Sound transmission through solids can be abated by varying the thickness and density values of several layers.
One solid piece of material, or composite of similar materials, of any type can abate sound transmission by attenuation. This is what Lucille is talking about when she mentions the substrate and sound absorption.
While a generous depth of absorptive material will help in the bottom of the enclosure, by using several material barriers of varying thickness and differing materials between the TV and enclosure, we can attenuate sound more effectively by removing resonance at the top.
The enclosure's top, for example, will resonate at a given frequency and pass this frequency, by virtue of its material characteristics and thickness. It also provides a certain amount of resistance to the frequencies outside of its resonance area. If we add a second layer of the same material, but at a different thickness, we provide a new resonant frequency. The double layered barrier no longer passes the first layer's resonant frequency easily. What gets through Layer #1 is not as easily passed through Layer #2, and vice versa.
If we take additional layers of varying thickness and materials, we further reduce the certain frequencies that will pass any given layer and it shows increasing resistance to sound by further restricting frequencies that are outside its resonance area.
By applying various thickness, we can actually form a 'filter' to eliminate some sound frequencies while passing others. When we vary materials AND thickness, we set up resonant barriers that become more and more selective, finally resisting almost all sound transmission.
We do this often in construction when we build home theater systems, and we once built a house at the end of a runway for the small airport's owner. As you might imagine, we learned a lot about acoustic resonance.
We will insulate the wall with a layer of foam, then a layer of batt insulation. This combination of barriers is more effective that twice as much as either one. We then apply 5/8" drywall, then a layer of furring strips to form an air gap, then a layer of 1/2" drywall. Again, the two layers of differing thickness drywall transmit different frequencies, so even if it "rings" through the 5/8", the same frequency cannot ring through 1/2". Frequencies that might pass both layers of drywall are unlikely to pass the air gap. In the theater system, we then apply acoustic wall coverings to the interior walls that are designed to attenuate and deaden sound.
In the case of the OP's ackie enclosure, a thin layer of padding, like bubble wrap, a thin carpet or shelf liner, then a layer of dissimilar material like a piece of 1/4" plywood, then another layer, such as an air gap produced by sitting everything on a couple spacer strips of wood or plastic, will provide much better attenuation than the same thickness of just one layer of anything.
