I just found out this blog and find it very well explained. May I know if the resonance frequency of the structure is known, can we predict/relate how the structure will react during random vibration.
Thank you in advance.

Please feel free to mention a few lines in the article on your web site, but please do make sure to refernce us here at Prosig and provide a link to our article or web site.

Thank you for asking a question on our blog.

For sure resonance will affect the long term reliability of your products.

Generally speaking products are designed so that in normal use the resonant frequency is never excited.

If your product is used at a range of frequencies that include its resonant frequency your testing should replicate the range of frequencies that the product will be exposed to during its working life. If you are trying to accelerate the life testing of a product then you can subject the product to higher levels of the same frequency that it would otherwise be exposed to. The increase in level is proportional to the acceleration and therefore life testing of the product.

Thanks for asking some questions on our blog.

Your questions are rather complex, perhaps you would be better to contact us directly and discuss I think for us to offer any advise we would need to have a lot more information about your structure and the tests you want to perform.

It is possible make a judgement on a natural frequency without excitation, but you have to consider many other issues.

Your better to isolate a structure and test it in isolation with measured excitation. That is the best way to find the natural frequencies.

You have tested the system separately and found its resonance frequency to be 2072Hz, these results you deduced from hammer impact testing.

If the system had a resonance at 100Hz this would be excited by your 100Hz input. But if your system had a 200Hz resonance this would also be excited by your input of100Hz as one of the harmonics would fall at 200Hz and so on.

I hope this is clear for you and this information helps.

Thanks for posting on our blog.

From what you have said you have a system which when tested had a resonant frequency of 2072Hz.

You then performed a separate test on a part of the system and found it had a resonant frequency of approximately 2000Hz.

And your question is that you are trying to find what had excited the system in the first place that then caused the resonance.

This is classical testing and analysis problem. I don’t know what your system is so I can’t offer you any specific advice, but generally if you have several inputs to a system you can simply remove them one at a time until you find the cause. There are more complex techniques but we would have to know more about your system to cover the more complex issues. You could for example consider something like Source Contribution Analysis, this could rank the inputs and show you which frequencies are being excited by which input.

If you would like to discuss this further perhaps it would be best to contact us directly.

Thank you for asking another question on our blog.

I don’t think we can give you advise on your project exactly, we do not know what your structure is for example. Generally I would not expect to see very small spikes, that would mean there is very little energy in those bands and that the structure was only being excited in a very small band, which is not impossible, but unlikely. I would suggest looking again at your test procedure and the signal processing techniques you are using.

If you would like to discuss this further perhaps it would be best to contact us directly.

Thanks for asking a question on our blog.

It sounds like the peaks your seeing are the natural frequencies of the structure your exciting.

Your Finite Element Analysis is giving you a natural frequency of 2700Hz, from the practical testing you have carried out it would appear to be inclusive. You say you have performed the test twice, but you have quite different results each time. Any practical test should be repeatable, no matter how many times you carry out the same test you should get the same results. I would suggest carrying out the practical tests again, perhaps repeating the test three times to make sure no experimental error is creeping into the testing.

Please feel free to come back and let us know your new results.

Benjamin Franklin,
Chennai, India.

Thank you for asking a question on our blog.

Your question is very interesting, it is however not possible for any damage to occur to a building structure from the co-ordinated playing of music.

From a structural point of view the buildings would not be affected for two reasons, the first reason is the frequency of the music is far too high to have any effect on structures of that sort. The sound waves would simply pass straight through.
Second, the magnitude of the sound wave would need to be very large to actually have enough power to excite a large structure.
With regards to constructive sound, this would happen if two points, or sound sources, had exactly the same music, and played it at exactly the same time. Exactly in between the two sound sources as the sound waves passed through each other there would be a constructive effect.
This is at that exact point the music would be louder.

I hope this answers your question.