Normally, one must choose between sound absorption and diffusion. However, researchers have designed a metamaterial that combines both functions in the form of an acoustic panel. Published in the journal
Advanced Materials Technologies, this work could find applications in architectural and industrial acoustics.
Traditional acoustic materials generally fulfill one of the two crucial functions for acoustic comfort. They absorb sound to prevent it from being reflected or transmitted through a partition, or they diffuse sound to homogenize it within an enclosed space, such as in a recording studio, an orchestra pit, or a meeting room.
Principle of operation of the panel in absorption mode.
© Éric Ballestero
In traditional materials, such as porous foams or diffusers, these two functions are indeed antagonistic, because the first requires dissipating sound energy, while the second must scatter waves with minimal dissipation. Acoustic metamaterials allow these limitations to be overcome while remaining passive, meaning without needing an external energy input, such as electricity.
Researchers from the Acoustics Laboratory of Le Mans University (
LAUM, CNRS/Le Mans University), the
Hong Kong University of Science and Technology (China), and the
Polytechnic University of Valencia (Spain) have developed a so-called "dual-function passive metamaterial" (DFPAM), which offers both efficient absorption (~85%) and high diffusion (~80%), across two adjacent frequency ranges. It alternates between these two functions through a simple mechanical reconfiguration of the panel, which, furthermore, at only about 4.7 inches (12 cm) thick, is half the thickness of traditional treatments.
The metamaterial was designed in three stages, with a theoretical phase, simulations, and experimental validation. The acoustics were thus modeled to perform multi-objective optimization, which allowed a balance to be found between the two antagonistic functions. The panel was then fabricated by stereolithography, a printing method using an acoustically rigid polymer resin here. Absorption and diffusion occur over two different but complementary frequency ranges, with absorption of low frequencies (200-1000 Hertz) and then diffusion above that (1000-2500 Hertz).
This work paves the way for innovative applications in architectural and industrial acoustics, particularly in confined spaces, such as those found in transportation, aerospace, recording studios, or meeting rooms. Companies have already shown interest in leveraging these results.