Porous Absorber Calculator is your best friend.
I need to make a keyboard macro with that sentence.
Dammit, is it so difficult do download the bloody thing and insert some bloody numbers?
OK, here are some values as being written in my secret documents (no actually derived from Porous Absorber Calculator,
e) for 48 Hz. Served on a silver tablet:
10 cm absorption, 10 cm gap, 16800 Pa*s/m²:
27 % absorption (equals a resulting dip of - 5.7 dB
per wall for an uneven mode)
30 cm absorption 5000 Pa*s/m², 20 cm gap:
60 % absorption (equals -2.2 dB)
30 cm absorption 5000 Pa*s/m², 40 cm gap:
68 % absorption (equals - 1.7 dB)
60 cm absorption 2000 Pa*s/m², 50 cm gap:
85 % absorption (equals - 0.7 dB)
All values given for normal incidence. Porous Absorption Calculator is based on the formulas of Delany and Bazley and requires the gas flow resistivity to be between 1000 Pa*s/m² and 50000 Pa*s/m² (as is the case here).
Hannes