Transmission Loss Measurement of Consolidated Granular Media (L)

The normal incidence transmission loss of a new class of sustainable acoustic materials, i.e. consolidated granular media, has been measured to determine their sound insulating performance.Tests performed on different samples in a standing wave tube suggest that the traditional methodemploying an anechoic termination assumption can yield serious errors because its accuracydepends significantly on the properties of both the termination and the tested sample. Since it is notpossible to implement a perfect anechoic termination over a wide frequency range, a well-developedalternative approach has been adopted. A simple theoretical analysis has been carried out to comparethese two methods. It has been shown that the two-load method, based on two sets of measurementswith different boundary conditions at the tube termination, can give more reliable and predictableresults. Greater discrepancies have been observed in the case of the anechoic termination methodapplied to samples with higher values of flow resistivity. Results of the two-load method match wellwith predictions based on a simple model for acoustic transmission through a porous plate.