Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
    Well some more extensive searching eventually led to this web page:

    http://www.ribaproductselector.com/Docs/5/22595/external/COL122595.pdf?ac= [Broken]

    I'm posting it here in case someone uses a search engine on these posts so they don't come up dry.

    Last edited by a moderator: May 3, 2017
  4. Aug 20, 2007 #3


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor
    Homework Helper

    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook