How should I process a signal before applying EMD on it?

I have a nonstationary signal obtained from a sensor on a structure. I want to use this signal for structure modal properties identification.

to decompose it to modal responses (since I'm going to use Hilbert transform) I use Empirical Mode Decomposition method(EMD). but In order to obtain a reasonably accurate resolution of the high frequency modal response, it is necessary to increase the number of sifting up to over 16. So I take the free vibration response and then implement EMD and the number of sifting decreases to 3.

I also have found that some use fourth-order band pass filters before implementing EMD to decrease the sifiting iterations. Can anyone tell me how should I do this?

I want to learn the difference between two methods(bans-pass filtering and free vibration response)