Koch snowflake-inspired acoustic metasurface for broadband sound diffusion in automotive loudspeaker systems

In confined automotive environments, the interaction between loudspeaker directivity and complex cabin boundaries further exacerbates the spatial non-uniformity of the sound field, especially at high frequencies. To address this limitation, we propose a fractal acoustic transmission metasurface (FATM) inspired by Koch snowflake geometry to mitigate undesired directivity effects. Through the integration of fractal-optimized edge structures, the FATM induces multiple secondary diffraction sources, facilitating broadband sound diffusion in the 10–18 kHz frequency range within the enclosed cabin space. By employing quantum genetic algorithm optimization, the proposed 2.33-mm-thick FATM achieves a 5 dB reduction in the spatial sound pressure levels (SPLs) disparity between axial and off-axis directions within automotive cabin environments. Systematic experimental validation conducted in both controlled anechoic environments and operational automotive cabins demonstrates a ∼5 dB reduction in on-/off-axis SPL disparity (10–18 kHz, in-cabin), indicating an effective strategy for broadband wavefront control in compact automotive cabins. This study highlights the potential of fractal metasurfaces as an effective strategy for broadband wavefront control in compact and complex acoustic systems.