SPCR

I have scythe Kamabay, where I've installed that particular fan. Its almost inauditable even with 12v just the air swoosh is auditable. Now I have scythe's own filter, but I find it rather stressful once a week remove fan and clips to take out the dust. (its a dusty place)

So I added 2nd self made filter outside. Its made thickish foam that passes pretty well air. The one you see as concussion material with motherboard packages etc. Now as soon as I added that extra filter, Noctua started to howl. I tried two different Noctua's and they howled when extra filter was added. When filter was taken howl stopped. Howls also stops when you lower voltages enough and reduces the spinning speed.

Now this is pretty intresting...

I would say that you are hearing air being squized thru those tiny holes in "filter"...

Hmmm, I am not that sure, since its similar than to that ~550 Hz howl someones have complained. I guess, fan has to take extra work to get that air which could cause the howl... Since sound is clearly electro-mechanical and not sound air.

Hello,

The higher static pressure that is induced by the second filter might be causing the high-pitch blades of the Noctua fan to "cavitate" and it may also stress the bearings to a point at which they start to make noise.

This confirms what has become the accepted conclusion about the design of the Noctua fans: they work well in situations when the static pressure is kept low, but they lose their advantage when used in a situation with higher static pressure; like on a HS with closely spaced fins, or a restrictive or filtered case.

Same with my 1200 exhaust. It makes a whine/howling noise if I give it full power from my Akasa Fan Control Pro and close the door on my Sonata II. Definitely a pressure issue, as opening the door or lowering RPM will quickly render it quiet, and the noise is most definitely not from air squeezing through passages.

The fan goes silent at what is reported as 1200 RPM by the controller.

The higher static pressure that is induced by the second filter might be causing the high-pitch blades of the Noctua fan to "cavitate"

possible, definitely possible. It would be interesting if someone could record this sound and find the frequency peaks, see if they correspond to the blade-passing frequency or some other obvious mechanism.

*looks at integrated microphone in Logitech Quickcam Pro 3000*

Ummm... not me.

When I feel more energetic and I find my cheapo-budget-no-name-mic, I could try give a shot of recording it... Its not a promise though.

It's most probably the blade design, it howls thru the top blow holes on the p180 when blowing in and either on the bottom cage or the top middle one (w/ brackets), if it's maxed to 920 with speedfan (2x800 here), the frequency is the same. Something in the middle registry, 600-900Hz'ish if i had to bet.. which is a bitch for a daw.. wish i'd gone with the s-flex 1200, the higher rpms would be more welcome than howling with my 3 fan config. Only way to work around it was sacrificing some temp with the scythe that came with ninja on the top cage for my passive gainward 8600gt, one noctua in the lower cage undervolted with the cable and another on the rear exhaust. Wish speedfan could read the 8600s temps since it's whats contributing more heat while idling. Scythes swosh@1150 is way better than the 920s howl, sounds like a muffled fog siren to me

puscifer wrote:It's most probably the blade design, it howls thru the top blow holes on the p180 when blowing in and either on the bottom cage or the top middle one (w/ brackets), if it's maxed to 920 with speedfan (2x800 here)--

Over 800 RPM is probably too much for the fan. I ran my 1200s maxed out, but it caused the howling to occur. Dropped to 1200 RPM and the howling stopped.

By "howling through top blow holes" do you mean that's where the sound is coming from, or that there's an 800 mounted horizontally there that howls?