Acoustic Analysis of Loudspeakers Cavity Including Viscothermal Effects

Abstract: The geometry of a loud speaker cavity often gets a complicated geometry with sharp corners and narrow ducts. For this kind of shape the internal energy losses caused by viscosity and thermal conduction play an important role in the acoustic behavior. Today there is no method that sufficiently predict these effects. In this report the analogy between the loudspeaker cavity, a simple mass and spring system and a Helmholtz’s resonator is very important. It shows that the damping at the eigen-frequencies corresponds to the acoustic impedance of the system. That was the reason that the aim of this master thesis was to create a finite element model of a loudspeaker cavity and with this calculate the acoustic impedance including the internal energy losses. The model was created by using a reliable approach of two equations. The first one describes the acoustic pressure and the second one describes the thermal conduction. The two equations can be used separated except at the boundaries where they are connected by the boundary conditions. This finite element model yields results that are totally damped, meaning that the model is not working. A reason for that could be that the mesh is too coarse. Another reason could be that the energy dissipating parameter bulk viscosity is not correctly implemented. One thing that made it difficult to implement was the lack of relevant values, another the fact that it seems to have different definitions for different kinds of applications.