I was actually trying to describe the sound that would be emitted from a HDD if it was suspended by elastic, rather than the air actually absorbing any of it. The air would, I presume, help a tiny bit with heat, but I'm not sure on that. Anyway, what are your thoughts? Could it be effective?
AFAIK all matter propagates vibrations (which we perceive as sound), period: any material is to some degree "transparent", with reference to energy waves.
If I'm not wrong, ideally, a system consisting of an hard drive suspended with an elastic suspension having a very low own resonance frequency (and at any rate, lower than the lowest frequency emitted by the hard drive) should work as an elastic pendulum, "dissipating" the vibrating energy in movement.
As far as I may recall, adding mass to the hard drive (this is what MDF and heatsinks does, for what I understand) changes the vibrational modes of such a composite system, but it does not actually "absorbe" the vibrations.