sound radiation efficiency?

[flyinghorse21]

when i use BEM in sysnoise(rev5.6) to compute the sound radiation efficiency of a cylindrical shell with two end plates in water, i found the efficiencies at several frequencies are negative. can anybbody help me to explain it.thanks!

 

[GregLocock]

Perhaps those parts are absorbing energy?

This is a real world effect if you plot the intensity around an engine - in the near field some parts of the engine absorb energy while others emit it at the same frequency.

My explanation for this is that the far-field response is essentially that of a point source, so we have to resolve the vibrations of the rather strangely 'shaped' source into a spherical wavefront. The system does this by shuffling energy behind the wavefront.

If that explanations helps, terrific. If it is right, even more terrific.



Cheers

Greg Locock

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

 

[flyinghorse21]

Thanks!At some frequencies, some parts of a structure absorbe energy and others emit energy,but we know the sound radiation efficiency is always positive.
are there anybody who are familiar with sysnoise and tell me the method to solve this problem?thank you very much!

 

[flyinghorse21]

if the driving force exerts at a point belong to the structural nodal lines,what value should the sound radiation efficiency at this modal frequency be? that is, is it possibly negative?thanks!

 

[GregLocock]

No, the vibration will be zero, but the radiation efficiency will still be positive.

On reflection I agree with your previous comment, radiation efficiency should always be positive.




Cheers

Greg Locock

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