three 120mm fans or two 140mm fans - which will have more airflow at low RPM?

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I have my Phanteks Enthoo ATX case and it comes with 2 x 140mm front intake fans but apparently they can be replaced with 3x120mm fans. Now the fans in the case are Phanteks f140sp and the 120mm fans i have are Noctua nf-p12 fans. Those Noctuas are designed for CPU coolers but they still shift air and are really quiet. Now the tricky thing im having issues with is they will be running on a cooling profile that will make my PC silent so they wont be running anywhere near full speed so the tech specs of speed/noise dont help me, the thing im wondering is are 3x120mm fans going to shift more air when they are running at low speed than 2x140mm fans. Does anyone have any ideas?
 
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If silence is your goal then fewer, larger fans should be better. If you're running all the fans slow then any additional airflow from a third, smaller fan will be negligible. It's very hard to find full range chart data for any fan and since the airflow to speed is not linear then it's hard to calculate. Stick with the 2x140 if minimal noise for performance is your aim.

EDIT: Think of it this way, assuming the same grade and design of fans: 2x140 will likely make less noise than 3x120 at the same speeds. This will allow you to run the 2x140 slightly faster to achieve the same noise level as the 3x120 and thus negating some of the advantage from the higher airflow from the 3x120.
 
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What Bugbait siad, 2x 140mm fans flow as much are and are usually a little quieter than 3x 120mm fans moving same amount of air. 140mm fans spin slower than 120mm fans with similar overall airflow. ;)

CoolingTechnique test fans every volt or every 10% PWM giving cfm and static pressure ratings. Still not as nice a if we could get cfm at specific resistance, but that is very hard to do.

Are the fans really PH-F140SP fans? Or they the newer ones that use PH-F140SP housing and motor with PH-F140MP impeller? Phanteks changed case fans over a year ago but still has old model showing in specs.

Here's image of HP and HP_II cooler fans, but same impeller differences between original SP case fans and new ones. New ones are slightly better, but not by much.
attachment.php
 
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What Bugbait siad, 2x 140mm fans flow as much are and are usually a little quieter than 3x 120mm fans moving same amount of air. 140mm fans spin slower than 120mm fans with similar overall airflow. ;)

CoolingTechnique test fans every volt or every 10% PWM giving cfm and static pressure ratings. Still not as nice a if we could get cfm at specific resistance, but that is very hard to do.

Are the fans really PH-F140SP fans? Or they the newer ones that use PH-F140SP housing and motor with PH-F140MP impeller? Phanteks changed case fans over a year ago but still has old model showing in specs.

Here's image of HP and HP_II cooler fans, but same impeller differences between original SP case fans and new ones. New ones are slightly better, but not by much.
attachment.php
The fans that i got are the PH-F140HP II according to your image, they have 7 blades. So i guess i should keep the two 140mm then, are these phanteks fans decent or would buying some Noctua fans be a better option?
 
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The fans that i got are the PH-F140HP II according to your image, they have 7 blades. So i guess i should keep the two 140mm then, are these phanteks fans decent or would buying some Noctua fans be a better option?
Like I said in above post, image is of cooler fans to show the difference in impellers. It is not an image of case fans.
 
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EDIT: Think of it this way, assuming the same grade and design of fans: 2x140 will likely make less noise than 3x120 at the same speeds. This will allow you to run the 2x140 slightly faster to achieve the same noise level as the 3x120 and thus negating some of the advantage from the higher airflow from the 3x120.
I think it's worth clarifying here. 140mm fans won't be quieter at the same speed, they'll be quieter at the same airflow, which is what matters. Speed is just a directly related number, but we care about noise, airflow or both right? As doyll said, you can run the bigger fans slower and so quieter, if aiming for a particular amount of airflow.

Think about a train at 100mph versus a car at the same speed. Bigger stuff is noisier
 
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That is fans of same Db pressure level. If I remember correctly 2nd fan increases noise level by 3dB, and adding a 3rd increases dB another 2dB. But it's not just about fans' own sound pressure level but also about airflow noise through grills and filters and what the sounds sound like to our ears. Some sounds even at low pressure level sound harsher to our ears than others at higher levels.
 
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