Modal Analysis correlations to Acoustics 1

[rfulky]

I am trying to coorelate the acoustic frequencies taken from impact testing to modes that I got from FEA. How doest the frequencies I measured from impact testing via microphone and oscilloscope coorelate with the frequences in modal analysis. Thanks for your help!

 

[elogesh]

Hai,

What is meant by acoustic frequencies?

Do you meant FRF from impact testing? or measuremnt of sound pressure vs frequency using microphone?

E.Logesh

 

[rfulky]

I mean the harmonic frequencies from the FFT that was recorded on the oscilloscope

 

[GregLocock]

Should be the same. I often use a mic as a quick way of instrumenting a complex assembly. You may miss some modes because they can't radiate sound very well (eg torsionals), but for simple plate modes you should get good agreement.

What sort of structure are you working on? what frequency range?

By the way, congratulations, an FEAnalyst who does their own experimental testing is off to a very good start.

Cheers

Greg Locock

 

[rfulky]

I am doing modal analysis on golf club heads. I am trying by preliminary sound analysis (without impacting a golf ball at 130mph) if a club has a good or a bad sound characteristic.

 

[GregLocock]

If you are testing the whole club then I'd expect the bending modes of the shaft to radiate reasonably well, because the shaft is long enough to couple well, and the head of the club will act as a small piston to help radiate the noise.

Torsional modes will radiate effectively IF the head is not nodal, but it may well be nodal.

I can't imagine that the head itself will have significant modes below 3-4 kHz at least. Cheers

Greg Locock

 

[elogesh]

Hai,

"I often use a mic as a quick way of instrumenting a complex assembly".

Can I have few information in this regard?

Have you carried out the sound spectrum in narrow band and then compared with the FRF of the assembly?

In my application I have a complex internal sound source covered by approximately spherical encluosure of 3mm thick.

In the assembly I measured the natural frequencies of this enclousre. The sound and vibration generated inside has to pass through the enclosure.I donot know the internal source characteristics.

I want to measure the sound spectrum while this assembly is operating and want to compare with the modal frequencies of enclosure. This is inorder to know which are the significant natural frequencies of the enclosure excited by internal source.

Whether this is a valid approach?

can I have your comments in this regard?

With Thanks and regards,
Logesh.E

 

[GregLocock]

That seems reasonable. When I worked in NVH we were very interested in finding sources of tonal problems, so I'd typically stick a mic in the engine compartment, or under the body, and hit everything in sight until I found a source of the right frequency.

Then we'd remove, modify, or cover that component and see if we had affected the noise. There are other ways to do this, but this one works, and you can do it by yourself.

Yes, we would often compare FRFs with sound spectrums, in the engine noise lab this was an important part of our source identification technique, since that tells you a lot about the radiation efficiency of each component.

It sounds like you are on the right track. Cheers

Greg Locock

 

[srinivasanpuri]

Hello All,

At the moment, Iam using an ANSYS Educational Version
to conduct a Harmonic Acoustic Analysis on
closed cavities (Eg. Simple Rectangular cavity).

I would like to apply a velocity amplitude on pne face of the 2-d rectangular cavity.

I have read through the ANSYS User Guide to perform a harmonic
acoustic analysis, but Iam still not able to apply a 'Velocity
Amplitude' on any face of the cavity (Rather, there seems to be
no options in the GUI to apply a velocity amplitude). #

I would be glad if anyone can help me out


Regards,

Srini

 

[Lnxman]

Hi everybody!

Hey rfulky, did you use a FEA software to model your structure? Which one was it? I am interested in doing something similar. I would like to correlate the frequencies measured by a hydrophone to those from the software but I still don't get good results using Femlab. Could you explain me a little more how do you do the modal analysis?

Thank you very much!

 

[yogeshb]

Dear Mr. Shrinivasan,
Please try to create new thread with proper title rather than appending to a thread which is one year old!!!
This will help others to view your thread and answer accordingly.
Don't mind to create new thread if you have query not related to the existing thread.
Cheers

Regards,
Yogesh Birari