Just constrained layer damping, nonoise. That, and an irregular shape.
Some few materials have constrained layer damping built in, like Panzerholz, or, to a lesser extent, slate, or Baltic birch plywood. The trick is to put two materials with different speeds of sound, in intimate contact, like hardwood and elastomeric glue. I used 2 1/2" of Baltic birch in three layers, bonded with elastomeric glue, bonded to a topping consisting of an inch of slate. Slate itself consists of very thin layers of rock, which reduce sound transmission and resonance.
The very best shelve material is Panzerholz, which I used in the plinth of my DIY turntable. So I have three different non-resonant materials, with different properties, which prevent resonance.