Cavity-based underwater lens focuses sound with 40% weight reduction

Researchers in South Korea have developed an innovative underwater acoustic lens that can precisely focus sound while being approximately 40% lighter than conventional designs. This breakthrough, led by Professor Junsuk Rho of Pohang University of Science and Technology (POSTECH) in collaboration with the Korea Research Institute of Ships and Ocean Engineering (KRISO), aims to enhance underwater communication and monitoring.

Traditional acoustic lenses for focusing underwater sound, especially at low frequencies with long wavelengths, tend to be large and heavy due to their solid structures. The new approach replaces solid components with cavity-based metamaterial structures that allow water to pass through. These cavities act as resonators, interacting strongly with sound at specific frequencies, and their strategic arrangement allows the lens to concentrate sound waves to a single focal point.

The team utilized metal 3D printing to fabricate a 240 mm-diameter lens. They successfully demonstrated its ability to stably focus low-frequency underwater sound within the 20–35 kHz range. Under comparable design conditions (280 mm diameter), the cavity-based lens weighed 17.2 kg, a substantial reduction from the 27.5 kg of a conventional solid lens.

Furthermore, the researchers observed the Willis coupling phenomenon, where sound reflects differently based on its direction of incidence. This asymmetric response suggests the lens can not only focus sound but also offer precise control over the direction of acoustic waves. The researchers anticipate applications in underwater sensor networks, communication systems, and wireless acoustic energy transfer, potentially improving signal clarity and reliability in future underwater IoT environments.