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Discussion in 'Electronic Design' started by Terry Pinnell, Oct 3, 2005.

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  1. 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?
     
  2. Michael Gray

    Michael Gray Guest

    It is usually the sides of the case behaving like a drum.
    Try damping both sides with your hands, and see what happens.
     
  3. 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?
     
  4. Dave D

    Dave D Guest

    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
     
  5. 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!
     
  6. John - kd5yi

    John - kd5yi Guest



    You are an exceptional person to be able to hear 1 Hz!

    John
     
  7. Dave D

    Dave D Guest

    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.
    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
     
  8. Thanks a lot for the follow-up.
     
  9. 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.
     
  10. Anna Daptor

    Anna Daptor Guest

    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£[email protected] to solve the problem, eventually
    cured by moving the second drive into an external USB2 enclosure.
     
  11. 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
     
  12. 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.
     
  13. 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?!
     
  14. I read in sci.electronics.design that Terry Pinnell
    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.
     
  15. I read in sci.electronics.design that Terry Pinnell
    I think you multiplied by 60. 7200 RPM is 120 Hz.
     
  16. I read in sci.electronics.design that Terry Pinnell
    Try loosening its fixing screws. If that stops the beat, tighten
    slightly at random until the beat comes back. Reverse last step. Relax.
     
  17. Oops! That makes a lot more sense, thanks <g>.

    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'?
     
  18. 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.
     
  19. It's just resting on the carpet and underlay at the side of my desk.
    Then wood flooring and plasterboard lounge ceiling below.
     
  20. I read in sci.electronics.design that Terry Pinnell
    Maybe, but you don't need to know. I've given you two ways to tackle it.
    Try them.
     
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