I saw your results and will try to interpret..
I have a good reference on such and but it refers to loudspeakers per say:
Basically it relates to the panel resonant modes due to torsion and panel flexure, their frequency and magnitude dependent on the Q of the material and also its thickness and density. With an increase panel thickness and density resonant modes are less likely to be excited.
Densities vary, lead being 11.3 x 10 3 kg/m 3, chipboard 0.81 x 10 3 kg/ m 3 and plywood depending on grade 0.67 x 10 3 kg/m 3..
Enclosure Resonance is extremely complex but essentially a clamped panel will have its own acoustic output when forming part of an energised loudspeaker, derived from sound within an enclosure.. The output consists of standing wave modes at higher frequencies and pressure modes at those frequencies where the wavelengths exceed the internal enclosure dimensions.
In theory, a clamped panel has a well defined vibrational series in both longtitudinal and bending modes. A further mode is due to panel mass resonating with its own and the enclosure's air volume stiffness.
Many have attempted to control panel resonaces with limited success. The results dissplace such resonances to more subjectively acceptable frequencies but not their magnitude. Sand can be effective but is awkward to use and mass soars dramatically. When damping is used to control resonance, the Q of the panel is reduced.
But where high mass was an advantage, it is detrimental with panel damping.
Resonance contol uses the principle of dissipating energy through friction and a given panel should be laminated with a layer of comparable mass. The mass equivalance ensuring that a good mechanical match is achieved b/n the layers to permit the effective transfer of vibrational energy from the panel to the damping material.
I hope this makes some sense.
I would also point out that an often overlooked fact is that adding fibreglass to cavity actively absorbs vibrational energy through frictional losses and therefore assists in reducing enclosure related energy transmission..