Dr.-Ing. Lennart Moheit

Modes in exterior acoustics and determination of sound power

The numerical treatment of the acoustic exterior problem requires consideration of the Sommerfeld radiation condition, which means that all sound energy must scatter to infinity and no reflexions lead to radiation of sound from infinity back into the field. Conventional Finite Element Method (FEM) is limited to bounded domains with clear definable finite external boundaries. The Boundary Element Method (BEM) allows the calculation of the sound pressure field for unbounded domains with the limitation to degrees of freedom at the surface of the solid structures amid the surrounding fluid. To obtain system matrices and pressure field in the whole fluid, the Infinite Element Method (IFEM) is applied. Collaboration of FEM and IFEM in the fluid leads to a discrete system of linear equations comprising stiffness, damping and mass matrices that can be solved for the sound pressure field in the whole fluid, finally.

Although, acoustic modes are commonly associated to bounded domains, even modes in exterior acoustics can be found in the literature. On the one hand frequency-dependent acoustic radiation modes can be obtained as eigenvalues and corresponding right eigenvectors from the acoustic impedance matrix. On the other hand, normal modes are determined by a state space eigenvalue problem and do not depend on the frequency. The calculation of the radiated sound power by modal superposition gives correct results for normal as well as for radiation modes. This research project aims at development of techniques of identification of neglectable and significant modes and their contribution to the total superimposed sound power.

Interactive Acoustic Apps

For a lively and vivid lesson, whether in study, exercises, school or follow-up at home, the Chair of Vibroacoustics of Vehicles and Machines develops Acoustic Apps, which can be executed and controlled in the web browser on a PC, tablet computer or smartphone. With these apps, physical phenomena from the fields of acoustics, vibrations and mechanics can be simulated and visualized. The effort for the set up and the dismantling of experiments is omitted and through the use of digital media, the topics are taught differently and are provided to interested people and those who have problems to imagine the application of physics.

The physical problem are solved in the background on a server by using the finite element method (FEM) and the software COMSOL Multiphysics. The apps can be found here or in the navigation bar on the left hand side. Further links: the project description as a sponsored teaching concept of the TUM Lehrfonds, an article in the online magazine ExploreTUM, an article in the COMSOL blog and a blog article of the TU Munich at lehren.tum.de.

If you have questions or feedback, please contact Lennart Moheit or write an email to  apps(at)vib.mw.tum.de.