Hi Aoliviero. Welcome back!
In regards to you comments, any structure under load resonates at a particular frequency. Such is the case of the cartridge/tonearm system.
The cartridge "compliance", determined by its suspension stiffness, and the tonearm "effective mass", determined by its component's mass distribution, must play at a frequency that does not overlap with the frequencies of the sound being reproduced. A general consensus is that the cartridge/tonearm frequency should fall in between 8Hz and 20Hz, so that it does not have much effect on the signal generated by the cartridge. Since the cartridge generates its electrical signal by the motion of the cantilever relative to the cartridge, it will be ideal if this interface is not being biased by external conditions.
One can refer to "cartridge compliance / tonearm effective mass" charts to determine what cartridge/tonearm combination will have a more effective frequency for playback. The problem is that cartridge compliance and tonearm effective mass are dynamic in nature. Therefore, it's not guaranteed that a particular cartridge/tonearm combination will play according to the "cartridge compliance / tonearm effective mass" charts. One of the reasons for this is that the cartridge and tonearm will be affected by outside forces, such as the frequency of the platter, the frequency of the sound, standing waves, resonance of the walls, etc. Another reason is that since effective mass is related to mass distribution, the effective mass can be changed on some tonearms by the relative configuration of its parts, such as the use of balancing weights. Also, the use of damping fluids on some tonearms can change the tonearm's frequency.
This is why it is so difficult to assert that one particular tonearm is better than the other. It is easier to accept that a particular cartridge/tonearm configuration is better. But for this statement to be true, the design goals must be defined first.
Best,
iSanchez
