Acoustic absorption and specific air resistance
The sound absorption is officially measured in a laboratory according to EN ISO 354 on a test surface of 12m² in a reverberation chamber. On a project basis, the specific air resistance Rs can be measured more quickly in a round tube with a diameter of 10 to 15 cm, before and after painting a sample of the acoustic surface. The measurement standard is then EN ISO 9053-1:2018. In the tube, an air flow with velocity v (m/s) is pushed through the material and the pressure drop p (Pa) over the sample is measured. Rs is then simply the ratio p/v (Pa.s/m). There is a direct relationship between Rs and the acoustic absorption of the material.
There is an optimum for the value of the specific air resistance. If the value is too low, there is insufficient resistance and there are too few pores to obtain high friction and ditto sound absorption. This is, for example, the case with open woven fabrics or metal mesh. If the air resistance is too high, the air particles cannot penetrate sufficiently into the material, which also results in low sound absorption.