A novel practical design of sheet-like acoustic metamaterials is described, which comprise a two-dimensional array of mutually connected cylindrical stubs with embedded metallic weights that can be processed with a simple integral molding. We investigate the elastic wave band dispersion of the acoustic metamaterial sheets to design the connected structure of the stubs so that each stub can work as spring–mass local resonators to open the out-of-plane acoustic bandgaps. For properly integrated materials, remarkable noise insulation performance over the mass law at the resonant frequencies is experimentally demonstrated. The sound insulation mechanism is numerically analyzed in association with the sound fields, the vibrational fields, and the dynamic effective mass using the finite element method by coupling the structures with acoustics. Furthermore, we show that the insulation frequency is tunable over a wide range of frequency by designing the physical and structural parameters of the resonator. The practically designed acoustic metamaterial sheets will pave the way for industrial use of acoustic metamaterials as advanced lightweight and compact sound insulators that can be tailored to meet individual needs on problems related to noise.

In the past two decades, many studies on piezoelectric nanofibers (NFs) prepared from poly(vinylidene fluoride) (PVDF) and its copolymers, including single NFs, randomly oriented nonwoven mats, and aligned NFs, have been reported. However, studies on the relationships between the PVDF NF diameter, the orientation of the β-phase crystals inside NFs, and the piezoelectric properties of the NFs are still limited. In this study, the effect of the fiber diameter on the internal molecular packing/orientation and piezoelectric properties of aligned PVDF NF thin films was investigated. Herein, piezoelectric thin films composed of densely packed, uniaxially aligned, PVDF NFs with diameters ranging from 228 to 1315 nm were prepared by means of electrospinning with a rotating collector and successive hot-pressing and poling. The effect of the diameters of PVDF NFs on their internal structures, as well as the piezoelectric properties of the thin films, was investigated. All prepared NFs mainly contained β-phase crystals with a similar total crystallinity. The orientation of the β-phase crystals inside the NFs increased with an increase in the fiber diameter, resulting in an improved transverse piezoelectric coefficient (d₃₁) for the thin films. The output voltage of the prepared thin films reached a maximum of 2.7 V at 104 Hz.