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Formulas for acoustical dampening?

  1. Aug 14, 2007 #1
    Formulas for acoustical dampening?


    A number of years back I worked at an engineering company where we needed to isolate very large, very loud motors from transmitting sound energy into the supporting structure. We did this with a series of elastic isolators that supported the motors and frames. Based on some very simple formulas provided by the isomount fabricators we could calculate the % of sound energy dampened out for a specific frequency if we knew the mass of the motor frame and the amount of pre-deflection of the supports. For obvious reasons dampening out the lower frequencies required larger elastic pre-loads but there was a critical point for each frequency where you could set up a harmonic resonance within the isolators and accidentally get your motor frame bouncing… very bad when it’s a pair of 500hp motors and a 20ft diameter wheel running at 45mph tangential velocity.

    Anyway, I have long since misplaced these formulas and don’t necessarily remember their correct application. If someone can recite the formulas or direct me to a web-page with a good explanation I would appreciate it. I now need to build a smaller machine that is essentially a hydraulic trip-hammer but it will have a variable speed setting and I need to assure myself before building it that I won’t have to pass through a dangerous amplification frequency while getting up to working speed.

    Thank you,
  2. jcsd
  3. Aug 17, 2007 #2
  4. Aug 20, 2007 #3


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    I can't say I have anything specific to isolation. The broad, general equations are stuff we tend to work with. I didn't get a chance to look through your link, but I would be interested to know if they are simply results of the ODE for a single DOF mass with a spring model.
  5. Aug 20, 2007 #4


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    That's mostly what they are, apart from a few general comments on coupled modes at the end.

    FWIW when I view the link, I don't see the symbol [tex]\pi[/tex] in the equations, which makes many of them look wrong. Beware!
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