Help with low frequency vibrations

ColdFlame · Post by **ColdFlame** » Mon Nov 15, 2004 3:25 pm

Hello all,

I'd appreciate if you could help me troubleshoot the behavior of my machine.

It is a "Ralf reference design", 3700AMB with 120mm fan @5v in the back, cool and quiet Celeron 2.4 Ghz and nice Fortron 300W PSU. It also has 3 hard drives (2x160GB Seagate 7200.7 and 1x200GB Seagate 7200.7) that I recently put there - the troubles began after I put the hard drives in (I used to have 2x200GB WD drives in there which were quite noisy at idle).

The problem:
I can hear my computer from another room (downstairs, the PC is on the 2nd floor) and all I hear is some low frequence rumble, like 1 second of noise and then about 0.5 seconds of silence. Turning that PC off gets rid of the sound. I can't hear much in the room the PC is in - and the PC is pretty quiet. There is some subtle noise but nothing like I hear from downstairs.

Any suggestions where to look? I tried disconnecting the hard drives and it seems if I disconnect all 3 then the sound goes away, if I disconnect just 1 then the sound becomes quieter but is still there.

The drives are grommeted using McMaster grommets (note: I've been using those grommets for another machine and reused them here - I hope they aren't single use, are they?)

Could it be that the PC case serves as the amplifier for the vibrations and "converts" quiet 7200 rpm into loud 50 Hz or something?

Any help or suggestions would be much appreciated. I have a lot of hardware at home so I could do some experiments.

Post by **Rusty075** » Mon Nov 15, 2004 4:59 pm

You've got resonance.

The frequency of the vibration of the HDD's is always slightly different, and if it works out just right (er, wrong)....you get what you've got.

Drives spin at very slightly different speeds, and therefore vibrate at very slightly different frequencies. The different waveforms alternate between cancelling each other out (the quiet times) and reinforcing each other (the loud humming).

The solution is pretty simple though: suspend them

You're only getting partial decoupling, which means that some of the vibration is still being transferred to the case.

The other solution may be to apply lots of mass dampening to the case itself, but that's significantly more work than just decoupling the drives.

ColdFlame · Post by **ColdFlame** » Mon Nov 15, 2004 11:14 pm

Oh thanks a lot Rusty. I will try to disconnect all hard drives but one and I assume the sound will disappear.

I'm afraid an option to suspend the drives is too difficult, I will try to do something with the case, like put it on a mat or something.

At least now I know what this is!!!

wim · Post by **wim** » Mon Nov 15, 2004 11:52 pm

probably he (Rusty075) knows that it's not, but the post reads a bit like he's calling this thing resonance..

just wanted to clarify that what he's talking about is not called resonance but interference (or more specifically beat cancel)

burcakb · Post by **burcakb** » Tue Nov 16, 2004 12:00 am

Coldflame, hdd suspension is actually quite easy in a 3700AMB.

Just take out the hdd cage, turn it on its side and suspend it. Put the drives in the middle, use the outermost grommets to suspend.

I'll post a link to what I did.

http://www.burcakbaskan.com/PC/Thor/HDD ... erview.jpg

http://www.burcakbaskan.com/PC/Thor/HDDInstalled.jpg

Post by **Rusty075** » Tue Nov 16, 2004 12:06 am

It's resonance that makes the noise, interference that makes it modulate.

maxxxim · Post by **maxxxim** » Tue Nov 16, 2004 12:44 am

Remove side panel and listen again. If vibration is gone then put something soft (rubber pieces or something like that) between side panel and the case.

Post by **Ralf Hutter** » Tue Nov 16, 2004 6:14 am

burcakb wrote:Coldflame, hdd suspension is actually quite easy in a 3700AMB.

Just take out the hdd cage, turn it on its side and suspend it. Put the drives in the middle, use the outermost grommets to suspend.

I'll post a link to what I did.

http://www.burcakbaskan.com/PC/Thor/HDD ... erview.jpg

http://www.burcakbaskan.com/PC/Thor/HDDInstalled.jpg

Hey, that's verrry cool!

burcakb · Post by **burcakb** » Tue Nov 16, 2004 7:58 am

Full report coming up - as soon as I mod the 7000B with an undervolted Nexus and put the fanless heatsink on the R9700. It's quiet as a cat atm.

hvengel · Post by **hvengel** » Tue Nov 16, 2004 9:00 am

Rusty075 wrote:It's resonance that makes the noise, interference that makes it modulate.

In music the ¨interference that makes it modulate¨ is called a beat. As the pitch of the sound sources gets closer (they become more in tune) the beat freq slows down. If you are tuning two strings on a piano and they are off you will hear something that sounds like wa wa wa wa wa and the wa wa´s might be at say 10 beats a second. As you pull the strings into tune with each other the beat will slow down until it goes away when the strings are in tune. A good piano tuner can listen to the beat rate and he/she can tell you how far off the pitch of the two strings is. As in those are about 10 cents apart (100 cents = 1 semitone).

Since in ColdFlame´s case he has three sound sources that are near the same freq he is hearing an irregular beat pattern that is divided into three parts and cycles about every 1 1/2 seconds.

peteamer · Post by **peteamer** » Tue Nov 16, 2004 9:19 am

Sweet, burcakb. Really like the ghettoness of it... although it looks so professional.

One burning question though... Why the little bit missing off of the bottom right of the second .jpg ? That's really got my curiosity going... (should I get out more

)

Pete

starcycle · Post by **starcycle** » Tue Nov 16, 2004 10:03 am

burcakb wrote:Coldflame, hdd suspension is actually quite easy in a 3700AMB.

Just take out the hdd cage, turn it on its side and suspend it. Put the drives in the middle, use the outermost grommets to suspend.

I'll post a link to what I did.

http://www.burcakbaskan.com/PC/Thor/HDD ... erview.jpg

http://www.burcakbaskan.com/PC/Thor/HDDInstalled.jpg

wow, that is quite a solution. does the cage move around at all? it doesn't look from the pics like it's achored anywhere.

lenny · Post by **lenny** » Tue Nov 16, 2004 11:29 am

peteamer wrote:(should I get out more )

I think you should

I think it's just the corner of a ribless fan, and the white's the background.

And *I* should get out more for responding to this...

burcakb · Post by **burcakb** » Tue Nov 16, 2004 11:35 am

The cage moves very little. I used the last of my elastic cord so it was a little shorter than desired. I'm stocked back right now and I'll probably redo the setup a little lower. The cables (power & sata) push the cage forward. That's why I put the rubber bumbers there - in case it moves too much. On the left side (the side panel) there's a bit of melamine foam. It doesn't move right enough to bang the right panel.

pete, if you mean why the last 2 cm of cabling isn't covered, I ran out of Dr.Cable and was too tired to sleeve. If you mean the little rectangular piece of the jpg, then YES YOU DO HAVE TO GET OUT MORE because that's just the white table cloth showing under the nexus fan !!!!

peteamer · Post by **peteamer** » Tue Nov 16, 2004 11:51 am

......

peteamer · Post by **peteamer** » Tue Nov 16, 2004 11:55 am

P.S. Hadn't even realised the last two cm were not covered...

... ought to pay more attention...

......