In the realm of acoustic metamaterials, a recent study has shed light on a phenomenon known as Willis coupling, an acoustic analogy to electromagnetic bi-anisotropy. The research, conducted by Philip A Cotterill, David Nigro, and William J. Parnell, explores the emergence and impact of Willis coupling in heterogeneous elastic slabs submerged in an acoustic fluid. This coupling is particularly intriguing due to its ability to link compressional and shear waves, a characteristic not yet fully understood in elastodynamic metamaterials.
The team investigated a unique microstructure within the elastic slab, composed of circular cylindrical voids. The asymmetry in the structure is achieved through two neighboring line arrays, each containing a repeating void of differing radius. The slab’s matrix is notably soft, with a Poisson ratio close to 1/2, which allows the voids to act as Giant Monopole Resonators. This configuration induces a robust dynamic response at low frequencies, a significant finding for potential applications.
The incorporation of Willis constitutive coupling in this structure enables the assignment of a unique set of effective material properties to the slab. These properties remain valid up to a maximum frequency, determined by the periodic spacing of the voids and the elastic properties of the substrate. The researchers also introduced loss in the elastic medium through its shear modulus, which resulted in strong directional-dependent absorption at low frequencies. Importantly, this absorption maintains reciprocity, a principle that ensures the consistency of physical laws when the direction of time is reversed.
The practical implications of this research are substantial for the field of music and audio production. The ability to control and manipulate sound waves with such precision could lead to innovative designs in acoustic materials and structures. For instance, these findings could inspire the development of advanced soundproofing materials that absorb sound directionally, reducing unwanted noise in recording studios or concert halls. Additionally, the unique dynamic response at low frequencies could be harnessed to create more effective bass traps, enhancing the quality of sound reproduction in audio systems.
Furthermore, the understanding and application of Willis coupling could revolutionize the design of musical instruments. By incorporating these principles, instrument makers could create devices that produce richer, more complex sounds, or that respond more dynamically to the player’s touch. The potential for innovation is vast, and the insights gained from this research could pave the way for a new era in acoustic technology. Read the original research paper here.
