Noise converted to vibration 4

[WARose]

First off, let me say: I'm (obviously) not a noise guy. Most of my work in vibration has been (low frequency) structural design for machinery vibration control. That being said, I am looking for a sanity check that some noise is not inducing some level of vibration I am measuring. (This is answering a suggestion some people who work at this facility have made....and I have to answer.)

More specifically: a piece of machinery in a room is believed to possibly be producing vibration in an adjacent area from its noise level. I am seeking to say it is likely more from unbalanced forces transmitted through the structure.

I think I can track down the absorption coefficients and all that.....but rather than make this a complicated acoustics problem....is there a way to just figure: hey, this is the max. noise this thing is producing....it can't cause more vibration in [widget x] than [this]...ergo we know....

Only one problem (possibly among many): I don't know how to go from a sound level to induced vibration. Can anyone help me with that? Thanks.

 

[Strong]

I have this document:
"Relationship between sound radiation from sound-induced and force-excited vibration: Analysis using an infinite elastic plate model" from

https://asa.scitation.org/doi/10.1121/1.4955308

Walt
w_f_strong@msn.com

 

[WARose]

I have this document:
"Relationship between sound radiation from sound-induced and force-excited vibration: Analysis using an infinite elastic plate model" from

https://asa.scitation.org/doi/10.1121/1.4955308

Thanks. It, however, looks like the reverse of what I am working on (i.e. sound produced by vibration rather than vibration produced by sound).

 

[Tmoose]

Without actual measured discreet frequency information I personally would not go down this road.

"The device I am using measures peak (or RMS) displacements, acceleration, and velocity."
Is that the extent of the instrumentation you have had available for your work " in vibration (low frequency) structural design for machinery vibration control." ??

If "The device" has a port to output the raw signal, that could be fed into a free PC base FFT program.
A cheap microphone or (my favorite) Sound Level Meter could be used similarly.
At that point, knowing the frequencies of the vibration, and the sound, I'd feel like I really knew what was going on.
If the frequencies do not fit, you must acquit.

The assumption the sound and vibration are largely sinusoidal is valid If, and only if the values for all three coincide on a chart like this -

http://azimadli.com/vibman/_aintroduction to machine vibration-171.png

For me, None of the synonyms for assumption are pretty, or sufficient grounds to design and build a bird house, or put ANYTHING in writing without a bunch of disclaimers.

https://en.oxforddictionaries.com/thesaurus/assumption

 

[Strong]

I am not a modeler, but I believe the energy transfer may be reciprocal within a defined frequency range. Probably GreqLocock would know more about that. The book "Noise and Vibration Control" edited by Leo Beranek (I have revised 1988 version) has a chapter (11) "Interaction of Sound Waves with Solid Structures" with equations for various situations along with discussion of SEA and energy flow. It covers vibrations induced by sound and sound radiation from vibrating structures. Good luck making a WAG on this problem!

Walt

 

[WARose]

thanks again Strong.

 

[GregLocock]

I don't have Beranek but when I worked in NVH on cars every company had at least one copy. It really is the go to ref for methods, I'd forgotten about it.

Cheers

Greg Locock


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[WARose]

Went ahead and bought the Beranek book. Looks pretty good. Thanks for the recommendation.

 

[WARose]

Reading the Beranek book (2nd edition, 2006), I found some equations that allow one to estimate the applied force from sound. (Se pages 462-465.) It's pretty straight forward (and (as you would think) it is a function of sound pressure).

thanks again for the book recommendation

 

[electricpete]

Without knowing the frequency, how are you going to rule out resonance of the excited mechanical structure?
Without ruling out resonance, how do you propose to place an upper bound on the mechanical response to the force from sound pressure? (think about an opera singer breaking a glass with her voice)

I know you're assuming a frequency (running speed), but that assumption is not necessarily conservative.

EDIT - Mapping the magnitude of vibration across the vibrating structure to build something like an operating deflection shape might go partway toward deciding whether it is / is-not a resonance. For that matter, it also may give clues about the source of the vibration.




=====================================
(2B)+(2B)' ?

 

[WARose]

I know you're assuming a frequency (running speed), but that assumption is not necessarily conservative.

That's something I've given a lot of thought to......but I don't see how the natural frequencies of the walls/items involved could be anywhere close to the source. (Even if the noise frequency was +/- 50% the operating speed.)

While we are on that point, does anyone have a generic rule of thumb for the +/- % of operating speed noise can happen at? When I've asked pump guys (for example) about that in the past they've said operating speed will be the noise frequency. Compressor guys (on the other hand) refuse to answer (they will only give noise levels).

 

[Strong]

"does anyone have a generic rule of thumb for the +/- % of operating speed noise can happen at?"

I have worked in the field of sound and vibrations for 55-years, but I do not understand the question! Sound pressure generation is determined by surface vibration level (perhaps amplified by resonance) and Radiation Efficiency (surface size and shape). A simple rule-of-thumb for sound radiation from a simple structure (not a woofer speaker)is that the sound wavelength should be approximately the same as (or larger) the radiating surface dimension. For a machine at 3600 rpm (60-Hz), the wavelength is about 18.3-ft, so poor radiation efficiency can be expected for smaller surfaces. Pump and compressors with vane pass frequencies and harmonics have smaller wavelengths and typical machines can more efficiently radiate the sound.

Walt

 

[Tmoose]

"When I've asked pump guys (for example) about that in the past they've said operating speed will be the noise frequency."

Note W Strong's comments about vane pass frequency.
I think you asked the wrong pump guys.

From the original post, "a piece of machinery in a room is believed to possibly be producing vibration in an adjacent area from its noise level.".

It would help an old man's understanding if the "machinery" was better defined.

 

[electricpete]

good points by Walt. I never thought about machine size in relation to noise emission efficiency. I'll add for op:
20hz, 30hz, and even 60hz are very low pitch. Try it with a sinusoidal tone generator on your phone or maybe this site:

https://www.szynalski.com/tone-generator/

I'm guessing if you walk by the machine and listen, those low pitches at running speeds of 20hz, 30hz or 60hz aren't what you'd notice. Typically (when people complain about noise anyway) it is the higher frequencies that get noticed. (I assume someone is noticing a very loud noise somewhere as basis for the customers question, but you haven't described the noise). For motors there are often slot-related electromagnetic noise where the frequencies are in the neighorhood of number of rotor bars times running speed. For open (ODP) motors there can be high frequency siren effects from the motor cooling fan or the rotor and stator core vent ducts. For TEFC motors there can be noise from the external cooling fan blade pass. . Reciprocating machines I'm sure can give high order harmonics of running speed in sound (they certainly do in vibration). Something like a ball mill might create noise at the resonant frequency of whatever is impacting. All of these (the things that usually get noticed) are much higher than running speed



=====================================
(2B)+(2B)' ?

 

[WARose]

Good feedback guys.

EDIT: What prompted my question was the Beranek book. He has some generic equations for sound level production per machine type. But it leaves the frequency of that production up in the air (no pun intended). So I wondered if there was something available.

 

[GregLocock]

For any type of machine it is just a case of listing all the forces that occur and working out their frequencies. Most of them will have harmonics. So for a cooling fan you've got balance, at 1 per rev, blade passing frequency, and multiples of that. If you've got a loose bearing then it'll show as odd harmonics of rotational speed. etc etc. There can be odd effects other than these, but that's where you start.

Identifying which frequency is annoying you is most easily done with a recording and a bandpass filter. I've only used posh top end gear for this (HEAD Acoustics or B&K) but I suspect these days it is within the capabilities of open source software such as Audacity.

Cheers

Greg Locock


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