Formulas for acoustical dampening?

1. Aug 14, 2007

Sean Powell

Formulas for acoustical dampening?

Hello,

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,
Sean

2. Aug 17, 2007

Sean Powell

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.

Thanks,
Sean

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

FredGarvin

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.

4. Aug 20, 2007

AlephZero

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 $$\pi$$ in the equations, which makes many of them look wrong. Beware!