Rooms with concrete floors can still have BIG problems with TT feedback at
Decoupling the speaker from the phono cartridge at the natural resonant
frequency of the system is essential. Usually this frequency is lower
than the LF cutoff off the transducer/cabinet making sound so the LF
feedback transmission mode is through objects more solid than air. If both
speakers and TT are on a giant cement slab with great mass, nothing
vibrates except the speaker cones in the air and the stylus.
If the floor is wood over joists the room turns into a giant sounding
board. Vibrations from the speakers easily overcome the damping mechanism
in the turntable and the speaker cones start flapping in unnatural efforts
to reproduce the infrasonic.
it is almost always a mechanical path. Speaker to floor. Floor to
turntable. Floors also couple to walls, walls to the ceiling; but there is
enough attenuation in this mechanical path to allow the damping in the
turntable base to take care of the residual vibrations.