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 method employing an anechoic termination assumption can yield serious errors because its accuracy depends significantly on the properties of both the termination and the tested sample. Since it is not possible to implement a perfect anechoic termination over a wide frequency range, a well-developed alternative approach has been adopted. A simple theoretical analysis has been carried out to compare these two methods. It has been shown that the two-load method, based on two sets of measurements with different boundary conditions at the tube termination, can give more reliable and predictable results. Greater discrepancies have been observed in the case of the anechoic termination method applied to samples with higher values of flow resistivity. Results of the two-load method match well with predictions based on a simple model for acoustic transmission through a porous plate.