HalfPower Bandwidth method

[vibract]

Hello members,
I am working on shell emitted noise and I am required to optimise the noise considering damping (modal).
As a part of that, I need to determine damping ratios experimentally. The simulated first natural frequency of the structure is of 300Hz. Now, I need to know which experimental method is best suitable for finding damping ratio. I read Experimental decay method and HalfPowerBand Width method suits well for this purpose. I gathered some info on the test rigs used for finding Frequency response function (FRF). For this, speaker is used as the exciting device. FRF is determined as the ratio of Velocity over the point on the surface to the Sound Pressure Level at the same point but just below the inner surface of the structure. i.e FRF=Vel/SPL. Using this FRF, damping ratio is found using Half Power Band width method.
Friends, I am in need of much information about this test rig/method. If there is any other method of determing damping ratio, kindly let me know. Your help in this regard is highly appreciatable.
Regards

 

[GregLocock]

half power methd is probably the best in this case. Exponntial decay is rather rough and ready.

Cheers

Greg Locock

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

 

[MikeyP]

Though it will only work if the modes are well separated in frequency (i.e. effectively single degree of freedom behaviour)

M

--
Dr Michael F Platten

 

[271828]

Like one of the other gentlemen mentioned, half-power will only work if you don't have modes right on top of each other.

For example, I have one specimen that has at least 5 frequencies between 5 Hz and 5.3 Hz. Half-power and log decrement are both impossible for it.

I have another specimen that has the first two frequencies of 7.8 Hz and 8.0 Hz and half-power works fine for it because it has a damping ratio of about 0.2% of critical, so the modes are well separated even though they're only 0.2 Hz apart. Just gotta look at the FRF and make the judgment. Log decrement fails on this one, though.

If it has modes right on top of each other, you'll have to try a tool like MEScope.

I always try log dec first if possible. Plotting semi-log makes it very reliable and fast.

 

[GregLocock]

Well, if the modal density is high then you just play suck it and see by some sort of least squares fit between the analytical and test FRFs. That's the way some of the older EMA software worked

Cheers

Greg Locock

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

 

[vibract]

Dear members,
I am very glad to receive your responses. I got some idea on the method to use. Can any one suggest on the test equipment which I explained before?
--Speaker as exciting source--
--FRF defined as the ratio of velocity just above the surface of the structure to the sound pressure level just below the inner surface of the structure keeping the location same for both parameters.
It would be highly appreciatable if any gentleman comments on this.
Regards

 

[GregLocock]

Another practical point - in vaguely homogenous systems the damping ratio for modes in a given small bandwidth will generally be similar. That's not always true for all systems- for instance you might have a small sub-system going off into resonance all on its merry own.



Cheers

Greg Locock

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

 

[Rob45]

You may run into problems using a speaker as an exciter; you might find it difficult to judge what the sound pressure is at the surface of the test object unless you have a rather good anechoic room.

 

[vibract]

If that's the case, is there any other better method of exciation in this case.

 

[GregLocock]

A shaker. A hammer. A tight string and a pair of scissors.

But in some respects for this problem a speaker is a good choice, as it will excite the modes of most interest for radiated noise, all other things being equal.

BUT there is a problem, as it is exciting the structure in a very complex fashion (simultaneously, all over and inside the structure). That's not to say it won't work - I've certainly done acoustically excited modals quite succesfully.

Cheers

Greg Locock

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

 

[vibract]

Hello 271828(Structural)
"
I have another specimen that has the first two frequencies of 7.8 Hz and 8.0 Hz and half-power works fine for it because it has a damping ratio of about 0.2% of critical, so the modes are well separated even though they're only 0.2 Hz apart."
Could you kindly explain about the figures you mentioned.
In what way 7.8 and 8.0Hz is related to 0.2% damping.
I am bit confused.
cheers

 

[271828]

The FRF peaks are much taller and narrower if damping is low. At 0.2% of critical, even two peaks at 7.8 Hz and 8.0 Hz are separated well enough so that half-power will still work great. If damping was higher, then the right side of the 7.8 Hz FRF peak would be on top of the left side of the 8.0 Hz peak, making a half-power estimate dubious.

 

[vibract]

Thank you very much for immediate response..
It's very clear about the methodology and its usage limits.
Now, I have one more question regarding the test rig which I am going to use.
Test rig consists of SPEAKER as an harmonic excitor. Can I replace the conventially used hammer/shaker with the speaker keeping the rest same (transducers,analyzers etc..).
Being a fresher in this test, I am not aware of the test methodology.
Can we get multiple modes in single excitation? If so, how should be the input?

 

[MikeyP]

If all you want is natural frequencies and damping, then yes you can use the speaker with the same instrumentation. In fact you could get away without the microphone and just use the voltage across the speaker as the excitation reference (provided the dynamic electrical impedance of the speaker varies smoothly across your frequency range - it probably will).

HOWEVER, your speaker must be a good approximation to a the point load that a hammer or shaker would provide. I have done this on large flat structures with a 125mm diameter speaker. The speaker was positioned less than 10mm from the surface of the structure. This only gave useful info for the first dozen modes or so where the "wavelength" of each mode was large compared with the speaker. If this is not the case for your application then you could still use a speaker but you would have to switch to some form of "operational modal analysis"

M

--
Dr Michael F Platten

 

[xydrex]

Yes I agree with previous post. It will be ok for operational deflection shapes & the damping for these but getting an FRF will be harder due to difficulty in measuring input accurately. Must keep it concentrated on a small area.
I would try the shaker as well in your relevant freq range. Just because end result is acoustic does not mean it is necessary to use sound as exciting source. You can use the sound source later to check the modal frequencies.

 

[vibract]

Thanks to one and all for their valuable suggestions. I will get back if I face any difficulties during experimentation.
Before that, I would like to know
---any method of selecting points over the structure. ( random or any mathematical method etc..)
---number of points required to get better values.

regards

 

[GregLocock]

No, random is not a good approach, unless you are doing SEA, which I don't think you are.

You need to pick up all the mode shapes in the frequency range of interest wihout spatial aliasing.

You need to select locations that are of interest.

You need to select locations that are not unduly sensitive to accelerometer mass.

You need to select locations that can be used for correlation with an FEA model.

Cheers

Greg Locock

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

 

[vibract]

Hello GregLocock,
Thanks for the response.
I am a newbie to all this stuff. I need some elaborate explanation on this. Can you suggest any relevant literature regarding this.
regards

 

[vibract]

Hello Members,
Can any one comment on the test rigs which I would like to use for my present project.
File is loaded in rapidshare.com and is available at

http://rapidshare.com/files/20851929/Test_Rigs.doc

Regards,

 

[GregLocock]

No, sorry. Try Ewins, I don't have a copy, but he probably discusses it. Or the B&K Modal analysis books.

Cheers

Greg Locock

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