Chinese Scientists Invent Fractal-Based Metamaterials to Improve Sound Field in Car Cabins

Car enthusiasts are willing to spend hundreds of dollars on audio systems that can enhance the sound quality inside the vehicle. However, no matter how advanced the audio system is, the inherent directionality of the speakers and the complex shape of the car's cabin will exacerbate the sound differences between the driver and passengers.

According to foreign media reports, researchers from multiple institutions in China have collaboratively developed a prototype system using fractal technology to alleviate this sound discrepancy. The related paper was published in the journal "Journal of Applied Physics."

Image source: Journal of Applied Physics

Fractals are unique shapes that can be infinitely divided into smaller pieces, each of which is similar to the original shape of the fractal. The famous Koch snowflake fractal resembles a hexagonal snowflake and can be iteratively constructed starting from an equilateral triangle. Researchers have used this shape to create a metamaterial capable of bending sound waves, placing it in front of speakers to manipulate sound.

"When sound waves interact with obstacles comparable in size to their wavelength, diffraction effects occur, especially at the edges," said the author Ming-Hui Lu. "The perimeter and morphology of fractal structures evolve with the increase in fractal dimension, providing a promising solution for controlling these diffraction effects."

As the fractal dimension of the Koch increases, the area remains unchanged while the perimeter can increase significantly, thereby enhancing edge diffraction (i.e., sound waves bending around the curves of the fractal) without increasing the size of the metamaterial, which would otherwise make it too bulky for automotive speakers. This achieves broadband wavefront control, ensuring sound energy is more evenly distributed. In a car, this means all passengers can enjoy a better sound field.

To manufacture this metamaterial, researchers used precision molds to shape fractals and then placed them on car speakers. They measured the sound pressure levels at different seats in the car and found that the differences in sound pressure levels were significantly reduced, including in the high-frequency range, which is a problem that many speakers struggle to address.

"These results indicate that the fractal supersurface successfully creates a more uniform high-frequency sound field, significantly enhancing the auditory experience for passengers," said Professor Lu.

Professor Lu was pleasantly surprised to find that both laboratory tests and in-vehicle tests demonstrated robust sound field control effects. "The in-vehicle test results closely match the laboratory results, indicating that the fractal supersurface exhibits reliable and stable performance under various conditions," Professor Lu stated.

While researchers plan to expand the working bandwidth of metamaterials, they are currently also in contact with automotive partner Chery Automobile to explore potential opportunities to bring them to market.