We propose a single-phase super lens with a low density that can achieve focusing of sound beyond the diffraction limit. The super lens has a star-shaped lattice structure made of steel that offers abundant resonances to produce abnormal dispersive effects as determined by negative parameter indices. Our analysis of the metamaterial band structure suggests that these star-shaped metamaterials have double-negative index properties, that can mediate these effects for sound in water. Simulations verify the effective focusing of sound by a single-phase solid lens with a spatial resolution of approximately 0.39 λ. This superlens has a simple structure, low density and solid nature, which makes it more practical for application in water-based environments.

Elastic metamaterials have promising applications in wave control and vibration isolation, due to their extraordinary characteristics, e.g., negative Poisson ratio, band gaps, effective negative mass density and effective negative modulus. How to develop new functional metamaterials using a special structure has always been a hot topic in this field. In this study, a three-dimensional (3D) star structure is designed to construct metamaterials with both negative static and dynamic properties. The results show that the 3D star structure formed a wide band gap at lower frequency and had a negative Poisson’s ratio. Different from conventional acoustic metamaterials, the main physical mechanism behind the low-frequency band gap of the 3D star structure is the resonance mode formed by the bending deformation of each rib plate, which made it easier to achieve effective isolation of low-frequency elastic waves with a low mass density. In addition, many structural parameters of the 3D star structure can be modulated to effectively adjust the band gap frequency by changing the angle between the concave nodes and aspect ratio. This study provides a new way to design the 3D acoustic metamaterials and develop the lightweight vibration isolation devices.

With the rapid development of animal husbandry, forage resources are increasingly scarce. Improving the utilization rate of forage products and silage efficiency of planting is an urgent problem to be solved. This experiment used high moisture alfalfa at the budding stage with a water content of 71.4% from saline-alkali and non-saline-alkali soils as raw materials, setting up four experimental groups: non-saline-alkali alfalfa without additives (HNS-CK), non-saline-alkali alfalfa with compound lactic acid bacteria (Lactobacillus plantarum + Lactobacillus buchneri + Pediococcus pentosaceus + Lactobacillus kimchi, HNS-L4), saline-alkali alfalfa without additives (HS-CK), and saline-alkali alfalfa with compound lactic acid bacteria (HS-L4). After 60 days of silage, the quality and microbial diversity of the silage were tested. The results showed that the dry matter (DM) and lactic acid (LA) of the HNS-L4 group were significantly higher than those of the HS-L4 (P < 0.05), while there was no significant difference in crude protein (CP) between the HNS-L4 group and the HS-L4 (P < 0.05). The neutral detergent fiber (NDF), acid detergent fiber (ADF), pH of the HNS-L4 group were all lower than those of the HS-L4. The results of the microbial community showed that compared with the non-additives group, the Shannon index decreased and the Simpson index increased in the compound lactic acid bacteria group, indicating a significant reduction in microbial diversity in the silage environment (P < 0.05). The dominant bacteria in the HNS-CK and HS-CK groups were Enterobacteriaceae, while the dominant bacteria in the HNS-L4 and HS-L4 groups were Lactobacillus. At the phylum level, the dominant bacteria in alfalfa after lactic acid bacteria treatment were Firmicutes, which were significantly higher than the control group (P < 0.05). Therefore, compound lactic acid bacteria can improve the quality of alfalfa silage in both saline alkali and non-saline-alkali soils, with saline-alkali soils being better than non-saline-alkali soils, and both can reduce microbial diversity.

We construct new quasi-three-dimensional fractal acoustic metamaterials based on adoption of the Menger structure, which offers extraordinary parameters such as double-negative properties and a near-zero density. The resulting metamaterials can thus achieve negative refraction, acoustic focusing and sound tunneling. Using the finite element method and the S-parameter retrieval method, the band structures and the effective parameters of these acoustic metamaterials are researched, respectively. The negative refraction property is numerically simulated using a Gaussian beam passing through a double negative prism. A plate lens with a refractive index of n = −1 is constructed to achieve acoustic focusing and the sound tunnelling ability is verified using the near-zero-density metamaterial. The results show that the Menger fractal structures have excellent acoustic properties and are promising for acoustic applications.

The acoustic characteristics of 2D single‐oscillator, dual‐oscillator, and triple‐oscillator acoustic metamaterials were investigated based on concentric ring structures using the finite element method. For the single‐oscillator, dual‐oscillator, and triple‐oscillator models investigated here, the dipolar resonances of the scatterer always induce negative effective mass density, preventing waves from propagating in the structure, thus forming the band gap. As the number of oscillators increases, relative movements between the oscillators generate coupling effect; this increases the number of dipolar resonance modes, causes negative effective mass density in more frequency ranges, and increases the number of band gaps. It can be seen that the number of oscillators in the cell is closely related to the number of band gaps due to the coupling effect, when the filling rate is of a certain value.