Edge scattering impact in free field estimation of surface impedance
(English)Manuscript (preprint) (Other academic)
Accurate experimental characterization of sound absorbing materials is important to ensure good quality in simulations of larger systems and to analyze materials with unknown acoustic properties. Free field methods allow characterization of material properties at arbitrary sound incidence, which is advantageous compared to standardized methods. The errors of these methods have been studied, in particular those related to the size of the test samples. These are typically seen as oscillations about the correct value, especially at lower frequencies. The errors have been related to the source position and the sample size, but the impact of the material properties has not been investigated. In this paper, the influence of these properties on the errors are investigated through measurements and numerical simulations. The studies show a dependance on the material properties. The error results both from the pressure scattered at the sample edges and the pressure reflected at the material surface. The scattered field is shown to be stronger for materials with high flow resistivity, although the impact of this field on the result is stronger on materials with low flow resistivity. In addition, a method to reduce these errors based on an analytical formulation of the scattering is proposed. The method is applied to numerical simulations and shown to signicantly reduce the impact of the scattered field on the accuracy of the surface impedance.
Sound absorption, Surface impedance, Edge scattering, Free field measurement
Fluid Mechanics and Acoustics
Research subject Vehicle and Maritime Engineering
IdentifiersURN: urn:nbn:se:kth:diva-183411OAI: oai:DiVA.org:kth-183411DiVA: diva2:910843
QS 20162016-03-102016-03-102016-03-11Bibliographically approved