I once had a rear-projection TV where the molded plastic cover in the back had a large rectangular section that sloped down and that faced toward the back wall but upward. Probably at least six square feet and not braced in any way. At some point I began to develop a vague sense that my system sounded funny, but it wasn't overt and it crept up on me kind of gradual. Eventually I was certain that something not right was going on, but I couldn't pinpoint what it was even though I would occasionally walk up close to the speakers. Finally one day I started putting my hands on everything that I thought could be vibrating. When I put my hand on the back of the TV there was a very obvious change in the sound. Sympathetic vibration of course. I don't recall how long I had been listening to it, but I think it was a pretty good while.