nvh

[zerodof]

Can anybody give me an idea on how to calculate point mobility using Nastran/FEA for automotive applications.

 

[GregLocock]

It is the transfer function (in the frequency domain) of velocity to force, at a point, in the same direction. If you need more than that (I think you do) then you'd be better off asking in the FEA group.

Ah, perhaps I see. You need to apply a unit force spectrum to one direction of a node, as a dynamic analysis.



Cheers

Greg Locock

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

 

[zerodof]

Should this be calculated in free free condition or constrained.

If constrained then where to constrain.

Or it does not matter where to constrain.

 

[GregLocock]

Well, you've kind of answered your own question. If constraints make a difference, then they matter.

On a vehicle body I'd use free-free.



Cheers

Greg Locock

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

 

[mert1]

I think there is a relationship between a NTF/VTF (noise transfer function/vibration transfer function and point mobility). Is it only because of the body sensitivities (resonances) or is there any other correlation between the two?

 

[GregLocock]

One way of looking at it is that the NTF needs a moving diaphragm to create noise, and the VTF shows the frequencies at which the structure is especially responsive, so there will be some correlation between the two.

I'm not sure if you meant NTF/VTF as a transfer function. That would be a very complex relationship, for a realistic structure, but very simple for a vibrating plate, where it boils down to P/V, which is a well known relationship.



Cheers

Greg Locock

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

 

[mert1]

I meant VTF to PM and NTF to PM. VTF to PM seems reasonable. What about PM to NTF?
For NTF I assume you hit the structure with an impact hammer and collect the pressure at the drivers ear.

 

[GregLocock]

"I meant VTF to PM and NTF to PM. VTF to PM seems reasonable. What about PM to NTF?"

Well PM is just one VTF, so the same answer applies


"For NTF I assume you hit the structure with an impact hammer and collect the pressure at the drivers ear."

yup, or anywhere else. We've done acoustic modals of vehicles, collecting P/F for a grid throughout the cabin. We've also done P/V, using a loudspeaker as excitation.



Cheers

Greg Locock

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

 

[mert1]

So if these make sense, how can I correlate PM to NTF? or can I?

 

[GregLocock]

What do you mean by correlate?

Cheers

Greg Locock

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

 

[mert1]

I mean if there is any relation between the two?

 

[GregLocock]

Yes. In a complex structure the peaks will tend to line up, in a simple structure you may see a trend in amplitudes as well.

But, can you substitute one for the other? only if you understand the system very well.

Cheers

Greg Locock

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