Triangular fractals can dampen low-frequency urban noise

Along with air and water contamination, noise pollution is one of the largest environmental hazards in urban areas. Though mid- and high-frequency noise can be mitigated, low-frequency sounds — originating from transportation systems, industrial machinery, construction activities, and building services like elevators and air conditioning units — are much more difficult, because their long wavelengths and strong penetration capabilities require bulky materials.

Wen et al. developed an ultra-thin metamaterial that can absorb low-frequency noise while avoiding the traditional bulk. Their 50 millimeter-thick structure relies on a third-order triangular fractal design, which elongates the propagation path of sound waves within a small physical space. Once a sound enters the fractal duct, it must travel through its entirety, dissipating into the material as it moves.

The researchers’ design offers easy tunability and flexibility. The metamaterial is comprised of three layers, which can be reordered to tune its fundamental absorption frequency. Its 3D-printed, modular structure can be assembled into panels to fit any desired space, making it particularly useful for weight- and space-sensitive applications.

“[The metamaterial] can be installed as a lining or cladding in confined spaces where low-frequency noise control is critical, such as aircraft cabins, vehicle interiors, and ship hulls; building partitions; machinery enclosures; acoustic windows; and barriers in urban environments,” said author Xiaodong Wen.

Wen says the group is currently focused on increasing the broadband nature of their design with different fundamental frequencies, further miniaturizing the material, and exploring how to apply it to more complex sounds.

Source: “Low-frequency multi-band sound absorption in triangular fractal acoustic metamaterials,” by Xiaodong Wen, Changxiang An, Ning Li, Siqi Gao, and Lei Kang, Journal of Applied Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0307772 .