Calculation of Visco-Thermal Losses in Thin Fluid Layers using BEM

Numerical modeling of acoustic structures containing narrow gaps or tubes requires considering the visco-thermal losses in the fluid. In condenser microphones, viscothermal losses in a 20- micrometer air layer behind the diaphragm control the response of the device, especially at high frequencies. The paper describes a numerical implementation using the Boundary Element Method (BEM) of a formulation that includes both viscous and thermal losses. The present development is an extension of previous theoretical work made by M. Bruneau. The sound field is divided into contributions from acoustic, thermal and viscous effects that must be coupled at the boundary. The viscous effects present special difficulties due to their vectorial nature and coupling through particle velocity boundary conditions. The base BEM formulation used is direct collocation. In addition, a special near-singular integration technique is used to overcome the high aspect-ratio problem.