Acoustic modal expansion for low-frequency sound in ducted atmospheres

• Applicants: Stefan Jacob, Ph.D.; Professorin Dr.-Ing. Sabine C. Langer
• Subject Area: Acoustics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536842818

Low-frequency sound can travel long distances in acoustic waveguides through the lower atmosphere, causing disturbance to both humans and wildlife even far away from sound sources. Those waveguides are commonly formed due to the presence of strong winds or temperature inversions, which tend to occur quite frequently during night hours. To support reducing noise pollution and the connected adverse health effects, we propose developing a method to characterize and predict the low-frequency noise propagating through atmospheric waveguides. The method is based on acoustic modes in the lower atmosphere. For low-frequency noise, it will be computationally very efficient, allowing us to include the effects of complex atmospheric conditions on sound propagation. Most importantly, our approach will cope well with the long-range nature of this noise propagation. In contrast, the computational complexity of existing noise prediction methods that are often developed for medium to high-frequency sounds renders them unsuitable for scenarios requiring fast and accurate predictions for low-frequency noise over long propagation distances. To archive accurate and fast calculations, we want to explore the feasibility of decomposing complex fields into well-interpretable mode spectra. Mode spectra are datasets particularly useful for far-field predictions and evaluation. Modal decomposition is a procedure that is widely used in other areas where guided waves are considered, for example, optics, ultrasonics, and aeroacoustics. Acoustic mode spectra can be used to predict noise under various unmeasured conditions and give insight into the physics and mechanics of the noise sources. Until now, modal approaches and modal decomposition have not been facilitated for noise characterization in the lower atmospheres. The project's objective is to address critical questions in this regard, including the efficient numerical computation of mode characteristics for complex atmospheres, the decomposition of sound fields into these mode characteristics, and the assessment of the approach's susceptibility to uncertainties. After the project's conclusion, we will recommend conceptual and computational frameworks to apply the method in experimental campaigns, hence, preparing it to be elevated to a fully functional tool for engineers in industries that require fast and accurate noise prediction methods.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Ulrich Römer