You can also improve the low frequency performance of thinner panels by spacing them out from the wall
In the context of audible frequency range in a high end home theatre or professional audio mixing room the distance from the front of the panel to the surface of the wall matters more than the absolute thickness of the panel (when compared between 2 panels of the same material but differing thickness)
For example a 4″ panel mounted flush to the wall and a 2″ panel with a 2″ air gap behind it will perform nearly identically when measured.
In the right conditions the 2″ panel with air gap can measure better than the 4″ flush panel due the higher number of material boundary changes the sound wave travels through (As I understand it the strength of this effect increases the closer to perpendicular to the panels face the sound wave is when it arrives)
