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Vibration/noise cancellation

  1. Jan 7, 2017 #1
    Hello gentlemen!
    So I have a situation. Concrete floor (ceiling actually) slabs like these
    8%20inch%20Hollow%20Core.JPG
    has many cylindrical spaces in it. And noisy neighbors on top! The slab receives shocks / impacts and human shouts.
    First I found Tuned Mass Damper. It is good for continuous vibration but not for first cycles after vibration applied. I mean shock is sort of "1-cycle vibration".
    May be if I pump some kind of oil or solution of calculated density inside that slab it could absorb impact energy and convert it to waves or something...
    Frequency range to be muted is about 10Hz - 3400Hz I think (it would be great to cover even short part of it).
    I would like to hear your advice what you think could be done here?
     
  2. jcsd
  3. Jan 7, 2017 #2

    tech99

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    I believe that for low frequencies the only way is to increase the mass or rigidity of the barrier. For higher frequencies, the slightest air gap will allow the sound through, so it must be totally sealed. Obviously a soft covering is going to help as well by absorbing impacts and higher frequency sounds.
     
  4. Jan 8, 2017 #3
    So the substance to be put in spaces must be as heavy as possible (tar/flux, may be something more realistic?) and putty all cracks carefully.
    Surely soft covering could help but I'm afraid giving carpets as present would cost me a lot :sorry:
     
  5. Jan 8, 2017 #4

    CWatters

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    I think your best bet is to build a false ceiling that is isolated from (eg not fixed to) the existing concrete. The false ceiling should have a high mass (perhaps use several layers of plasterboard/dry wall). Should also be sound absorbent mineral wool or similar between the new false ceiling and the concrete.
     
  6. Jan 8, 2017 #5

    Baluncore

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    Maybe partly fill the cavities with bedding sand that flows easily. It will be very lossy and so prevent oscillations following an impact.
     
  7. Jan 8, 2017 #6
  8. Jan 8, 2017 #7

    Baluncore

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    Speculation is good. Machine tool vibrations can be greatly reduced by partly filling some of the structure, or a bore in a tool holder, with free flowing sand or shot. Sand is cheap, garnet used for grit blasting might work well. The particles are thrown around by any vibration, so it dampens vibration and can prevent build up of oscillation or tool chatter.

    With a heavy concrete beam you should estimate the Q of the oscillation resulting from an impact. A high Q resonance will need less added free mass and so be easier to dampen than a low Q resonance. But the low Q resonance is not so much of a problem as it lasts less time.
     
  9. Jan 9, 2017 #8
    Thanks guys for your answers!
    CWatters, 256bits,
    There are services around building second ceilings. Just wanted to save height of the room. I'll pick heaviest from their options if I'll choose to use them.
    Baluncore,
    I like idea about sand. Is there a simple way to estimate the Q and subsequently mass of sand?
    Speculation is good indeed :)
     
  10. Jan 9, 2017 #9

    Baluncore

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    Does a hammer impact to the slab make it ring like a bell or does it sound like a short dull thud.
    Loading with a sand should quickly turn the ring of a bell into a thud.
    Any “thud” will be hard to stop. What is the most annoying sound?
    What is the dominant frequency or musical note? How long does it ring for?
    https://en.wikipedia.org/wiki/Q_factor#Physical_interpretation
     
  11. Jan 9, 2017 #10

    CWatters

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    It won't make any significant difference putting stuff in the hollow voids and could make things worse. The sound is being transmitted by the solid parts not the hollow. That's why any solution involving a false ceiling should be isolated from the existing ceiling (eg the false ceiling should ideally be supported by the walls NOT the existing ceiling). There are things called "resilient bars" and "isolated/acoustic hangers" intended to allow plasterboard to be fixed to the existing ceiling. These can be effective but totally isolated is better.

    If you really don't have the height for a false ceiling (may only take 4") then I think the only alternative that will be effective is to give the folks above carpet with sound reducing underlay.

    Any penetration between floors for services or walls can bypass sound insulation making it less effective.

    Here in the UK new houses have to meet Part E of the Building Regulations that covers sound transmission both between rooms and independent dwellings. This can be done by using proven floor and wall designs called "Robust Details". Some are free, others not. Typically companies that make hollow concrete floors and or sound insulation will have a drawing that shows how to use their products to make a floor that complies with UK regulations. Using one of these detail drawings can avoid the need for a sound test. Here is one example...

    http://www.knaufinsulation.co.uk/sites/knaufuk/files/downloadfile/4_3-separating-floors_1.pdf

    They seem to recommend a combination of underlay and resilient bars for new floors. For upgrading an existing floor they suggest an isolated/false ceiling as I suggested.
     
  12. Jan 9, 2017 #11
    Those are primarily dull thuds. Slabs don't really oscillate. And voices - another half of the problem. So considering also possible procedure to inject stuff inside slabs would be a challenge and CWatters scepticism I should go with carpets and false ceilings. Will keep in mind all details CWatters provided (Now I want to move to UK :smile: )
    I was just imagining jelly inside a slab and how it absorbs energy. But if this so easy someone would already had invented appropriate technology.
     
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