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  1. #21
    Wammer mosfet's Avatar
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    Is there an argument of coupling something to a large mass limiting vibrations in the first place due to increased inertia?
    Coupling something to a large mass wouldn’t limit the propagation of vibration because of an increase in inertia. At least I can’t see how it would. Take a big lump of concrete, say a few cubic meters, and hit it with a sledgehammer. The energy imparted would not be sufficient to overcome the inertia of the lump of concrete and cause it to move (or rather change its velocity) but still a wave of energy (vibration) would propagate through the concrete.




  2. #22
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    OK. So I went back to the drawing board, and I couldn't come up with anything better than the original flexirack design. It was arrogant of me to think I could, especially given my lack of any engineering knowledge or skills.

    I'm hopefully going to add a layer of damp9ing material and a smaller board of MDF to the top of each shelf, as in my original plan, and favoured by MartinC.

    Now I'm wondering about the CD Player shelf.

    As it would be a good idea to couple this to the frame I had two thoughts:

    1) I could put it straight on the main shelf without the additional damping and MDF which would presumably de-couple.

    2) I could insert another shelf directly above the CD Player which could be carefully screwed down onto the top of the player to hold it firmly in place in relation to the frame.

    What do you think about doing either or both of these?

  3. #23
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    Quick Illustrative (I hope) diagram.


  4. #24
    Wammer mosfet's Avatar
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    If you’re going for a simpler design Garnett (no reason not to, and often you simply have to accept the limitations of what you’re capable of) I’d suggest you combine two basic principles.

    Constrained layer damping (that’s the bit you’ve already looked into with shelves made of a sandwich construction) and high rigidity. Lose the spikes between the platforms; these look good and are very hi-fi but in terms of rigidly coupling one shelf to another are not as good as using bolts or threaded rod. I’d recommended aluminum as a material for this if you can find or make suitable parts. If not mild steel.

    Have a look at the Quadraspire website for designs using high rigidity – and then combine this with shelves using constrained layer damping.


    Edit: Yep, that's what I was thinking off, looks good!



  5. #25
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    My concern would be that all that damping material will suck the life out of your system, resulting in wooly and sluggish performance.

  6. #26
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    How so, Dexter? I can't see how the damping material can cause a problem anywhere but in the case the CD player (since that is perhaps the only thing that will benefit significantly from coupling), and my thoughts there are that the weight of the shelf above will suffice to secure it to the frame.

    Here's another pic, since I'm thinking now the two shelves above the CDP can be combined and the frame could benefit form the weight of thicker MDF.



  7. #27
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    mosfet wrote:
    Is there an argument of coupling something to a large mass limiting vibrations in the first place due to increased inertia?
    Coupling something to a large mass wouldn’t limit the propagation of vibration because of an increase in inertia. At least I can’t see how it would. Take a big lump of concrete, say a few cubic meters, and hit it with a sledgehammer. The energy imparted would not be sufficient to overcome the inertia of the lump of concrete and cause it to move (or rather change its velocity) but still a wave of energy (vibration) would propagate through the concrete.
    True. I can't quite get this sorted in my mind, but I wonder if there is a distinction to be made between an acoustic wave travelling though a solid, and a bulk vibration of the object as a whole? If say I put a piece of granite and a jelly on a vibrating floor, the top of the jelly would I suspect vibrate with a larger amplitude than the granite?

    What got me wondering about an inertial argument is that what are probably the most sensitive measuring balances I've seen* sit on huge lumps of granite. I assumed for insensitivity to external vibration and thinking in terms of inertia is the best justification I've come up with yet...

    *At TNO in the Netherlands, the Dutch equivalent of NPL.

  8. #28
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    Garnett - the only slight concern I'd have with the CD player clamping scheme you've drawn is whether it might get too hot as a result? For what it's worth my gut feeling is that it wouldn't be worth going to this level of complexity, but I'm not knowledgeable enough to say for sure. I'd be inclined to think though that looking at damping the CD player case internally, along with damping the transport mechanism directly, might be more beneficial.

  9. #29
    Wammer mosfet's Avatar
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    If say I put a piece of granite and a jelly on a vibrating floor, the top of the jelly would I suspect vibrate with a larger amplitude than the granite?
    Whichever way you look at it Martin you can’t get away from the fundamental law that energy cannot be destroyed, only converted to another form. So if the jelly and the granite are subject to the same amount of energy (in the form of a waves propagating through them at a molecular level) the amount of energy passed depends upon the efficiency (or inefficiency) of the material as a conduit for doing so.

    This is where things begin to get complicated and as far as I understand concerns frictional heat losses at a molecular level. I’d think the jelly is a far less efficient conduit in this respect since it is more comparable to those materials usually described as ‘lossy’ absorbers of wave energy (or vibration). In other words the jelly would de-couple more than the granite - if used as an equipment support - because it is a less efficient conductor or conduit for vibration.

    What got me wondering about an inertial argument is that what are probably the most sensitive measuring balances I've seen* sit on huge lumps of granite. I assumed for insensitivity to external vibration and thinking in terms of inertia is the best justification I've come up with yet...
    Adding mass? When you increase the mass of a structure (a structure being any bodies that are coupled together) you alter the natural or resonant frequency of that structure. This is the frequency at which a structure is most excited by an external stimulus. The answer I suspect is somewhere here but can't explain further than this.






  10. #30
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    mosfet wrote:
    Whichever way you look at it Martin you can’t get away from the fundamental law that energy cannot be destroyed, only converted to another form.
    Err, I don't see that anything I said was suggesting violation of energy conservation. I was just talking of the energy staying in the floor rather than being transfered to what was sitting on it. I think I'll just remain confused for now.

    mosfet wrote:
    What got me wondering about an inertial argument is that what are probably the most sensitive measuring balances I've seen* sit on huge lumps of granite. I assumed for insensitivity to external vibration and thinking in terms of inertia is the best justification I've come up with yet...
    Adding mass? When you increase the mass of a structure (a structure being any bodies that are coupled together) you alter the natural or resonant frequency of that structure. This is the frequency at which a structure is most excited by an external stimulus. The answer I suspect is somewhere here but can't explain further than this.
    Doesn't adding mass lower the resonant frequency of a structure though? For a balance I'd have thought you'd probably want a higher resonant frequncy if anything. I mean it would be LF vibration that could pose problems I'd have thought. The resonant frequencies of the lump itself will be lower as it gets bigger too I suppose...

  11. #31
    Wammer mosfet's Avatar
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    I was just talking of the energy staying in the floor rather than being transfered to what was sitting on it.
    Possibly. It’s another way to look at it. Soundproofing materials (lead, bitumen-loaded rubber) would maybe make good isolation platforms if this is the case.

    For a balance I'd have thought you'd probably want a higher resonant frequncy if anything.
    Those isolation tables I’ve seen use large slabs of granite (or similar) supported on pneumatic suspension pistons. The structure is tuned to a very low natural frequency of around one or two Hertz.





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