Elastic metamaterials possess band gaps, or frequency ranges that are forbidden to wave propagation. Existing solutions for impeding three-dimensional (3D) wave propagation largely rest on high-volume fractions of mass inclusions that induce and tailor negative effective density-based local resonances. This study introduces a class of elastic metamaterials that achieve low-frequency band gaps with a volume fraction as low as 3% (mass density as low as $0.034\mathrm{g}/{\mathrm{cm}}^3$). The working of the proposed design hinges on a 3D trampolinelike mode behavior that gives rise to wide, omnidirectional, and low-frequency band gaps for elastic waves despite very low-mass densities. Such a 3D trampoline effect is derived from a network of overhanging nodal microarchitectures that act as locally resonating elements, which give rise to band gaps at low frequencies. The dynamic effective properties of the metamaterial are numerically examined, which reveal that the band gap associated with the trampoline effect is resulted from a negative effective modulus coupled with a near-zero yet positive effective density. The experimental characterization is then made possible by fabricating the metamaterial via a light-based printing system that is capable of realizing microarchitectures with overhanging microfeatures. This design strategy could be useful to applications where simultaneous light weight and vibration control is desired.

In the functional 3D-printing field, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) has been shown to be a more promising choice of material over polyvinylidene fluoride (PVDF), due to its ability to be poled to a high level of piezoelectric performance without a large mechanical strain ratio. In this work, a novel presentation of in situ 3D printing and poling of PVDF-TrFE is shown with a d${}_{33}$ performance of up to 18 pC N${}^{-1}$, more than an order of magnitude larger than previously reported in situ poled polymer piezoelectrics. This finding paves the way forward for pressure sensors with much higher sensitivity and accuracy. In addition, the ability of in situ pole sensors to demonstrate different performance levels is shown in a fully 3D-printed five-element sensor array, accelerating and increasing the design space for complex sensing arrays. The in situ poled sample performance was compared to the performance of samples prepared through an ex situ corona poling process.

This study introduces a framework for the design and implementation of acoustic black holes (ABHs) in airfoils. A generalized multiparameter damped-ABH generation function is mapped onto airfoils based on the National Advisory Committee for Aeronautics series. Representative geometries and a uniformly distributed baseline, all with the same mass of structure and damping are fabricated using multimaterial PolyJet three-dimensional printing. Laser Doppler vibrometer measurements along the airfoil chord in response to a broadband 0.1–12 kHz excitation show a decrease in trailing-edge vibrations by as much as 10 dB, a broadband 5-dB reduction across the entire chord, substantial spatial and temporal modulation of flexural waves by ABH foils. Finite-element analysis (FEA) models are developed and validated based on the measured data. Furthermore, a parametric FEA study is performed on a set of comparable designs to elucidate the scope of modulation achievable. These findings are applicable to trailing-edge noise reduction, flow control, structural enhancement, and energy harvesting for airfoils.

Acoustic metamaterials, which manipulate acoustic propagation using architected, discrete structures, have wide ranges of applications but typically have a fixed response upon design and fabrication. We present a novel tunable acoustic structure with a granular jamming-based approach, where the jammed state acts as a solid with a well defined resonant response and the unjammed state acts as a liquid with a broad, weakly resonant response. In between these two extremes, resonance peak tuning is observed. Our approach to particle jamming is uniquely controlled with a dielectric elastomer actuator (DEA), whose applied confining pressure is governed by voltage. Mathematical modeling of the DEA and the confining pressure is used to understand the device behavior. Samples are fabricated and tested acoustically, revealing a novel solid–liquid like transition at an applied voltage of about 6.0 kV and a preceding “rattler” behavior at around 4.25 kV.

Dirac points in the band structure of acoustic systems are essential features affording classical analogs of quantum condensed matter states. We show that measured dispersion curves near and at the Dirac point of an acoustic graphene analog can be suppressed by strong variations in the impedance boundary between free field and surface wave regimes under certain conditions. Increased Rayleigh scattering and diffractive excitation are shown to increase the dispersed surface wave pressure amplitude, circumventing the impedance-based wave suppression. The improved excitation and scattering conditions for observing acoustic Dirac points for two samples with two distinct operational frequency ranges are reported.

In this work, we report amplitude-dependent band structures in mechanical lattices with spatially modulated weak cubic stiffness. In contrast to lattices characterized by spatially uniform nonlinearity, the lattices studied herein possess a remarkable ability to filter, guide, and redirect the propagation of nonlinear acoustic waves. We use a multiple scale perturbation analysis to derive amplitude-dependent dispersion relationships in a generalized spatially modulated lattice. Nonconventional results are presented: the existence of hardening and softening within the same branch, the merging of branches at specific amplitudes, and the broadband cancelation of dispersion shifting altogether. Direct numerical simulation of the lattice equations of motion provides validation for the analytical predictions, and a method of physically realizing the system is proposed. Findings may inspire nonlinear devices for enhanced vibration control and mitigation.