That is exactly it. It is the contact area that is the culprit. Think of it like a car antenna in the old days where they were telescopic. When at the non extended short length it was really ridgid. Extended all the way which effectively made the base less stiff it whipped all around just by tapping on a fender. Same principle here really less adhesive contact area makes the block of sorb less ridgid.. I doubt a strain guage is necessary. You can get there with a cheap mic and some recording software on the pc. The mic will pic up vibes from wherever you touch it to the outside cup, headband or wherever. The software can show you the waveform and frequency of the vibration. This is by no means an engineering test as the results are going to be subjective to your set up. It will however tell you where the vibrations are and give a more than adequate guide for damping them out. For this purpose even a cheap capsule mic from an old headset would do.