Painted block reflects most sound; unpainted block is porous and absorbs a little more at high frequencies, but it also leaks sound through its pores.
Absorption by the numbers
A concrete block (CMU) surface absorbs about 7% of the sound that hits it at 500 Hz (coefficient 0.07). A fabric-wrapped 2 in panel absorbs about 100%. Covering even a fifth of the wall with panels changes the room dramatically.
| Surface | Approx. absorption at 500 Hz |
|---|---|
| Concrete block (CMU) | 0.07 |
| Carpet on pad | 0.30 |
| Heavy curtains | 0.50 |
| 1 in panel | 0.79 |
| 2 in panel | 1.14 |