On 01/06, afternoon we went to the central area to perform some tapping tests in order to identify possible sensitive spots on the vacuum vessels.

We had he opportunity to explore the contour from the PR tower to the SR tower following the East/South perimeter (see first picture) and tapped onto several structures. Besides the already analyzed "B4 ghost" flange on the PR north side, we found the most sensitive spot to be the bellow in the link between PR-SR towers (big arrow in first picture).

When knocking on the bellow with bare hands, a remarkable increase of noise occurs in MICH and DARM (see second picture: fft of DARM, MICH, and SSFS_B4). It is especially worth mentioning large and broad peaks  at 23.9Hz and 32.7Hz  excited both on DARM and MICH (third plot: close-up of the previous). We should make a proper noise injection with the shaker on this spot, possibly by trying to excite transversal resonances of the tube, e.g. by actuating with the shaker with a vertical orientation.

Other areas were not as sensitive as this spot when tapped upon.

The inspection was at some point stopped by the fact that the spectra became largely non-stationary, and it was not possible to discriminate our interventions anymore.