Characteristics and functioning of membrane absorbers covered with micro-perforated panels
The use of sound absorbers is a standard treatment in noise control engineering. Absorbers
are available in a variety of basic configurations, e.g. membrane absorbers,
porous absorbers, and micro perforated panels. There are also configurations that combine
two or more of these basic configurations to achieve more efficient absorbers.
This study focuses on the fundamental behaviour of an absorber configuration that is
mounted in the sub-frame under the engine in a passenger car. This configuration consists
of a plastic material forming closed volumes with a flexible membrane directed
toward the incident sound field. These membrane absorbers are partly covered with
a micro perforated plate that also covers regions without any membrane absorbers.
Therefore, three different cases were analysed: the membrane absorber alone, the micro
perforated panel alone, and the combination of both. Absorption coefficients were
measured with the Kundt’s tube for a variety of conditions. The absorber configurations
were studied with theoretical models and results from measurements and theory
were compared giving good agreement. A parameter study was made to determine
the most important parameters when optimizing the absorber configurations. Hence,
calculations were done to study the influence of the characterising parameters such as
size of the cubes, size of the micro holes of the perforated plate or the distance between
them. In addition, since absorbers are commonly used in diffuse-like fields, the absorption
coefficients for oblique incidence and diffuse fields were calculated.
numerical models of absorbers