Do Locally Reacting Acoustic Liners Always Behave as They Should?

Abstract

BY definition, a liner is locally reacting (or point reacting) when the local response of the liner is dependent only upon the local acoustic pressure and not upon the structure of the acoustic field in the duct. This implies that no sound propagation may take place in the liner parallel to the liner surface. Helmholtz resonator-type acoustic liner materials are normally assumed to be locally reacting. However, it is shown that different behavior arises when neighboring cells are interconnected (e.g., by water drain holes) or when the cell cross dimensions are not small enough with respect to acoustic wavelength. The assumption of local reaction is then violated and the liner intended to be locally reacting will not behave as it should, that is, not as expected. In these cases the twomicrophone technique of in-situ impedance measurement will provide erroneous data. Further, it is shown that when using the two-microphone technique on really locally reacting liners the location of the surface microphone has to be chosen carefully in order to provide accurate data.