Measurement and modelling of acoustical properties of open-cell metal foams for acoustic liner applications

As one kind of porous materials, open-cell metal foams are suitable for absorbing sound in much harsher pressure and temperature environments where no ordinary material can survive, due to their superior features such as fire resistance, low moisture and high stiffness. The first thing before utilizing such a material is to measure and model the acoustical properties, which is the main work done in this paper. By using the transfer-matrix method, the equivalent wave numbers and characteristic impedances for a particular metal foam were measured in a rectangle duct with an approximately anechoic termination. With the accurate measured parameters, the well-known empirical power law was used to establish a single parameter model. During the procedure, measurements of absorption coefficients of such a material with a given depth was conducted to judge the accuracy of the measured data and the reliability of the model. It is found that some physical constraints must be satisfied during data processing to assure physically right results and the length of the sample must be selected carefully when the transfer-matrix method is applied. It is also found that this simple model based on the low-frequency measured parameters gives a very good prediction of absorption coefficients in the higher frequencies, proving the reasonability of the power law. Finally, the established empirical model was used to calculate the transmission loss of the metal foam liner while the experimental measurements were underway at the same time. The measured data and the predicted curves match well, which again shows the reliability and effectivity of the established model for acoustic liner applications.