A boundary element method based procedure to calculate boundary admittances from measured sound pressures

Owing to the rapid development of computers in the past few years, boundary and finite element methods have become a powerful tool for acoustic calculations of internal and external problems. The calculation of closed domains (i.e. vehicle cabin) is often done using rigid acoustic boundary conditions (Neumann condition) since there is hardly any information about boundary admittance available. The determination of admittances using Kundt's tube does not consider the diffuse sound field. The calculation from the measured reverberation time provides an average admittance of the whole boundary only. This paper presents an application of the boundary element method which can be used to calculate boundary admittances at the nodes of the boundary element mesh from sound pressures measured at mesh points or at internal points. Additionally, this method is suitable for the determination of admittances at openings of the domain, i.e. open windows or the ear canal opening. Herein, one can receive the sound pressure from the calculation of the external acoustic problem or from measurements. The method is validated by consideration of the boundary admittance of a uniformly pulsating sphere.