NumCalc: An open-source BEM code for solving acoustic scattering problems

The calculation of the acoustic field in or around objects is an important task in acoustic engineering. The open -source project Mesh2HRTF and its BEM core NumCalc provide users with a collection of free tools for acoustic simulations without the need of having an in-depth knowledge into numerical methods. However, we feel that users should have a basic understanding with respect to the methods behind the software they are using. We are convinced that this basic understanding helps in avoiding common mistakes and also helps to understand the requirements to use the software. To provide this background is the first motivation for this article. A second motivation for this article is to demonstrate the accuracy of NumCalc when solving benchmark problems. Thus, users can obtain an idea about the accuracy as well as requirements on the memory and CPU requirements when using NumCalc. Finally, this article provides detailed information about some aspects of the actual implementation of BEM that are usually not mentioned in literature, e.g., the specific version of the fast multipole method and its clustering process or how to use frequency -dependent admittance boundary conditions.