I submit that rapping or knocking the stand or the iPad seismic app is actually ineffective for determining whether a particular stand design is effective or whether seismic vibration is getting into the component, especially a turntable.
First, as far as I could tell, the seismic apps are insensitive to the very lowest frequencies - the ones that are the biggest problem. These are the ones in the range of 0 to 20 Hz. Second, rapping or tapping a surface doesn’t generate the seismic vibrations of concern 0-20 Hz, only higher ones, ones that the turntable is relatively immune to, including acoustic waves. For turntables, the resonant frequencies of the tonearm, cartridge and platter are usually around 10-14 Hz, so the isolation stand resonant frequency should be much lower than 10 Hz to be able to significantly attenuate those frequencies 10-14 Hz, assuming a mass-on-spring device, since it’s a low pass mechanical filter. For example, if your isolation stand has a resonant frequency of around 4 Hz, attenuation at 10-14 Hz will be only around 50%. But if you can get the resonant frequency of the stand down to 2 Hz the attenuation will be up around 80%.
Of course the analysis is more complicated, since there are six count em directions of motion that should ideally be isolated. Holiday Hint: The effectiveness of isolation in a particular direction is proportional to how easily the component is able to move in that direction. This is why one should be wary of overdamping. Thus, undamped springs can be more effective than rubbery type materials or rubber air bladders or air springs.
First, as far as I could tell, the seismic apps are insensitive to the very lowest frequencies - the ones that are the biggest problem. These are the ones in the range of 0 to 20 Hz. Second, rapping or tapping a surface doesn’t generate the seismic vibrations of concern 0-20 Hz, only higher ones, ones that the turntable is relatively immune to, including acoustic waves. For turntables, the resonant frequencies of the tonearm, cartridge and platter are usually around 10-14 Hz, so the isolation stand resonant frequency should be much lower than 10 Hz to be able to significantly attenuate those frequencies 10-14 Hz, assuming a mass-on-spring device, since it’s a low pass mechanical filter. For example, if your isolation stand has a resonant frequency of around 4 Hz, attenuation at 10-14 Hz will be only around 50%. But if you can get the resonant frequency of the stand down to 2 Hz the attenuation will be up around 80%.
Of course the analysis is more complicated, since there are six count em directions of motion that should ideally be isolated. Holiday Hint: The effectiveness of isolation in a particular direction is proportional to how easily the component is able to move in that direction. This is why one should be wary of overdamping. Thus, undamped springs can be more effective than rubbery type materials or rubber air bladders or air springs.