An ultra-thin acoustic absorber for underwater low-frequency noise

Human activity in the oceans has been steadily increasing over the past several decades, and today, underwater vehicles, pipelines, and other equipment are a common sight beneath the waves. Human activity brings noise, however, which can be harmful for nearby marine life.

Wang et al. developed a method for suppressing low-frequency underwater noise using folded-cavity extended-neck Helmholtz resonators. Their ultra-thin design effectively reduces noise at a wide range of frequencies, offering an improvement over traditional bulky sound absorbers.

“Low-frequency underwater sound creates a severe size problem,” said author Kai Zhou. “Many conventional designs are either too thick, too narrowband, or not dissipative enough for compact low-frequency underwater noise control.”

The researchers took a different approach, adding several modifications to a standard Helmholtz resonator to improve its performance. They extended the resonator neck to better target low-frequency noise, while folding the cavity to reduce its thickness. Then, they added a second resonator cavity to widen the targeted frequency range. They also incorporated a rubber interlayer to help tune the resonances and supply additional damping.

The result is an acoustic absorber that achieves strong absorption from 250-450 Hz with only 32-42 mm of thickness. The team also designed a broadband absorber with a several hundred Hz range with only 52 mm of thickness.

The authors hope their design will be used as a thin acoustic coating on underwater vehicles and equipment.

“We plan to fabricate prototype absorbers and test their performance in controlled water-based acoustic measurement facilities,” said Zhou. “We also want to examine how robust the design is when real-world factors are included, such as manufacturing tolerances, material-property variations, hydrostatic pressure, and possible nonlinear effects at high sound pressure levels.”

Source: “Deep-subwavelength broadband underwater sound absorption: Modeling, optimization, and mechanism elucidation,” by Jiayu Wang, Yuanze Li, Xiang Yu, Badreddine Assouar, Siqi Ding, Yi-Qing Ni, and Kai Zhou, Journal of Applied Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0339570 .