I was just wondering: is it better (i.e. quieter) to have one 140mm front intake case fan running at a higher rpm, or two 120mm fans running at lower rpm? (everything else being equal).

I'm assuming two 120mm fans would create more airflow, but not sure if the acoustics would be simply double the db/A of each fan?

The two 120mm fans would have roughly 50% more air flow, all else being equal, so they would be quieter. Two fans are slightly louder at the same RPM as the single fan, but they could be significantly slower RPM.

The other piece, though is whether the system would be close to balanced intake vs exhaust, or slightly negative or positive pressure. I prefer slightly negative pressure cases, as an exhaust fan is more efficient at pulling air, than an intake fan is at pushing air.

So the noise of two fans would not be a simple linear doubling of a single fan, then? Great, that's good to know. Thanks.

No - I think the rule of thumb is if they are running exactly the same RPM, then there is about a 3dB increase of noise. If they are quiet fans - say 20dBA, together they would be 23dBA. Since they could be run at about 25-33% slower than the one 140mm fan, the pair of 120mm fans would likely be quieter.

+3 dB = double the SPL.
The main problem with running two (or more) fans is that there's a risk of interference that might lead to a low frequency variation or humming in the overall noise. Controlling the speeds of those fan independently can help minimising the problem but then require the fans to run at a constant speed independent of computer load and room temperature.

My mistake - I thought that 10dB was a doubling.

It's messy...if dealing with SPL meters, 6dB is a doubling. Here's a fun read.

Ok, what about two 140 mm fans running slower than two 120 mm fans?

I guess my questions all boil down to, what is the best configuration of front intake fans for the best balance of airflow and quietness?

I have a Fractal R5. I'm still not sure whether to go with one or two fans in the front, or what size of fans.

Here's my opinion: more exhaust air flow is better / quieter - for two reasons. The main one is that exhaust fans are more efficient - since lowering pressure is going to shift air far away from the fan. The best illustration of this is a whole house fan - it exhausts air from the attic, and if the only windows open are in the basement, air will flow all the way through the house.

The second reason is simply that the exhaust fans are a a bit farther away from the user, typically.

For each setup is different, usually the first fan is the ones that will net you the biggest benefit, the second usually is also a good gain, from then on the diminishing return scale up quickly. Read Bit-Tech The Big Air Cooling Investigation, and if you can test it with your own setup, at least one to three fans is what i would try. Personally i prefer positive pressure regardless because im lazy and dislike cleaning too often the pc.

So it seems, in general, it's a little more effective to pump the heat out of a case than to draw cool air in. And also that article found that having a side panel fan can be very beneficial.

I will first try putting one 140mm at the front, mounting a 140mm on the side panel, and putting a 120mm rear roof exhaust nearest the CPU (in addition to the standard back rear exhaust fan).

The good thing about the Fractal R5 case is that there is a side panel mount, plus the modu-vents on the roof come off in three pieces, so you can adjust the size of the roof opening. With the R6, the roof is one piece, so the whole roof is either on or off.

Thanks Abula.

Here's another variable: what you are trying to cool inside the case. A high powered gfx card may benefit from a directed case fan allowing the gfx card's fans to run slower/quieter and have a total system that's quieter than with one input fan.

If you don't have a gfx card, then really, we're just typing here. One input fan (or for Neil, no input fan) is fine.

As far as 120 vs 140mm, it's a cfm vs rpm (noise) thing. I don't have time to go data mining, but gut feel is two 140's running at minimal speed can provide more cfm at lower SPL than two likely 120mm candidates having to run faster for same cfm.

The GFX card will be an MSI 1060. So, maybe I won't need a side panel fan, but the article said it was good for cooling the chipset as well.

I think I will at least try the two rear exhaust configuration. Would two 120mm be better?

Generally I agree with this, but with one important caveat:
Make sure to close off all unwanted air intakes to get optimal paths for the air from any (remaining) opening past some hot part before reaching the exhaust fan.
* Most cases today have perforated rear plates that can be bad if they work as intakes and draw warm air just expelled from the case. Worse if the entering air isn't used by the graphics cooler.
* Top openings are also extremely common, and should be closed if not used as exhausts.
* If dust is a problem, so filtered air intake is mandatory, then it's very important to seal any opening that isn't filtered or has an exhaust fan.

When dust protection is a high priority it's easier to just have some relatively powerful fans pushing filtered air into the case, since then one can be more relaxed about the placing of unfiltered openings.

Olle, I agree about having controlled air flow through the case. Something I learned recently is that top exhaust fans are more effective than back exhaust fans, since warmed air rises, and top fans "work with" this.

The graphics card is a key area - if the case is narrow, and depending on where intake air can come from, it is important to prevent a "hot pocket" of stagnant air from being formed below the graphics card.

As a schematic design, I really like the Silverstone FT-05 - and ironically, this is an exception to having more exhaust fans; with its two 180mm intake fans located at the bottom of the case. The key aspect is the position of the motherboard - rotated so the I/O panel is at the top of the case. This lets the warm air from the graphics card(s) flow easily upward.

That case though, has very awkward hard drive bays, and not great cable management.

