I hit your turntable or stand holding your turntable do you think its gonna feel it and your gonna hear it thru the speakers? OF COURSE!
No, Undertow, if the rack & TT is built well then you will NOT! That's the point I'm trying to make.
If you know a friend/dealer/acquaintance try this on some well-built rack like a Mapleshade Samson, Grand Prix Monaco, HRS. The knocking does not get thru to the speakers. Also try this with a well-built TT like a Micro-Seiki, HRX, Rockport, Goldmund. Again, the knocking does not get thru even when you knock on the platter. The top-end racks & TTs are made with far more inert material than the less expensive units but (& this is reason to tweak & DIY upgrade) with some work in the right areas it is possible to (significantly) improve the less expensive units. That's precisely what Timtim was doing & sharing & that's where I was going too.
the source of speakers to induce some kinda airborne or vibration in the room structure....
....but are your speakers that are not physcially touching even with heavy bass making the same "Knuckle Rapping" sound thru your system?
thanks for the clarification. I didn't understand what you were trying to say the 1st time around.
I agree, this is another good test - get the speakers to flex the sheet-rock walls & see if that couples into the rack & gets picked up.
This would again highlight how well (or not) the rack is decoupled to the floor & how well it is damped.
When the walls flex they induce resonances in the rack & these travel to the TT & get picked up.
Another test that I have used is to jump on the floor near my listening chair which a worst-case scenario of foot-falls or running across the room. If you have children this is definitely happening while you are listening. Again, this induces resonances in the rack that get/do not get picked up.
What my knocking test did was simply bypass all these resonance inducing mechanizms & get directly into inducing the resonances in the rack by knocking on it. The idea is to induce the resonance - flex the walls with the speakers & get that to couple into the rack or knock on it. Yeah, the knocking might be an extreme case but it does give you an idea what's happening. Taking variable to the extreme is often (not always) a good way to check out a "what-if" idea.