The seismic vibration is continuous, micro seismic activity as opposed to earthquakes. Plus traffic, wind, subways, etc. The reason these relatively small amplitude low frequency vibrations are a problem for audio is that much of the signal, e.g., phono cartridge, tonearm wire, preamps, DACs, are low level signals and completely susceptible to low frequency seismic vibration. I.e., electromagnetic waves are distorted by vibration. Even high level signals are susceptible to vibration as we’ve seen with cable risers, etc. Thus, even for rooms on cement slabs vibration isolation improves the sound.
