NRC

Noise Reduction Coefficient — the panel honesty number, 0 to ~1.

NRC averages a material’s absorption at 250, 500, 1k, and 2kHz: 0 reflects everything, 1 absorbs everything that hits it. A proper 2-inch mineral-wool panel runs NRC 0.9+; thin sculpted foam often lands near 0.3 while looking the part. Note what NRC omits: everything below 250Hz — which is why bass traps are specified separately.