Acoustic properties of unconsolidated granular mixes

Simple empirical relations have been proposed to relate a limited number of directly measurable non-acoustic properties of an unconsolidated granular mix to its characteristic acoustic impedance and propagation constant. These properties are: characteristic particle dimension, porosity, tortuosity and the density of the grain base. It is believed that the model accounts heuristically for the mechanical friction between the elements of the rigid frame, the absorption in the frame micro-pores, and the degree of compaction. These effects can be important and are linked to the value of material density. This work presents practical applications of the proposed model for the prediction of the acoustic characteristics of hard-backed layers of loose granular mixes which can be used for acoustic absorption and insulation. It is shown that the predictions are in excellent agreement with the measured data for a representative range of loose granular mixes. A comparison of the results of the 4-parameter Attenborough model for the acoustic properties of porous media and the experimental data is made also. This model is used extensively to predict the acoustic properties of porous ground and granular media. The accuracy of this model with respect to loose granular materials is discussed.