I am performing modal analysis of an electronics enclosure and using Steinberg’s Octave rule to decouple the chassis and the PCBs. According to Steinberg’s book “Vibration analysis for electronic equipment”, the chassis fundamental and the PCBs fundamental frequencies ONLY should be separated by an octave (i.e. Mode 1 = 200 Hz chassis, PCBs should be Mode 1 400 Hz or Mode 1= 100 Hz). This rule does not mention matching the frequencies which coincide on the same axis.
My question is:
Steinberg only states that the fundamental frequency be compared and mentions nothing of coincidental axes (i.e. I may have a chassis fundamental frequency in Z direction and a PCB board mounted perpendicularly in the chassis who’s fundamental frequency is in the X direction).
In my opinion I should be comparing the chassis Mode (Hz) in the X axis with the “critical” axis of the PCB, the X-axis in this case. This may be, for example, Mode 3 of the chassis, which has dominant mass participation in the X axis with the PCB fundamental (X axis also).