The Nonlinear Eigenfrequency Problem of Room Acoustics with Porous Edge Absorbers

Using porous absorbers is common practice in the acoustic treatment of rooms. To reduce low-frequency reverberation in rooms, the absorber material is often placed in one or more edges of the room. Such an arrangement of porous absorbers is commonly known as bass trap or edge absorber. Edge absorbers have the advantage that, with relatively little porous absorber material, they efficiently reduce low-frequency reverberation, for which there are modal peaks, in the room, and do not overattenuate high-frequency reverberation. However, standard room acoustic simulation methods based on ray tracing and mirror sources cannot predict the edge absorber's influence on the acoustic field for low frequencies because the frequency limits of geometrical room acoustics are not met. Hence, a finite element model is employed to simulate the edge absorber's influence on a room's eigenfrequencies, wherein the edge absorber material is modeled using an equivalent fluid model. This results in a nonlinear eigenvalue problem due to the frequency dependence of the porous material parameters. The paper presents an iterative solution approach to the nonlinear eigenvalue problem and its application to acoustic predictions of edge absorbers. It is shown how edge absorbers shift and damp the eigenfrequencies of a room.