Originally Posted by Carnivale
now I’d like to put some felt on top of it. Will it make my traps less absorbing?
No, felt is fine. You don't need "acoustically transparent" material to cover an absorber. If the material absorbs a little more on its own no harm is done. It's not like speaker grill cloth in front of a tweeter. However, Bob is also correct that too much absorption at mid and high frequencies can suck all the life out of a room. This is one reason "high end" professionally made bass traps are built with a membrane of some sort. They absorb as much bass as possible, and absorb less at higher frequencies. But for a cloud overhead, having it fully absorbent at all frequencies is appropriate. So go ahead and use the felt.