Measurement of transmission loss with a standing wave tube in porous materials with and without open cells porosity

The ability of the standing wave tube to measure the sound transmission loss of porous materials, which is the subject of the standard ASTM E2611-17, is studied with six porous materials, three of them with open cell porosity, the other three with closed cell porosity. We find an intrinsic obstacle in the procedure with the latter materials because the high variability of the measured data makes the results unreliable.We have also tested the two different ways of performing the measurements, by means of two or four microphones, both of which are allowed by the standard, and compared the results. The outcome cannot be distinguished in the case of open-cell porosity materials, which is the expected behaviour, while it gives different results in some of the closed-cell materials, although at this point we already know these materials not to be well suited to be measured with the tube.