How to measure or calculate natural frequescy of a machine

[nkb]

Hi

I am designing an electrical machine which has 3 cubical of size (30”x20”x10”) connected together side by side.

Each cubical is full of 20 to 30 electronic components like capacitors, resotors etc. . Each cubical has a fan mounted on its front for cooling.

I was told that I should find out the natural frequency of the structure so that it does not create resonance with the FAN frequency.

My question is:
Can any one tell me how the natural frequency of this type of structure is calculated and also if possible tell me material to read for my knowledge in this area.

Thanks in advance,

nkb