Good Modal Extraction

To get good modal extraction from tap tests and the like, You should have at least 5 (better to have 10) spectral lines within the modal bandwidth.

Modal Bandwidth: the area between the half power points of the mode

Half Power Points: take the peak of the mode, normalize it to 1, come down to .707 on either side. Whatever width that is is the modal bandwidth. (.707 because you are looking at an FRF, a linear scale. If you were looking at a PSD it would be 0.5)