Fractal-based metamaterial improves sound fields in car cabins

WASHINGTON, Nov. 11, 2025 — Car enthusiasts will pay hundreds of dollars for stereo systems that will improve the sound quality in their cars. However, the inherent directionality of speakers and complex shapes of car cabins can exacerbate sound disparities between the drivers and passengers, no matter how advanced a speaker system.

In the Journal of Applied Physics, by AIP Publishing, researchers from a collaboration of institutions in China created a prototype using a fractal to mitigate the sound differences.

Fractals are unique shapes that can be split into infinitely smaller pieces, each of which resembles the fractal’s original shape. The famous Koch snowflake fractal resembles a six-pointed flake and can be built iteratively beginning with an equilateral triangle. The researchers used this shape to create a wave-bending metamaterial that can be placed in front of a loudspeaker to manipulate sound.

“When sound waves interact with obstacles comparable in size to their wavelength, diffraction effects occur, particularly at the edges,” said author Ming-Hui Lu. “Fractal structures, whose perimeter and morphology evolve with increasing fractal dimension, offer a promising solution to manage these diffraction effects.”

As the Koch fractal dimension increases, the area remains constant while the perimeter can grow significantly, allowing increased edge diffraction — the bending of the sound waves around the fractal — without increasing the metamaterial’s size, which would make it too clunky for car speakers. This creates broadband wavefront control, which ensures a more even spread of acoustic energy. In a car, this translates to a better sound field for all riderss.

To create their metamaterial, the researchers used precise molding to shape the fractal and then placed it over a car speaker. They measured the sound pressure levels at different seats in the car and found their disparity significantly decreased, including at higher frequencies, something many speakers struggle with.

“These results demonstrate that the fractal metasurface successfully contributed to a more uniform high-frequency sound field, leading to a noticeable improvement in the auditory experience for passengers,” said Lu.

To Lu’s surprise and satisfaction, both the laboratory and in-car tests showed robust results for sound-field control.

“The in-vehicle test results were remarkably consistent with the laboratory findings, indicating that the performance of the fractal metasurface was both reliable and stable across varying conditions,” said Lu.

While the researchers plan to expand the operation bandwidth of the metamaterial, they are also currently in touch with their automotive partner, Chery Automobile Co., for potential opportunities to bring it to market.

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The article “Koch snowflake-inspired acoustic metasurface for broadband sound diffusion in automotive loudspeaker systems” is authored by Zhi-Han Li, Long-Xiang Xie, Xinhua Gao, Weichun Huang, Youzhong Xu, Jin Yang, Ming-Hui Lu, and Xu Zhong. It will appear in the Journal of Applied Physics on Nov. 11, 2025 (DOI: 10.1063/5.0277003). After that date, it can be accessed at https://doi.org/10.1063/5.0277003.

ABOUT THE JOURNAL

The Journal of Applied Physics is an influential international journal publishing significant new experimental and theoretical results in all areas of applied physics. See https://pubs.aip.org/aip/jap.

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