Please watch our short introduction of the BEM acoustics MATLAB toolbox using the CHIEF method to improve results and stability.
Using a sphere the acoustic radiation of a monopol source will be simulated. The results show small disturbances because the internal eigenmodes of the sphere are exited and influence the outer sound field. For longer simulation times the results may also become instable. This effect strongly depends on the choosen time step Δt. If the sampling frequency 1/Δt coincides with the frequency of an internal eigenmode of the structure the simulation will get instable. In case of the sphere we know the eigenmodes. But for more complicated structures this is not the case and choosing the time step Δt will become kind of a lottery game. By increasing the spatial discretisation the influence of the internal eigenmodes will be reduced. This will also increase the computational time of course.
An other way is to use the CHIEF method. To do so define some points inside of the structure (the so called CHIEF points). In the solution process the solver trys to find a solution with a zero sound pressure in these CHIEF points. Use arround 30% of the number of surface elements as a estimate for the number of CHIEF points.
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