Normal acoustic foam does not damp the panels, since it does not change the stiffness of the panel, nor does it change the mass. It is a filter that isolates the panel from some of the sound coming from inside the box. It filters out mostly higher frequencies though.
Take a guitar and set a piece of acoustic foam 1"x1"x4" under the strings does it change the resonance of the strings? It doesn't change the mass of the strings nor does it change the "stiffness".
speaking of relativity...
3/4 plywood has an SAC of .2 at 125Hz and .15 at 500Hz, a bare concret block wall has a SAC of .36 at 125hz, 0.44 at 250Hz and 0.31 at 500Hz. You'd be better off lining a subwoofer box with drywall (sac .29 at 125Hz, .13 at 250Hz)
than "sonic barrier"
btw, What was the amplitude of the harmonics at 300Hz?
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It's true about the bare concrete block, but you have to realize that the other materials you mention are measured while installed on a stud wall with hollow air cavities behind it. I'm pretty sure their properties would change a lot if the were mounted directly on a solid material.
Coefficient Chart
The rigid fiberglass board looks the best to me.
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Is this an acoustic guitar or electric? I don't have a guitar so I can't do your experiment.
Like I said before, the foam does not damp the panels. It's an acoustic filter.
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