Sound proofing: mass-spring-mass or mass-mass-spring? I am trying to improve the sound proofing of a metal box. The box is made of steel of thickness $0.8 \;\mathrm{mm}$. I have additional sheets of steel with $3 \;\mathrm{mm}$ thickness for reinforcement. For vibration reduction, I have access to some bitumen mats (anti-vibration mats for damping cars) and some alubutyl (butyl rubber with an aluminum foil on top).
What would be the best way to improve the insulation properties of the box?
(A) Glue both steel plates together $(0.8 \;\mathrm{mm}+3.0 \;\mathrm{mm})$, then add bitumen/alubutyl to reduce vibrancy.
(B) Put the anti-vibration mat right into the middle to achieve kind of a mass-spring system.
The bitumen mats are pretty stiff and thus will probably be not sufficient to act like a damping spring. The alubutyl however is more like gooey grubber and might be better for attenuating the mechanical vibration. On the other hand: It is only $2 \;\mathrm{mm}$ thick.
My take on that is that (A) might be a more rigid construction (if glued tightly) while (B) might be better for blocking mechanical vibration to travel from the inside to the outside.
The noise source I want to isolate (computer hard drive) adds both structure-borne and air-borne resonance in/to the case.
I am looking forward to your suggestions.
 A: Here are some general rules of thumb:
Before you start designing your sound blocker, you need to know the spectrum of sounds you wish to block. This will determine which wall design will do the trick. For example, if the noise spectrum consists primarily of high frequencies (above, say $3000 \;\text{Hz}$), then two sheets of steel with a stiff rubber layer in between will be the best. If the spectrum consists primarily of low frequencies (say, $500 \;\text{Hz}$ and below), then sheets of steel with soft rubber sandwiched in between will be the best. For very low frequencies $100 \;\text{Hz}$ and below), lots of mass (many steel sheets with soft rubber in between) will be needed. For very high frequencies (say $6000 \ Hz$ and above), a single sheet of steel and a single sheet of stiff rubber will be the best.
A: Try sheets of cardboard. They don't transmit sound well.
Consider if you leave an opening for cooling air flow, you have an opening for sound. Open cell foam might help? Pointing the opening away from listeners?
Computer fans are typically noisier than hard drives. But if it is really the drive, you can get a solid state drive.
