Neunert U., Kathan R., Sattelmayer T.

Due to their special combination of material and structure, novel porous materials like open-cell metal foams are increasingly often used in technical applications. In many cases, they are inserted in systems with mean air flow. For this reason, the influence of the flow on the damping characteristics of the metal foam is of particular interest. One method to describe the acoustic properties of a whole system is the network model, which combines the transfer matrices of acoustical elements. A transfer matrix relates the sound pressure and acoustic velocity on each side of the element. The transfer matrix of sound-absorbing materials is usually expressed as a function of its bulk properties (complex characteristic impedance and complex wavenumber). In the literature, several analytical models exist to predict the bulk properties of common porous media, which mainly differ in the values of their model parameters. In this work the multi-microphone transfer matrix method is used to measure the bulk properties and the flow resistivity of open-cell metal foams with varying pore structure and pore volume density for different element length. The experiments are carried out over a frequency range up to 950 Hz at different mean flows with Mach numbers below 0.1. The effects of flow velocity on the damping characteristics and on the flow resistivity are investigated. The frequency dependent complex characteristic impedances and complex wavenumbers for the measured metal foams are compared to those calculated by analytical models of related porous materials. Specific model parameters are determined by using a least-squares regression based on their measured bulk properties. A relation for the influence of the mean flow on the acoustic damping is provided. The results indicate that the multi-microphone transfer matrix method combined with a least-squares regression is a simple and reliable way to describe the acoustic properties of porous materials.