EXPERIMENTAL VALIDATION OF ACOUSTIC FOCUSING BY A DOUBLE LAYERED ACOUSTIC GRATING

Acoustic focusing plays an important role in various domains such as non-destructive evaluation, acoustic imaging and structural health monitoring. In the past decades, acoustic metasurface lens have been developed intensively and extensively based on various designs of engineered structures like space coiling up structures and Helmholtz-resonator-like structures. The reflected or refracted phase of acoustic wave existing from each unit structure can be controlled within 0 to 2π by tailoring its geometrical size. If the unit structures with different tailoring phase profiles are selected and arranged properly, the acoustic wave existing from each unit can arrive in phase at a predefined focal point, so that acoustic focusing can be achieved. However, the cumbersome geometrical shapes of these structures inevitably cause fabrication difficulties in practice and may result in low energy transmission due to the thermoviscous loss. To overcome these drawbacks, the paper proposes a double layered acoustic grating (DLAG) which consists of two layers of rigid panels perforated with multiple subwavelength slits. By optimizing the slits' positions, the acoustic energy can be converged into a predefined focusing region. The paper will first introduce how to predict the wave propagation controlled by the DLAG based on the surface coupling approach (SCA), where the SCA was proposed to predict the sound radiation in multiple connected spaces. Thereafter, the optimization of slits' positions for acoustic focusing will be investigated. Using a 3D printed DLAG with optimized slits' positions, the acoustic focusing was successfully validated by experiments and matches well with the predicted result obtained by the SCA, which demonstrates the feasibility of acoustic focusing by the DLAG in practice.