80mm U1A we more than 40% the price of the single 120mm fan. Holy Carp!!!

Now are you as convinced as I am about better airflow, much quieter running and it even saves you money!!! Now you have an excuse to upgrade your case to one that uses 120mm fans entirely :thumb:

You can send me my Nobel prize now. Same place as my paychecks.

- The BEon

PS - now that I have proved that bigger is better I am off to get drunk and then become a monk. I can't stand the lies any more :cry:

[Dave3d]

hey Snafu.
Did you do any real world temp diffs?
Or is this all based on paper?

The reason I am asking is:
I have 4 Aerocool X-Blaster "The Heat Terminator" 80mm Fans (down from 6 !!!) that run at RPM: 4000 rpm, Air Flow: 52.8 cfm, Noise Level: 45.6 dba :coocoo:

I was thinking of modding my case to use some low dba 120's, but if temp changes are negligible then I wouldnt bother going through the hassle (seeing as how my fans are on a controler).

Anyway, interesting topic.

[Hawk]

Size Does Matter!!!



I'm not touching this. . . . . . :o

[Snafu]

Did you do any real world temp diffs?
Or is this all based on paper?
It's all paper. Now if someone tossed an armor case down in the cellar then I would run some tests to give some numbers.

One thing you should to keep in mind. If you can get a 120mm fan delivering the same CFM as your 80's you should not see any temp difference than what you have now but it will be quieter.


I'm not touching this. . . . . . :o
Chicken :na:

[Snafu]

bumping an old thread as FYI for those looking at fans.

[Dave3d]

You are not catching me with this one again.
:lol:

[Snafu]

I think I just did :lol:

The 120's delivering the same CFM will be quieter. Can't guarantee cooler cause there are more factors than just CFM that contribute to cooling such as room temp.

:thumb:

[RotorHead]

I can back this up with my real world testing.......:p

I'm sitting at my desk, which has 3 computers running, 2 underneith and 1 on top. Of the 2 below the desk, 1 has 4 120mm fans the other has 2 El cheapo 80mm fans. the 1 on top of the desk also has 4 120mm fans and the case side removed.

All I can hear is the 2 El cheapo 80mm fans.

Bigger is always better :rock:

[FB]

i have been thinking about modding my case to replace my 2x80mms in the back with 1x120, but i have been lazy. my fan controller does pretty well, and keeps my x2 to 39-40C under max load with all fans on low, and if i crank all 4 up to full speed i have what sounds like a wind tunnel and keeps my CPU 5C cooler

[Freddy]

Attached is a table of some fans and their specs just for comparison. the "cCFM" and "cDBA" are "corrected" CFM and "corrected" DBA when using two 80mm fans (basically a fancy word for doubling the single fan values). There is no science here so don't start telling me about sound waves cancelling each other or whatever. Just a warning: if you make me pull out my physics text books I will just gonna have to throw it at you - HARD!

Now if the MFR specs listed here are correct then these values prove that
Size Does Matter!!! :yup:

Panaflo fans were about the only ones worth comparing (toss the SilenX fans out unless you mod them to run like a real fan). Two of thier U1A (ultra) 80mm fans can push out 94CFM at 76DBA. Its bigger brother the U1A 120mm fan pushes out more air (115CFM) and does so much quieter (45.5DBA). Take a look at all the other specs. You will come to the same conclusion and if if that doesn't prove it then I don't know what would (maybe a physics book ;) ).Hi Snafu, :wave:

please forgive me (and don't throw that heavy physics book at me), :hide: but... you are just a "little" :p bit wrong here...
You say you simply doubled the dB(A) ratings when using two of the same fans, but two fans produce only about 3 dB(A) more than one fan does; that is the rule of thumb. So in your example your U1A 80mm fan produces 38 dB(A) and two of those would give you 41 dB(A), meaning they would still produce less noice than your U1A 120mm fan producing 45.5 dB(A)!

The reason why those 120mm fans move more air with a lower rpm than two 80mm fans is that, as you say, the area is about the same, but the effective area is a lot bigger! With that I mean:
- first, the area simply is a little bigger: 1x80mm=50cm2, 1x120mm=113cm2, making the difference 63cm2, which is more than one 80mm;
- second, that extra area does not contain an axis, which means it is completely used to pump air through, whereas the second 80mm fan lacks the inner space, which contains the axis, making the effective difference even bigger;
- and third, most of the air that is being moved is moved through the outer parts of a fan, not the inner parts, because the outer parts of the blades move a lot faster than the inner parts do and a 120mm fan has a lot more of those outer parts, that on top of that move even faster than those of an 80mm fan with the same rpm, making the effective area even bigger again.

So to move the same amount of air, or even more, compared to two 80mm fans, a 120mm fan can have a lower rpm, causing it to have a lower pitch, which is usually perceived by people as less annoying, making it seem to be quieter! In your example the 80mm fans make 3450 rpm, whereas the 120mm fan only makes 2750 rpm.

