We present the concept design of a new class of acoustic metamaterial structure based on a combined heterogeneous double-split hollow sphere (CHDSHS). These structures are local resonators possessing subwavelength band gaps. The present CHDSHS metamaterial structures are made of plastic sphere structures with two different holes. The main novelty of a CHDSHS relies in the significant noise reduction obtained and the simplicity of manufacture. Their characteristics in band gaps are influenced by the volume sizes of the plastic sphere and the areas of the two holes. The CHDSHS metamaterials are combined to increase pressure attenuation over a wide frequency range. The frequency shifts in the band gap due to the changes in size are formulated and discussed in detail. Furthermore, the band gaps of the present CHDSHS metamaterials are numerically and experimentally demonstrated.

This study develops a new acoustic negative-refraction metamaterial that utilizes a synthesized double split hollow sphere (DSHS) for its unit cell. Recent relevant research has affirmed the concept that acoustic metamaterials can show unusual behavior that has not been observed in nature previously. However, as some hypothetical metamaterial designs have material properties not found in nature, the realization of practical metamaterials requires practical and complicated models. As a contribution to the development of acoustic metamaterials, the present study proposes a new anisotropic unit structure that encompasses Helmholtz resonators. This structure is referred to as the DSHS, is easy to manufacture, and has the advantage in that it uses the natural medium in its original form. By drawing the equifrequency or isofrequency contours of the designed two-dimensional (2D) anisotropic unit structure using the Floquet–Bloch’s principle, the properties of the present metamaterial can be understood. Numerical simulations are also conducted to identify and present the characteristics of the presented acoustic metamaterial. Through these, a new refraction phenomenon is identified that deviates from Snell’s law, and an acoustic hyperlens is numerically implemented that overcomes the diffraction limit.