04-23-09: UndertowUndertow, why not??
Okay, so here is what I want to know knuckle rapping cannot be very well proven to tell you anything with a turntable isolation.
first anything physically "Knocking" on something attached to your cartridge will most likely feedback the sound thru your speakers..not necessarily. if the TT isolation is good any knocking that excites any resonances should get dissipated (if your rack is damped, etc) or decoupled (if you have brass cones, roller-blocks, squash balls, etc) from the TT thereby disallowing it to get to the TT in the 1st place.
(I think the only place where this would not hold is if you knock on the LP itself when the stylus is in the groove).
However are your speakers playing in the room doing it self induced via airborne or not?The knocking is not creating any significant air pressure changes to modulate the drivers & the speaker terminals are connected to the amp. The amp is on during this test thus the primary input signal feed to the speakers is an electrical feed from the amp output.
so, how are the speaker drivers modulating via airborne if one is knocking on the rack/shelf/plinth, etc??