I think I got a resonance problem... suggestions?

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Thomas
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I think I got a resonance problem... suggestions?

Post by Thomas » Thu Sep 10, 2009 10:06 am

The loudest components in my PC, is 3 S-Flex's at approx. 500 rpm. I thought that would be very nice - but it's still noisy.

It sounds crazy, I guess, and I dont understand why it's still annoying...

Problem: With case side panel off, my PC is very quiet. When panel is on, and the PC is under the desk, there's an annoying sound. Sometimes it's like a constant humm, at other times it's more like a woh-woh-woh at a frequence I'd guesstimate to 2-5 times pr. second. The last scenario is the most common - though it depends on where my ears is positioned relative to the PC.

It also depends on how well the room is dampened. Unfortunately my new home office is less dampened - a consequence of being far less messy ;-)

The front of the PC is approx. 1 meter from my ears, and it's below my desk.

I dont hear it much, if I listen to quiet music, but when music is off, I can easily hear it during daytime. (I live in a quiet area)

System details:

Antec P180 - fan grills cut, and the front inner doors is removed.

AMD 4850e with Asus Triton 75 bend, so the radiator part is directly in front of the exhaust fan @ the rear.

Seasonic S12 380 modified with a softmounted S-Flex, which never exceeds 500 RPM's.

2 x Hitachi 5K160 2,5", each in a Scythe Quiet Drive, soft mounted with the stock P180 silicone grommets in the lower chamber.

I've done the usual stuff with the P180: Top exhaust closed, all PCI slots closed, vents next to PCI slots and around PSU is also closed. There's a front intake S-Flex, and a ditto rear exhaust. Both softmounted. Cabling aint beautiful, but the air path is clean.

No vibrations can be felt on the case.

The rear of the PC is less than 12 cm from the wall behind it, and I've tried some clothing on the wall, in order to dampening reflections... Didnt work.

I tend to try some kind of dampening material... But I dont want to waste money and time, if it's the wrong way to go.

Oh, the front fan run about 50, maybe 100, RPM slower than the rear - can this cause problems? I tried without the front fan, but then the rear fan got a more grrr-like sound - I dont recall if the woh-woh-woh was there...

I'm a bit confused about what to do, and I'd really appreciate some suggestions :-)


Thanks,
Thomas

MikeC
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Re: I think I got a resonance problem... suggestions?

Post by MikeC » Thu Sep 10, 2009 11:10 am

Thomas wrote:I tend to try some kind of dampening material... But I dont want to waste money and time, if it's the wrong way to go.
Worth trying temporarily, even with some packing open-cell foam. Put some somewhere not touching any components, probably the inside of the cover. See what happens.
Oh, the front fan run about 50, maybe 100, RPM slower than the rear - can this cause problems? I tried without the front fan, but then the rear fan got a more grrr-like sound - I dont recall if the woh-woh-woh was there...
This could be an issue -- intermodulation between two close-freuqnecy sounds. But the fans are far apart...

It could also be those drives, even tho they are encased.

This is a situation where you have to get creative to identify the source of the noise first of all. I think you're right that it's some kind of cavity resonance -- in which case, even a small amount of high absorption (dense) foam in the right places could help. But it could also be interaction between noise sources.

Thomas
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Location: Denmark

Post by Thomas » Thu Sep 10, 2009 11:37 am

Thanks for a fast response, MikeC :-)

Though I do feel quite comfortable with english, I'm unsure what packing open-cell foam is? A link to a pic, would help :-) Same goes for high absorption (dense) foam :?:

Sorry... :-)

K.Murx
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Post by K.Murx » Thu Sep 10, 2009 12:09 pm

How about playing with the fan speeds through undervolting/...? That should clear up whether the sound is due to a beat of the fans.

MikeC
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Post by MikeC » Thu Sep 10, 2009 12:25 pm

open-cell foam -- the spongy foam used for shock damping in shipping cartons. it's open cell -- which allows sound to be absorbed (and water too, in a sponge) -- as opposed to closed cell.

high absorption (dense) foam -- like http://www.silentpcreview.com/AcoustiPack_delux and similar (all of which incorporate open cell foam)

Thomas
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Post by Thomas » Thu Sep 10, 2009 12:46 pm

Super, thanks to both of you :-)

Now I have a couple of things to try out - I have fanmate, I could use on the front fan, and see, eh, hear how it interacts with the rearfan at different speeds.

I'm also going to disconnect one of the drives, the one with more platters are the easiest to disconnect, since it aint the system drive.

And I have to check if I have some foam laying around.

Thanks again 8)

Quietstick
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Post by Quietstick » Fri Sep 11, 2009 7:53 am

Sounds like the desk could be amplifying it! I've got the same resonance issues - not loud at all but gets annoying, and made worse when I put the case in the desk computer hole. Mine also comes and goes!! Let me know what you get with the different HDD, fan speed combos!

Going to suspend my drives tomorrow as the sounds changed when I swapped HDD. :)

(I've got a dampened Sileo, which probably helps although not too silent a case at all!) :(

Thomas
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Post by Thomas » Fri Sep 11, 2009 9:28 am

Both S-Flex in the upper chamber are now excluded - I found out I could stop them, while my PC was closed and below the desk - didnt made any change in the sound.

I suspect one of my harddrives is the problem. I vaguely recall, that I used one 80 gig drive for a while, and when I added the second one (160 gig edition) there were more noise. Afterwards I made several other changes to the system and forgot about. I think it's been there for a long time, and after I moved the PC to another room, it's more apparent.

So now I'm down to the noise comes from the lower chamber, which only contains the 2 disks and the PSU. And maybe the location amplifies the noise...

It might take some time, before I have time for further testing, but I'll keep you updated :-)

new2spcr
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Post by new2spcr » Sat Sep 12, 2009 12:08 am

Thomas wrote:Both S-Flex in the upper chamber are now excluded - I found out I could stop them, while my PC was closed and below the desk - didnt made any change in the sound.

I suspect one of my harddrives is the problem. I vaguely recall, that I used one 80 gig drive for a while, and when I added the second one (160 gig edition) there were more noise. Afterwards I made several other changes to the system and forgot about. I think it's been there for a long time, and after I moved the PC to another room, it's more apparent.

So now I'm down to the noise comes from the lower chamber, which only contains the 2 disks and the PSU. And maybe the location amplifies the noise...

It might take some time, before I have time for further testing, but I'll keep you updated :-)

I'm glad you found the noise source. Yeah, I've also found some harddrives actually sound like a spinning fan.
I've also concluded that in my setup (P182) the HDD makes more noise if I have it in the lower chamber together with the PSU.

Thomas
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Joined: Mon Jun 20, 2005 11:21 pm
Location: Denmark

Post by Thomas » Sat Sep 12, 2009 8:25 am

new2spcr wrote:I'm glad you found the noise source. Yeah, I've also found some harddrives actually sound like a spinning fan.
I've also concluded that in my setup (P182) the HDD makes more noise if I have it in the lower chamber together with the PSU.
I still have some investigation to do :wink:

Your observations about HDD's in the lower chamber are interesting, thanks 8)

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