The best we can hope for out of these kinds of calculations is a rough estimate. How rough? Well, maybe the answer to the question I asked MikeC below will give us some idea.
Regarding whether a system with a GTX680 can be made to run quietly, here's one that's pretty good, done without resorting to extreme methods:http://www.silentpcreview.com/AVADirect ... _PC_GTX680
That configuration is different from the one we're considering here, but not so different that we can't make a rough comparison. When I reduce the number of GPUs and case fans to match the system you linked, substitute a Caviar Green for the Barracuda XT, and so on, my calculations come to 29dbA at idle an 39dbA at full load. But the review measured 16dbA and 26dBA. The difference matches but the level is off by 13dbA. Of course, I haven't taken into account any noise damping effects of the case.
Could it be that this is causing me to overestimate noise production by as much as 13dbA?
First of all, your method of caculating combined SPL is way off. Referring back to this...
I did some calculations for the configuration I'm currently favouring, which is this:
CPU: 3960X + Noctua NH-D14 SE2011
GPU: 3 x ASUS GTX 680 DirectCU II 4GB
PSU: 2 x Kingwin LZP-1000 1000W
disk: WD Caviar Black 2TB
fans: 7 x Yate Loon 140mm
case: Rosewill Blackhawk Ultra
This system will put out noise as follows at idle:
CPU: 14 dbA x 1 = 14 dbA
GPU: 15 dbA x 3 = 25 dbA
PSU: 10 dbA x 2 = 16 dbA
disk: 29 dbA x 1 = 29 dbA
fans: 10 dbA x 7 = 27 dbA
total: 38 dbA
This will increase to the following at full load:
CPU: 28 dbA x 1 = 28 dbA
GPU: 28 dbA x 3 = 38 dbA
PSU: 10 dbA x 2 = 16 dbA
disk: 34 dbA x 1 = 34 dbA
fans: 22 dbA x 7 = 39 dbA
total: 48 dbA
Not sure where you got the SPL numbers from, but assuming they are accurate, your SPL "addition" is way off. The way SPL adds is like this: Assume identical noise sources, say 15 dBA@1m (cited for your GPU). Two of them close to each other (and still 1m away from you) will measure 18 dBA. Four of them will add up to 21 dBA.
But when you have multiple noise sources will making different levels of noise, all with different frequency spectrum balances, then it becomes almost impossible to "calculate".
We can, however extrapolate from the data I measured for the AVADirect single GTX680 system:
1) assume that the 2011 CPU you chose can be kept adequately with a HSF that makes no more noise than the one in the AVAD system (it can, I guarantee you)
2) The single loudest noise source in the system is the video card. The HDDs, etc are all way below the noise level of the video card, and contribute virtually nothing to the overall SPL. My guess is that if all other noise sources in the AVAD system I tested were eliminated, the overall SPL might drop by 1-2 dBA. And pulling out the video card would bring the overall noise down to perhaps 20-22 dBA at full load. Odd, isn't it? But that's the way it works -- and measures & sounds.
3) So with your 3-GPU system (which strikes me as an expensive way to get marginal performance benefits, but that's my opin & it's not my work)... if all the video cards could be cooled equally well with the same fan speed
as in the single GTX680 AVAD system, then in a even with a couple more case fans, the overall SPL would be under 32 dBA@1m at full load. At idle, I expect it might be 20~21 dBA (assuming good soft mounting for the HDDs and intelligent selection of quiet case fans).
But the biggest challenge with 3 GTX680s is how to prevent one or more of them from running way hotter & louder. The pics of the Silverstone case system posted earlier show the 3 cards jammed up against each other; assuredly, these cards' fans will not all run at the same speed, my guess is that the one in the middle would run way faster (and still be much hotter) than the one at the end whose fan has open access. Are there any motherboards w/3 or more PCIe slots that have two slot spaces between them? Without that, you'd have to mod all the cards with massive passive aftermarket coolers and some way of getting large amounts of outside air directly onto them w/ fairly high pressure and without making a huge racket. Big challenge, but this would be quieter than leaving any stock GTx680 coolers to fend for themselves in a tight 6-card-slot space.