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I am new to silent pc building so if I am making some lame assumptions please correct me.

For example this is claimed properties of Noctua NF-S12-1200 fan.

          airflow     noise 
 1200 rpm   81 m³/h   17 dB
  600 rpm   41 m³/h   <6 dB

If I recall high school physics correctly two 6dB are summed as this:

 10 * log ( 2 * 10^ 0.6) = 9dB

So two Noctua fans running 600rpm should make considerably less noise that one fan running 1200 rpm while airflow is unchanged.

Is this just a property of particular Noctua fans or is this a general principle?

Also how does my theoretical assumption about noise addition stand in reality? What about case resonance?

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Rubber gaskets. – Ignacio Vazquez-Abrams Nov 25 '10 at 19:55
The #1 resource on silent PC building i Silent PC Review. They have tests (with normalized measurements) on many fans including the NF-S12-1200, as well as in-depth articles. And yes, your reasoning does hold out well in practice. – Gilles Nov 26 '10 at 0:00
up vote 12 down vote accepted

Adding of two identical sound sources will increase the total sound power level with 3dB (source)

So yes, two fans at half speed will shift the same amount of air with considerably less noise. This (with minor variations) will be true for all fans.

Any case resonance issues is pure speculation as it varies on a case by case (pun intended) basis. I doubt it will be a problem.

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Exception: 1U servers are +3db per each device, when using with 6 hdds and lots of coolers – kagali-san Nov 26 '10 at 2:01
Yes. You did not say, and could have added, that fan noise goes up (not always linearly) as fan RPM goes up. – quickly_now Nov 26 '10 at 6:08

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