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Age Old Case Front Intake Question: 2x140mm or 3x120mm?

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post #1 of 13 (permalink) Old 02-07-2019, 01:31 PM - Thread Starter
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Age Old Case Front Intake Question: 2x140mm or 3x120mm?

This is a question I've seen asked a million times but I still don't feel like I've seen a great answer on the subject. For sake of discussion, let's assume the 120mm fans are Noctua NF-A12x25 fans and the 140mm fans are Noctua NF-A14. To boil down all the different arguments I've read into what I consider the consensus, it would be that 3x120mm fans offers a higher maximum potential CFM, while the 2x140mm fans offers a superior noise-to-CFM ratio.

So, if that's the consensus as it were, then 2x140mm setup would be superior in all situations, except in situations where volume is not a concern. Yet my intuition feels differently. While yes, 2x120mm fans would have spin a good deal faster to match 2x140mm, I'm not exactly sure how is it that 3x120mm wouldn't be able to match the CFM at the same or less volume. I've read a few different people trying to break it down with math, but to put it shortly, I wasn't quite sold on their math.
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post #2 of 13 (permalink) Old 02-07-2019, 01:38 PM
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Three NF-A12X25 are capable of 180 CFM at max, while the two NF-A14 are capable of 164 CFM at max, a very negligible difference. So, really, the question you are asking here is "why are three 2000 rpm 120 mm fans louder than two 1500 rpm 140mm fans, when all of the fans in question have an almost identical dBA output?". That is a far simpler perspective than any math, and should have an obvious answer.

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post #3 of 13 (permalink) Old 02-07-2019, 01:42 PM
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The hubs are dead spots. Fewer fans = less dead spots.

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post #4 of 13 (permalink) Old 02-07-2019, 01:59 PM - Thread Starter
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Quote: Originally Posted by ciarlatano View Post
Three NF-A12X25 are capable of 180 CFM at max, while the two NF-A14 are capable of 164 CFM at max, a very negligible difference. So, really, the question you are asking here is "why are three 2000 rpm 120 mm fans louder than two 1500 rpm 140mm fans, when all of the fans in question have an almost identical dBA output?". That is a far simpler perspective than any math, and should have an obvious answer.
Since when did all 120mm fans and 140mm fans have the same volume output?

I'm not sure if you understood the question I'm asking, or if you mistyped your response. I'm not asking how can something can be louder than something else with the same dBA output, which is what it sounds like you thought I asked.

Quote: Originally Posted by The Pook View Post
The hubs are dead spots. Fewer fans = less dead spots.
Well if you are thinking of this in purely of terms of area which air can pass through, sure.

But I think there are more variables involved here.

Last edited by t1337dude; 02-07-2019 at 02:06 PM.
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post #5 of 13 (permalink) Old 02-07-2019, 02:11 PM
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I personally wont even consider a build unless 12 60mm fans are being used in place of three 120s. It's simple math.

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post #6 of 13 (permalink) Old 02-07-2019, 03:23 PM
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Quote: Originally Posted by t1337dude View Post
This is a question I've seen asked a million times but I still don't feel like I've seen a great answer on the subject. For sake of discussion, let's assume the 120mm fans are Noctua NF-A12x25 fans and the 140mm fans are Noctua NF-A14. To boil down all the different arguments I've read into what I consider the consensus, it would be that 3x120mm fans offers a higher maximum potential CFM, while the 2x140mm fans offers a superior noise-to-CFM ratio.

So, if that's the consensus as it were, then 2x140mm setup would be superior in all situations, except in situations where volume is not a concern. Yet my intuition feels differently. While yes, 2x120mm fans would have spin a good deal faster to match 2x140mm, I'm not exactly sure how is it that 3x120mm wouldn't be able to match the CFM at the same or less volume. I've read a few different people trying to break it down with math, but to put it shortly, I wasn't quite sold on their math.
Simple answer is 2x 140mm fan perform about the same as 3x 120mm fans of same design usually running 2-300rpm faster and making 2-5dB more noise. A 120mm fan has about 2/3rds as much airflow area as a 140mm fans .. so basically 3x 120mm fans have about the same airflow area as 2x 140mm fans. Also 3x fans of same dB each make about 2dB more noise than 2x fans of same dB each.