The FT05 is a very weird case, in paper it should be the Air King, but in reality its not, on the CPU yes, it perform extremely well, even dropping the included fans below 500rpms, but on the GPU.... idk its like between the HDD mount/PSU location it affects the airflow that the GPU needs, on GTX970, then GTX1080, and finally on a GTX1080ti, i never manage to get as good temps as i get with the Corsair Air 740, i still have the FT05, i want to test without he hdd cage and the PSU, i want to try to have an external PSU to see if this is the reason, the FT02 design is much better, i feel FT06 should adopt the reverse side, that way the CPU cooler is next to PSU, this would allow the GPU to have the full airflow of the AP181, there could be even an extreme version with side case fans.

While it's true that warm air rises (if left undisturbed) this effect is so small that it's negligible if there are any fans involved.


With air intake at the front (as is the norm) top mounted exhaust fans will draw the cool air out of the case while leaving the graphics card in such a hot pocket.
That's why I above suggest having intake fans at the front, sealed top and sides, and no exhaust fans but mesh all over the rear. That way the general airflow inside the case will go straight from front to rear while passing all fanned coolers and leave no hot pockets.

The FT05 use the same principle, but has directed fans that might actually reduce the cooling efficiency.

So you recommend having only front intake fans, and no other fans, with everything sealed up except the back vents?

Why no rear exhaust fans?

(Assuming a regular tower case, or similar, is used.)
It's "difficult" (read: as good as impossible) to place an exhaust fan behind the graphics card.
Exhaust fans above the graphics card will a) reduce the flow of fresh air to the volume below the graphics card (where its cooler typically need it), and/or b) draw some (unfiltered) air in trough vents at the rear, some of which will be the heated air just expelled.

Using intake fans only you'll get a slight overpressure inside the case. All fans inside will have lots of air to draw, while the general flow is from front to rear. The common mesh/grill areas at the rear of today's cases will come to their right.

My own case, an Antec P183, has only one intake fan. To make the flow efficient I've made an internal duct/funnel that direct the intake air towards the motherboard below the graphics card.
That way it's all available for the graphics cooler. Then an exhaust fan (and the CPU cooling fan) draw some/most air upwards, through the CPU cooler, and out at the case rear.

My thought is that greater air flow out of the case will be better at minimizing hot areas, including around the video card. Positive pressure = slower air flow, and is much more likely to have stagnant pockets of air.

More fans will generate more flow.
No general difference between positive and negative pressure, other than that of filtered vs unfiltered air.
Hot pockets should only emerge where no cooling is needed anyway.

To put it simply, if you have a frontal air intake and a rear air outlet, then some of the intake fans will blow air straight into the outlet, and you'll have a loss of fresh air. I think that having the outlet fans on the top of the case would make sense as hot air rises, and then you could optimise this by having your intake fans located quite low on your case.
If you can't have your outlet fans on the top of the case, then having them on the side (perpendicularily to the air flow of the intake fans) would be a decent solution, that's what I run (I modified the outer shell on my computer in this purpose) and it works fine.

With a case that has an inch or so of space underneath the bottom, should the PSU be placed fan up, or fan down?

PSU fans are intake fans. So, if there is a mesh opening for the PSU at the bottom of the case, then by all means use it by facing the PSU fan down. The unasked question is should you put a case with a 1" clearance on carpeting...and the answer is no.

Thanks. Just thought I'd ask here because Jayztwocents generally recommends placing the fan up regardless of space underneath the case, as he feels the PSU can usually take advantage of cool air coming in from the front intake. Also because with the fan down, the PSU could potentially be drawing it's own warm exhaust air in from the bottom.

https://www.youtube.com/watch?v=h8XP9AksAP8

When I've build systems for my gamer family members they always want the PSU positioned fan up. Their reasoning is that under gaming load conditions the PSU will act as an additional exhaust, clearing hot air away from the region below the graphics card. I have pointed out that this means that the PSU fan will probably run faster (and potentially noisier) but the attitude is that given the noise already generated by the CPU, case and GPU fans under heavy load it does not make much difference.

I agree to not making so much difference, but since moving to Corsair Air 740, having two 140mm fans on the bottom drawing fresh air to GPU there is two gains, lower GPU idle temps (since now most of them stop their fans) and 4C lower load temps with the fans below 850rpms, overall its not a huge change, but allows the GPU to remain with its fans on 1300rpms or so, where its not audible on my seating position, the PSU with the fan side up on load, will have a reverse effect on this, probably is very small and might not affect the performance or noise of the gpu, but on my planing its backwards, i try to help the gpu by any means that i can. But lower tpd gpus might have even a lower effect or difference, so i do agree in terms of having little impact.

I only have one build left with PSU on the bottom, R6, and its drawing air from the bottom, specially since it also has a shroud and ElGato 4k60 on the bottom PCIe slot, so passively it would heat up this section, actively.... maybe it helps, hard to say, but i prefer to have it down.

I always put my psu fan underneath my case.

Firstly there are dust filters there.

Secondly I use solid Corsair PSU's (RM or better) and they are efficient, not only that but their fans only come on under extreme conditions so 99% of the time exhaust fan heat going back into the case is moot.

During the three + years that I ran my pc completely passive (in a case with a lot of ventilation) I never heard the PSU fan come on, and I would have as it was the only fan in the system; and I never had a problem either.