Need to make a simple mechanical system quiet 1

[kelroy]

I am trying to make a very simple system quiet - in essence, a simple thin-cross section beam banging the thin edge into piece of soft rubber. Can't change the rubber.

Looking at natural frequencies, etc. of the beam. Looking at the peaks in the acoustic measurements. Any resources online, recommmendations, etc? Not looking for complex solutions -- looking at simple things first.

Thanks!

 

[GregLocock]

which mode of the beam is causing most of the objectionable noise? what is its radiation efficiency? why not put damping pad or constrained layer damping on the beam?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kelroy]

which mode of the beam is causing most of the objectionable noise?
Not exactly sure -- looking into that now via FEA (it's really not a beam, but conceptually, for this discussion, it is.)

what is its radiation efficiency?
Don't know -- what is this, or where can I read up?

why not put damping pad
It's needs to be precision located (start and stop). Might be possible to put something along the width dimension

or constrained layer damping on the beam?
Not sure what this is.

Clearly, I'm a rookie at this noise stuff -- thanks for the help.

 

[MikeHalloran]

Uh, don't "bang" it? Use a less enthusiastic drive? Brake before impact?

Make the beam flexible, e.g. like a piece of music wire, so it bends elastically at impact? It will eventually stop ringing; the rubber will help a little with that.

The presence of rubber as a stop seems inconsistent with (continued) precision.

Immerse the whole mechanism in a liquid?






Mike Halloran
Pembroke Pines, FL, USA

 

[kelroy]

immerse the whole thing in liquid? That's kinda like the suggestion to fill my electronics box w/ sand to dampen vibrations in high shock....sorry....no liquid. I can't reveal the application unfortunately, or you'd understand some of my points.

 

[MikeHalloran]

I love that old show display with the television working perfectly while completely immersed in Fluorinert(r).

We don't expect you to give away the store. Maybe you can find a way to reveal the core of the problem you are trying to solve, without revealings its 'gozinta'. Then you will get suggestions that are more directly useful.





Mike Halloran
Pembroke Pines, FL, USA

 

[sms]

It would at least help to have an idea of how big this thing is. A thin cross section beam could be 6 inch thick by 60 inch deep by 50 feet long....

-The future's so bright I gotta wear shades!

 

[kelroy]

that's a good question, sms. But, the underlying science should be the same no? It's on the order of an inch w/ a .010 Xsection or so.

 

[sms]

The underlying science is the same, the practical issues of implementing a fix are much different.

That tiny cross section in some ways makes things difficult, and in others makes it easier. Are you sure the sound is coming from the beam and not the rubber it is contacting? Can you put some duct tape on the beam to see if it helps (by providing damping to the beam mode if there is one).

Can you drill holes in the beam?

Can you change the edge profile or the beam dimensions?

-The future's so bright I gotta wear shades!

 

[kelroy]

Are you sure the sound is coming from the beam and not the rubber it is contacting?
No. It could also be coming from the mechanism moving the beam.

Can you put some duct tape on the beam to see if it helps (by providing damping to the beam mode if there is one).
Been done -- not sure of the results yet.

Can you drill holes in the beam?
yes.

Can you change the edge profile or the beam dimensions?
Somewhat, yes.

Just trying to understand the underlying physics a bit. I didn't see a lot of straight forward references out there.

Thanks for the interest and questions.

 

[Rob45]

This is almost funny: you're trying to get help in solving a problem without actually defining the problem!

"Is it bigger than a breadbox?" is a parlor game.

 

[Strong]

Rob45 is right on the money!

We don't even know the problem frequency or how much noise reduction is needed. It may be that the beam is not an efficient sound radiator, and the dominant sound is from another part of the structure. Sure the beam impacts may be the excitation force, but what is actually radiating most of the sound? Consider reducing impact energy or sound radiation or a combination of both.

Walt

 

[kelroy]

Strong -- guess I should have worded my original question differently. I'm a green-as-can-be rookie at this noise stuff. my problem is that a system comprised of a simple "mechanism" swinging a "beam" against another material is "too loud." I'm not looking for specific solutions, just where to even start reading up on this stuff, where to start sniffing, etc. I'm not looking for E=mc^2.

As a side note, good call -- testing seems to suggest the noise source may not be a resonating beam, but something in the mechanism, as there is a peak at a certain frequency when using multiple beam materials....just different in magnitude. If it were a resonating beam, I'd expect the peak frequency to shift with materials having drastically different E/Density.

Sorry guys/gals -- I'd love to give you more but can't.

 

[Tmoose]

Noise Control - Principles and Practice
Brüel & Kjær (188-81)

 

[SME456]

Well after reading the comments posted it doesn't appear that you are getting any closer to a solution. Here's a question for you.

Where is the observer located in relation to the mechanism? Perhaps you can do a better job on enclosing this mechanism and reduce any noise perceived by the observer. While this is obviously not the ideal solution, given then fact that you are apparently severly constrained in the actual mechanism design maybe this is more practical.

 

[Strong]

Kelroy,

Here is another book that may help your knowledge base:

"Machinery Noise and Diagnostics" by Richard Lyon 1987 Butterworth Publications ISBN 0-409-90101-6

Walt