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post #7 of 13 (permalink) Old 02-08-2019, 01:30 AM - Thread Starter
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Quote: Originally Posted by doyll View Post
Simple answer is 2x 140mm fan perform about the same as 3x 120mm fans of same design usually running 2-300rpm faster and making 2-5dB more noise. A 120mm fan has about 2/3rds as much airflow area as a 140mm fans .. so basically 3x 120mm fans have about the same airflow area as 2x 140mm fans. Also 3x fans of same dB each make about 2dB more noise than 2x fans of same dB each.

I like the answer - seems in line with what everyone is saying.

I just wish there was a more detailed answer. For as often as I've seen this question asked in the past, I'm surprised nobody has linked me some detailed graphs or charts by this point.

You could argue I'm being lazy by not doing the math myself but I'm not good at math. If I were to figure this out, at bare minimum I would need the fans in question, a SPL meter, and whatever I'd need to measure CFM (or I could measure the temps directly because that's what we care about in the end). And if I wanted to know for more general usage instead of the Noctua examples I referenced, I would use a variety of 120mm and 140mm fans. In a round-about way I suppose I'm trying to say so far I'm not convinced anyone figured it out more than a rough guesstimate because I'm having difficulty finding anyone who has done the testing. If I knew that the fan RPM's correlated to CFM and decibels in a completely linear fashion, I would feel more comfortable with the rough math people do.
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post #8 of 13 (permalink) Old 02-08-2019, 02:34 AM
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depends on the fans and the case. I recently did two PH-F140MP v 3 arctic F12 fans. The arctic's were quieter and the was cpu cooler. no idea what it would have been like with two arctic F14 fans or a different case. far too many variables
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post #9 of 13 (permalink) Old 02-08-2019, 05:07 AM
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Quote: Originally Posted by t1337dude View Post
I like the answer - seems in line with what everyone is saying.

I just wish there was a more detailed answer. For as often as I've seen this question asked in the past, I'm surprised nobody has linked me some detailed graphs or charts by this point.

You could argue I'm being lazy by not doing the math myself but I'm not good at math. If I were to figure this out, at bare minimum I would need the fans in question, a SPL meter, and whatever I'd need to measure CFM (or I could measure the temps directly because that's what we care about in the end). And if I wanted to know for more general usage instead of the Noctua examples I referenced, I would use a variety of 120mm and 140mm fans. In a round-about way I suppose I'm trying to say so far I'm not convinced anyone figured it out more than a rough guesstimate because I'm having difficulty finding anyone who has done the testing. If I knew that the fan RPM's correlated to CFM and decibels in a completely linear fashion, I would feel more comfortable with the rough math people do.
What you are asking for is impractical at best .. more like impossible.
All case venting is a little different for other cases.
All filters are different.
All fans are different.
All systems airflow is different
All systems use different fan curves and different component TDP output
TDP output also depends on component load.
Etc.
Etc.
I agree with you, you are too lazy to do your own work so instead ask us to do it for you. Saying you are not good at math is a lame excuse. The math is simple and I'm sure you have a little calculator handy or 10-key pad on your computer. Then look up manufacturers' fan specs for the basic data.

But to help you on your way:
PH-F140MP spec 68.1cfm, 1.62mm H2O & 25.3dB(A) .. 2x fans = 136.2cfm, 1.62mm H20 & 28.3dB(A)
PH-F120MP spec 53.3cfm, 1.72mm H2O & 25.0dB(A) .. 3x fans = 159.3cfm, 1.72mm H2O & 29.8dB(A)
For MP fans 3x 120mm are slightly better than 2x 140mm and slightly more noise
PH-F140XP spec 85.19cfm, 1.77mm H2O & 17dB(A) .. 2x fans = 170.38cfm, 1.77mm H2O & 20dB(A)
PH-F120XP spec 61.1cfm, 1.72mm H2O & 27dB(A) .. 3x fans = 183.3cfm, 1.72mm H2O & 30.8dB(A)
For XP fans 2x 140mm are similar but much quieter
PH-F140SP specs 82.1cfm, 1.33mm H2O & 19dB(A) .. 2x fans = 164.2cfm, 1.33mm H2O & 22dB(A)
PH-F120SP specs 54.4cfm, 1.29mm H2O & 24.2dB(A) . 3x fans = 163.2cfm, 129mm H2O & 29dB(A)
For SP fans 2x 140mm have similar performance but much quieter.
There has been all kinds of fan testing done with all kinds of data out there, including built systems with 120mm and/or 140mm fans and their resulting temps.

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post #10 of 13 (permalink) Old 02-08-2019, 05:29 AM
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Quote: Originally Posted by anticommon View Post
I personally wont even consider a build unless 12 60mm fans are being used in place of three 120s. It's simple math.
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