Abstract

Behaviour of Rigid and Elastic Porous Materials at High Levels of Continuous Acoustic Excitation: Theory and Experiment

O.Umnova, K.Attenborough, A.Cummings (University of Hull, Cottingham Road, Hull, UK)

e-mail: o.umnova@hull.ac.uk

A model for the propagation of high amplitude continuous sound through rigid and elastic porous materials has been developed which allows for Forchheimer's correction to Darcy's law. The nonlinearity associated with this is shown to be the dominant one in the wide range of frequencies for most real porous materials. The model for rigid materials requires 4 parameters: tortuosity, porosity, flow resistivity and Forchheimer's parameter. Knowledge is required also of the Young's modulus and loss factor of the frame in elastic porous materials. Depending on the material parameters, sample thickness and frequency range the model predicts either growth or decrease of reflection coefficient value with sound amplitude. Measurements of the impedance and reflection coefficient has been performed on 0.15 m long samples of a high porosity rigid material (aluminium foam), low porosity rigid materials (porous concrete) and high porosity elastic materials (polyurethane foam) for incident sound pressures up to 250 Pa in the low-frequency range (up to 800 Hz). Good agreement between model predictions and data for rigid-porous materials is demonstrated. Based on this, calculations have been carried out for layered systems including several porous materials with different parameters to find the configuration that gives maximum absorption of high-amplitude sound at low frequencies.

Section : 3