Of course two fans running at 6V are more silent than 1 fan at 12V (pushing the same amound of air). Two fans are, well, twice as large as one fan, giving the effect of a 17 cm fan running at 6V.
In fact you have to count the sound of the fans self doubled. This can be ignored however, as with good quality fans the only sound is the sound of the moving air.

You mentioned Frequency Balance in the first post and i think this is an important issue we need to get back to and discuss. Two identical fans spinning at the same RPM with have roughly the same spectral balance of sound if all else is equal, vent openings, air impedance (loading) etc. and acoustically will follow the +3dB rule.

However. several other factors also come into play. one is the point source rule. no two objects can share the same point in space and when you have two noise generating devices you will also have additive and subtractive acoustic effects due to the distance between them. this is called comb filtering. Without getting mega technical (If you wish, start here http://en.wikipedia.org/wiki/Comb_filter).

So in our example. we have two fans and lets say they are spinning the same speed. depending on where the microphone is in relation to the two fans. the spectral content will change. if you "pan" the microphone across a plane from the fans, you will see certain areas of the spectrum change as you move between peaks and nulls of a particular frequency.

Fans generate an almost white noise and as such this is harder to measure then it sounds in free air. But any monotone steady state noise such as harmonics, hum or noise created from stationary fins on the fans case, vent holes etc. that the fan creates can have effects beyond whats expected.

But vary the speed of one fan slightly and you can spread those peaks and nulls out over a larger area so that less are bunched up all at the same areas. just make sure the fan speed is not a harmonic multiple of the first fan speed. IE 1000rpm and 500 rpm. you would be better off with 1000rpm and say 850rpm or 625rpm etc. and by varying one fan speed you can find what works best.

or if you need to keep the total CFM the same, raise one fan speed and lower the other. if the fans are side by side you can also "direct" air flow slightly this way.


Zc

I know this is an old topic but i thought i'd throw in my piece.

The +3dB doubling only applies when you have coherent wavefronts. In the case of two subwoofers next to one another, it is possibly to achieve because the frequencies are low, and the wavefronts combine to produce a higher pressure waveform.

For the waveforms to be exactly coherent with two fans opposite ends of a pc case, you would have to be seated exactly on axis between the two of them. The case and componentry would have to be symmetrical, they would have to run at a synchronised rpm, and the same blade would have to be facing you when they started. These conditions woud be physically impossible to achieve, the +3dB rule would only applies to very low frequencies, you would have to measure the distance between the 2 fans, and then apply the 1/4 wavelength rule blah blah.

the result with two non-identical sources is the comb filtering effect described previously. Next time you see big flown speakers, the great big vertical lines, try and get a look through the grill. You'll see that the smaller the speakers get, the closer they have to be to get coherent, until the tweeters just fire through narrow slots.

Onto far more subjective evidence, i've found that a push-pull configuration front and back has always sounded quieter. I think it has been illustrated in the fletcher munson curves, keeping the frequencies low to avoid that sensitivity in the critical range keeps a system inaudible.

wanted to post to this old topic too.

Agree, with the above.

The interactions between 2 fans can lead to a more dissonant sound than a single fan that is 3db twice as loud.
You may hear the "beats" from the wavefront interference as compared to just a constant noise of a single fan. You can't fine tune fans well enough, so the closer you get the fans to identical, actually will result in the more annoying and obvious "beats".

This comes into play even more if you have another noise sources such as an HDD, and the slow interval "beats" wavelength of the 2 fans will at some frequency cancelling out that sound of your HDD in periods which leads that noise to cycle up and down compared to the single fan that would have just resulted in at least a more acceptable constant noise.