Acoustic Textiles
The same 50 mm of polyester absorbs twice as well hung 50 mm off the wall. Air against a hard surface is not moving, and stationary air cannot be damped.
— α
Normal-incidence absorption at the chosen frequency
This is the thin resistive sheet idealisation: the panel is treated as a lumped resistance at the face of the cavity, which holds while the panel is thin against the wavelength and understates absorption for thick panels at high frequency, where the material is better modelled as a bulk propagating medium. The Bies and Hansen resistivity fit was established for glass and mineral fibre and carries real scatter on polyester. Reactance runs to infinity at multiples of the half wavelength, where this model correctly predicts nothing is absorbed — those nulls are real but shallower in practice than the formula shows.
Nonwoven Acoustic Baffle Absorption & Flow Resistivity — free, with the formula and a worked example, at Textile School.