So to be of any comfort to some of you: :p bigger is NOT always better! Sometimes it can be, but not by far! :D

Freddy :thumb:

[Snafu]

Nah. I aint gonna throw no heavy books at ya. But you are going to have to show me that rule of thumb without throwing any heavy books :lol: I cannot see how 2-80mm fans would be quieter than a single 120mm fan (U1A)

Now I never brought pitch or blade design into the equation but tried to compare similar fans hoping they woudl be fairly similar in design.

BTW 2x80mm= 100cm2 vs. 1x120mm=113cm2 for a difference of 13cm2

Oh and you lost me on the second point :o

[Freddy]

Nah. I aint gonna throw no heavy books at ya. But you are going to have to show me that rule of thumb without throwing any heavy books :lol: I cannot see how 2-80mm fans would be quieter than a single 120mm fan (U1A):) Quieter here means less noice, not necessarily less annoying! The rule of thumb (that comes from the fact that the dB scale is logarithmic) is that for every doubling of power (for example by using two of the same sound sources) the sound level rises with 3 dB. 10 dB corresponds to a doubling of perceived loudness.
You can easily "see" ;) there is a difference between doubling the power and doubling the loudness by listening to for example a running car engine: when you are close to a running car engine with the hood up it is very loud! But when on your other side a second car starts it only gets a little louder, not much and not nearly twice as loud.
Another simple way to see you can not just double the dB rating is by comparing your ratings to a dB scale: your U1A 80mm fan produces 38 dB(A), which corresponds to a "Residential area at night" (could be just about right for a rather quiet fan), but 76 dB(A) is even more than "Busy traffic at 5 m" (very unlikely for two fans, even a Delta)! :p

Now I never brought pitch or blade design into the equation but tried to compare similar fans hoping they woudl be fairly similar in design.

BTW 2x80mm= 100cm2 vs. 1x120mm=113cm2 for a difference of 13cm2

Oh and you lost me on the second point :oIn the first point I said "1x80mm=50cm2, 1x120mm=113cm2, making the difference 63cm2, which is more than one 80mm" and I meant that the difference between a 120mm fan and an 80mm one is more than a second 80mm fan: 63cm2 v. 50cm2.
And in the second point I meant that when you add a second 80mm fan, you also add a second axis, through which no air can go of course: meaning you add unuseable area, whereas the extra area a 120mm fan brings (63cm2 v. 50cm2) contains no obstacles, so it can be fully used to blow air through.
The third point makes it even stronger, by showing that the extra area a 120mm fan brings is on the outside, where the fan blades go much faster (m/s) than those of an 80mm fan with the same rpm and thus move even more air per minute per cm2.

So the area is more than twice as big, contains less obstruction and is more efficient. :thumb:

For those of you who are really interested in the dB scale, :rolleyes: here you can find more about it:
http://www.phys.unsw.edu.au/~jw/dB.html

[RotorHead]

Smarty pants :na: .......bigger is still better :D

:hide: :hide:

[Freddy]

:lol:

[Snafu]

Doh - I forgot about the logarithmic scale thing.

So what's with the second axis? Suggestion cases with dual 80mm fans out back don't work?

For your responses I appoint you with THIS (http://img.sparknotes.com/content/snafu/pdf/snafu_diploma.pdf) ;)

[RotorHead]

The Hub area of the two 80mm fans is greater then the hub area of a single 120mm fan, further reducing the total effective opening as compared to the 120mm.

Blade tip speed could actually be higher in a 120mm fan due to the time and distance traveled. But most of the time this may not cause an increase in noise because the larger blades will not need the same pitch angle to move more air.

I think the 120mm fans are statistically more efficient then the 80mm fans but noise level could be more subjective.

[Freddy]

Doh - I forgot about the logarithmic scale thing.

So what's with the second axis? Suggestion cases with dual 80mm fans out back don't work?With the second axis I meant what RotorHead explained above: "The Hub area of the two 80mm fans is greater then the hub area of a single 120mm fan, further reducing the total effective opening as compared to the 120mm."

For your responses I appoint you with THIS (http://img.sparknotes.com/content/snafu/pdf/snafu_diploma.pdf) ;)http://members.chello.nl/h.h.j.f.beens/temp/thankya.gif :lol:

Freddy :thumb:

[Snafu]

Cool - thanks. Got it now that it aint all spelt out in sigh-n-teefic talk :lol:

Basically, need to consider that the hub (motor) reduces the overall effective area. If there is a hub - seen some fans without a central hub or not much of one but these are definitely not the norm.

Can we say that usually size does matter?

[jsimmons]

My system has nine 120mm fan holes. :)

[Snafu]

:lol: is that all? I thought the UFO could hold far more :lol: