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Linear spreading of a wave packet or a Gaussian beam is a fundamental effect known in evolution of quantum state and propagation of optical/acoustic beams. The rate of spreading is determined by the diffraction coefficientDwhich is proportional to the curvature of the isofrequency surface. Here, we analyzed dispersion of sound in a solid-fluid layered structure and found a flex point on the isofrequency curve whereDvanishes for given direction of propagation and frequency. Nonspreading propagation is experimentally observed in a water steel lattice of 75 periods (~1 meter long) and occurs in the regime of anomalous dispersion and strong acoustic anisotropy when the effective mass along periodicity is close to zero. Under these conditions the incoming beam experiences negative refraction of phase velocity leading to backward wave propagation. The observed effect is explained using a complete set of dynamical equations and our effective medium theory.

 

A coupled resonant acoustic waveguide (CRAW) in a phononic crystal (PnC) was engineered to manipulate the propagation of ultrasonic waves within a conventional phononic bandgap for wavelength division multiplexing. The PnC device included two, forked, distinct CRAW waveguide channels that exhibited strong frequency and mode selectivity. Each branch was composed of cavities of differing volumes, with each giving rise to deep and shallow ‘impurity’ states. These states were utilized to select frequency windows where transmission along the channels was suppressed distinctly for each channel. Though completely a linear system, the mode sensitivity of each CRAW waveguide channel produced apparent nonlinear power dependence along each branch. Nonlinearity in the system arises from the combination of the mode sensitivity of each CRAW channel and small variations in the shape of the incident wavefront as a function of input power. The all-acoustic effect was then leveraged to realize an ultrasonic, spatial signal modulator, and logic element operating at 398 and 450 kHz using input power.

 

The principle of the conventional ultrasound test states that the detectable voids cannot be smaller than the acoustic wavelength. However, by using effective medium approximation, the fraction of small voids can be estimated by the variation of the effective density. In this study, a non-contacting ultrasound-based porosity fraction mapping methodology is developed for estimated small voids in coal with long operating wavelength in air. This novel ultrasonic technique based on the mechanical properties of coal offers a rapid scan of the effective density mapping and distribution of void fraction over a large sample area, which overcame the limitation of small voids detection in the conventional ultrasound testing. 

Metals are excellent conductors for phonon transportation such as vibration, sound, and heat. Generally, metal sound insulators require multimaterial structure or defects and unimetal sound insulators are challenging. Therefore, a design of a defect‐free sound insulator made by single alloys with multiple friction stir processes (FSPs) is proposed. Periodic friction stir processing can induce superlattice‐like local mechanical properties’ modifications. By experimental acoustic characterization, it is observed that FSP can introduce clear acoustic–elastic property contrast on an aluminum plate by the presence of stir zone and heat‐affected zones. In numerical simulations, the signature FSP‐induced property profile is periodically and parallelly arranged on a long aluminum plate. The transmission gap frequencies are present on the frequency spectrum with the sound propagation direction perpendicular to the FSP paths. Disorder offsets on FSP periodicity are further introduced. Anderson localization is found on a resonance frequency, which provides −11 dB sound reduction by an exponential decay. Due to the finite design length, the slight disorder can also enhance sound insulation in the periodic transmission gap frequency. With analysis and comparison with different configurations, the best performance in the models can achieve −30 dB sound insulation in the 350 mm‐long aluminum alloy plate with 14 parallel FSPs.

 

The functionality of thermally active phononic crystals (PnC) and metamaterials can be greatly enhanced by utilizing the temperature-dependent physical characteristics of heat-sensitive materials within the periodic structure. The phase transformation between water and ice occurs within a narrow range of temperatures that can lead to significant changes in its acoustic transmission due to the modification of the elastic properties of periodic phononic structures in an aqueous medium. A phononic crystal with acrylic scatterers in water is designed to function as an acoustic filter, beam splitter, or lensing based on the device’s temperature due to changes in the phase of the ambient medium. The transition from room temperature to freezing point reduces the contrast in acoustic properties between the ice-lattice and the scatterer materials (acrylic) and switches off the metamaterial of the water-based PnC. The numerically simulated equi-frequency contours and wave propagation characteristics demonstrate the switchable meta-material to the periodic phononic structure’s normal behavior due to the phase transition of water. Effects such as Van Hove’s singularity and filamentation-like effects in an acoustic meta-material system can be thermally tuned. 

A reconfigurable phononic crystal (PnC) is proposed where elastic properties can be modulated by rotation of asymmetric solid scatterers immersed in water. The scatterers are metallic rods with a cross section of 120◦ circular sector. Orientation of each rod is independently controlled by an external electric motor that allows continuous variation of the local scattering parameters and dispersion of sound in the entire crystal. Due to asymmetry of the scatterers, the crystal band structure possesses highly anisotropic band gaps. Synchronous rotation of all the scatterers by a definite angle changes the regime of reflection to the regime of transmission and vice versa. The same mechanically tunable structure functions as a gradient index medium by incremental, angular reorientation of rods along both row and column, and, subsequently, can serve as a tunable acoustic lens, an acoustic beam splitter, and finally an acoustic beam steerer. 

Defect mode induced energy trapping at the bandgap frequency of a phononic crystal has been widely explored. Unlike this extensively used mechanism, this work reports the use of nonreciprocity in the transmission band to trap energy inside a phononic crystal cavity. Passive nonreciprocity is due to natural viscosity of the background liquid (water) and asymmetry of aluminum scatterers. The level of nonresonant energy trapping was compared for three cavities with different symmetry. Enhancement of energy trapping at a frequency of 624 kHz was observed experimentally for the cavity where nonreciprocity suppresses acoustic radiation into environment. Experimental results were further investigated and confirmed using finite element numerical analysis. 

Using analytical results for viscous dissipation in phononic crystals, we calculate the decay coefficient of a sound wave propagating at low frequencies through a two-dimensional phononic crystal with a viscous fluid background. It is demonstrated that the effective acoustic viscosity of the phononic crystal may exceed by two to four orders of magnitude the natural hydrodynamic viscosity of the background fluid. Moreover, the decay coefficient exhibits dependence on the direction of propagation; that is, a homogenized phononic crystal behaves like an anisotropic viscous fluid. Strong dependence on the filling fraction of solid scatterers offers the possibility of tuning the dissipative decay length of sound, which is an important characteristic of any acoustic device. This article is part of the theme issue ‘Wave generation and transmission in multi-scale complex media and structured metamaterials (part 2)’. 

We designed and characterized a 3D printed acoustic shear wave polarization rotator (PR) based on the specific nature of the fused-deposition-modeling printing process. The principle of the PR is based on rotation of the polarization axis of a shear wave due to the gradual change in orientation of the axis of anisotropy along the direction of wave propagation of a printed layered structure. The component of the shear modulus parallel to the infilled lines within each layer is significantly higher than that in the perpendicular direction. As the PR was printing, a small angle between neighboring layers was introduced, resulting in a 3D helicoidal pattern of distribution of the axes of anisotropy. The polarization of the propagating shear wave follows this pattern leading to the rotation of the polarization axis by a desirable angle. The total rotation angle can be tuned by the number of printed layers. The fabricated [Formula: see text] rotators demonstrate high performance that can be improved by changing the infill fraction settings.

 

The square lattice phononic crystal (PnC) has been used extensively to demonstrate metamaterial effects. Here, positive and negative refraction and reflection are observed simultaneously due to the presence of Umklapp scattering of sound at the surface of PnC and square-like equifrequency contours (EFCs). It is found that a shift in the EFC of the third transmission band away from the center of the Brillouin zone results in an effectively inverted EFC. The overlap of the EFC of the second and third band produce quasimomentum-matching conditions that lead to multi-refringence phenomena from a single incident beam without the introduction of defects into the lattice. Additionally, the coupling of a near-normal incident wave to a propagating almost perpendicular Bloch mode is shown to lead to strong right-angle redirection and collimation of the incident acoustic beam. Each effect is demonstrated both numerically and experimentally for scattering of ultrasound at a 10-period PnC slab in water environment.