PC CPU fan

I'm getting a sort of 'beat' effect from my PC. It seems to have been triggered (or maybe just increased to a level at which I've become aware of it) by adding a new (third) HD.

I'm at my desk, and the PC is on the carpet 3 feet away, at the side of the desk. I can just hear this beat from here, behind the familiar high frequency fan and HD spinning sound. Kneeling on the floor with my ear by the PC case, I can hear the beat distinctly, at about 1 cycle per second. But my real gripe is that directly below, downstairs in the lounge, it's surprisingly distinct. By no means deafening, but it is intrusive. I'm aware of it even while watching TV. This is through carpet, underlay and 2 layers of plasterboard.

I did also have mechanical noise from the fan (which itself may have been due to multiple causes, such as bearings, dust and one loose screw). But I've fixed that, and am left with this pesky beat. It doesn't appear to be anything I can stop by touching any PC panels, or even holding the fan itself. So I assume it's some sort of beat in the air itself, caused by the rpm speeds of various components being very close together? Rather like 'heterodyning' in radio terms?

I'm not sure what the best approach is. Presumably I can take the beat frequency out of this apparently sensitive LF region by lowering the CPU fan speed rpm a bit? I could wire a couple of series diodes in the supply. Maybe as an initial step I should buy a new 60mm CPU fan, which is surely unlikely to be similar in rpm speed to the present one.

Any advice would be appreciated please. Anyone else had similar behaviour?

--
Terry Pinnell
Hobbyist, West Sussex, UK

Not at all, you can do it too. The beat he's hearing is the same as tuning a musical instrument. My question is: how accurate is the '7200' RPM? Would different models be phase locked or are they just 'close'? AND, is it a balance issue with the drives (or fans)? GG

At 1Hz? I doubt it, unless it is beating against something else (like another drive ;). I'd suspect the fans before the drive(s). Disconnect one at a time and listen for any change.

I've had Windows change drive letters for no apparent reason too. It's easy to change 'em back though.

--
  Keith

That makes sense, but holding both sides doesn't diminish the beat. I can now *feel* it as well as hear it (~ 1 Hz). Any other ideas on stopping it? How about that rpm reduction I mentioned?

--
Terry Pinnell
Hobbyist, West Sussex, UK

You'll need to isolate the cause of the sound. Try stopping the CPU fan briefly either by putting your finger on the centre hub of the fan or unplugging it. Don't worry about overheating, an idle CPU will be OK for several minutes without a fan before it gets uncomfortably warm. If the noise is still there, go through all the other spinning devices one by one. Disconnect hard drives and case fans and see if you can isolate the offending item. Still no luck? Disconnect the cpu fan, case fans and all the hard drives and power up. The only rotating item should be the power supply fan. If the noise is still there, that's the problem.

I've seen quite a few noisy PSU fans, and the way they are mounted means they can transfer a lot of noise and vibration to the case so it acts like a resonating chamber. To confirm a noisy PSU fan, you can temporarily disable it safely by powering off, inserting a plastic rod carefully into the blades and powering back on. I've even stopped moving blades with a plastic implement without any damage, but it's unnecessary for your purposes. Never insert anything metallic or conductive. Don't run the PC for longer than absolutely necessary with a disabled PSU fan, they get very hot rather quickly!

Dave

Thanks, I'll schedule that for when I don't need the PC for a while!

But if this is a *beat*, presumably there are two components, not one?

It may be coincidence, but I've only become aware of this since installing a third HD - a 200GB Maxtor Diamond, 7200 rpm, in addition to my two 60GB Maxtors.

BTW, when it comes to disconnecting a HD to test its impact on the noise, can I just close WinXP, remove the power plug from it and reboot? On later reconnecting it, will WinXP have to go through any hardware wizardry? My experience of that is mixed, so I like to avoid it whenever possible!

--
Terry Pinnell
Hobbyist, West Sussex, UK

Not necessarily, it could be the natural resonance of the component/PC casing.

Could be the new hard drive isn't fantastically well balanced. For example, I've had UW SCSI drives create quite a racket as they can vibrate a bit more than slower devices.

Yes, but you don't have to let Windows boot to test for noise, just hit pause at the POST screen, or hit F8 to stop Windows starting.

No.

Best to not let Windows boot then. I've seen windows change drive letters under these circumstances, which may or may not be a problem for you.

Dave

Thanks a lot for the follow-up.

--
Terry, West Sussex, UK

How many HDD's have you? I had a similar problem when I added a second 7200 HDD to my system. It was a complete ba£T@rd to solve the problem, eventually cured by moving the second drive into an external USB2 enclosure.

This new one is the third. I had to buy a 5.25" tray for it, to install it below my DVD & CD drives. All 3 are 7200 rpm.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Thanks Glenn. All 3 of my HDs are Maxtors (2 x 60GB, and this latest

200GB), and all are 7200 rpm. Are you speculating that one might be spinning at 432.000 kHz, and the other at 432.001 kHz?!

--
Terry Pinnell
Hobbyist, West Sussex, UK

It is usually the sides of the case behaving like a drum. Try damping both sides with your hands, and see what happens.

I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'PC CPU fan', on Mon, 3 Oct 2005:

Yes, with my old DIY computer with two hard drives.

What is your computer standing on? If it's rigidly connected to the building structure, the beating frequencies will get into the structure and appear, amplified, in all sorts of strange places, as you found.

Try standing the computer on a THICK (think 75 mm) of none-too-soft PU foam, that doesn't collapse under the weight.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'PC CPU fan', on Mon, 3 Oct 2005:

I think you multiplied by 60. 7200 RPM is 120 Hz.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Terry Pinnell wrote (in ) about 'PC CPU fan', on Mon, 3 Oct 2005:

Try loosening its fixing screws. If that stops the beat, tighten slightly at random until the beat comes back. Reverse last step. Relax.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Oops! That makes a lot more sense, thanks .

So is it plausible that a HD1 could be spinning at 120 Hz and HD2 at

121 Hz (7260 rpm) to cause such a 'beat'?

--
Terry Pinnell
Hobbyist, West Sussex, UK

OK, will try that, thanks. As the tray has to be removed to get at the screws, that potentially implies means a lot of rebooting though.

It's too soon to be sure, but I think I've made an improvement by not using the 'locking knob' after attaching the side panel of this MESH UK Athlon 1800 PC. The LF hum is still there, but it sounds almost steady now, rather than beating.

I may also try some self-adhesive insulation of some sort on that side panel.

--
Terry Pinnell
Hobbyist, West Sussex, UK

It's the resonant frequency of the floor itself. I'm ass-u-me-ing that you're talking about the beat frequency, which sounds like, wOwwwOwwwOwww..., is that accurate? If it's just going thumpa-thumpa-thumpa, then you need to fire your yard boy. ;-P

Go either to a scrap yard or a construction site, and get something very massive, like a paving tile or slab of metal, put it on the floor where the computer is now. Put some kind of acoustic insulation on top of it, like get a discarded wet suit or some old inner tubes, and put the computer on top of that pile. Don't use "acoustical tile", because you want the opposite of rigidity here.

Good Luck! Rich

(of course, you could overwhelm it by playing that tape of the barking dog that seems to come up here periodically. ;-) )

It's just resting on the carpet and underlay at the side of my desk. Then wood flooring and plasterboard lounge ceiling below.

--
Terry Pinnell
Hobbyist, West Sussex, UK