Need some explanation in tuning_the_ecl_ek chapter about vibration

i got some sentence in tuning_the_ecl_ek chapter

An isolation mount has 6 degrees of freedom, and therefore 6 resonant frequencies. As a general rule, the 6 resonant frequencies of the autopilot on the isolation mount should be above 25Hz to avoid interaction with the autopilot dynamics and below the frequency of the motors.

which is very according with our flight controller anti-vibration modifing experiment

if the anti-vibration is not proper,it very commont to occur a very large 25Hz acclerator noise, that make flight controller very unstable.

the data can be saw here

the question is how that 25Hz be derivated, and are there any deatail explaniation in theory for that sentence referenced above