A porous absorber works best where air particles move fastest, about a quarter wavelength from a hard surface. At 125 Hz that is about 27 in; at 1,000 Hz about 3.4 in. Thicker panels, or panels spaced off the wall, reach further into the low frequencies.
| Thickness | 125 Hz | 250 Hz | 500 Hz | 1 kHz | NRC |
|---|---|---|---|---|---|
| 1 in | 0.08 | 0.31 | 0.79 | 1.01 | 0.80 |
| 2 in | 0.24 | 0.86 | 1.14 | 1.07 | 1.00 |
| 4 in | 0.70 | 1.20 | 1.21 | 1.08 | 1.15 |