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  1. 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. 
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    Free, publicly-accessible full text available August 21, 2024
  2. 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. 
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  3. 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. 
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  4. 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. 
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  5. 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. 
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  6. null (Ed.)
    This work demonstrates the detections and mappings of a solid object using a thermally tunable solid-state phononic crystal lens at low frequency for potential use in future long-distance detection. The phononic crystal lens is infiltrated with a polyvinyl alcohol-based poly n-isopropyl acrylamide (PVA-PNIPAm) bulk hydrogel polymer. The hydrogel undergoes a volumetric phase transition due to a temperature change leading to a temperature-dependent sound velocity and density. The temperature variation from 20 °C to 39 °C changes the focal length of the tunable solid-state lens by 1 cm in the axial direction. This thermo-reversible tunable focal length lens was used in a monostatic setup for one- and two-dimensional mapping scans in both frequency domain echo-intensity and temporal domain time-of-flight modes. The experimental results illustrated 1.03 ± 0.15λ and 2.35 ± 0.28λ on the lateral and axial minimum detectable object size. The experiments using the tunable lens demonstrate the capability to detect objects by changing the temperature in water without translating an object, source, or detector. The time-of-flight mode modality using the tunable solid-state phononic lens increases the signal-to-noise ratio compared to a conventional phononic crystal lens. 
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  7. null (Ed.)
    The primary noise sources of the vehicle are the engine, exhaust, aeroacoustic noise, and tire–pavement interaction. Noise generated by the first three factors can be reduced by replacing the combustion engine with an electric motor and optimizing aerodynamic design. Currently, a dominant noise within automobiles occurs from the tire–pavement interaction over a speed of 70–80 km/h. Most noise suppression efforts aim to use sound absorbers and cavity resonators to narrow the bandwidth of acoustic frequencies using foams. We demonstrate a technique utilizing acoustic metasurfaces (AMSes) with high reflective characteristics using relatively lightweight materials for noise reduction without any change in mechanical strength or weight of the tire. A simple technique is demonstrated that utilizes acoustic metalayers with high reflective characteristics using relatively lightweight materials for noise reduction without any change in mechanical strength or weight of the tire. The proposed design can significantly reduce the noise arising from tire–pavement interaction over a broadband of acoustic frequencies under 1000 Hz and over a wide range of vehicle speeds using a negative effective dynamic mass density approach. The experiment demonstrated that the sound transmission loss of AMSes is 2–5 dB larger than the acoustic foam near the cavity mode, at 200–300 Hz. The proposed approach can be extended to the generalized area of acoustic and vibration isolation. 
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  8. null (Ed.)
    Abstract Rapid thermokinetics associated with laser-based additive manufacturing produces strong bulk crystallographic texture in the printed component. The present study identifies such a bulk texture effect on elastic anisotropy in laser powder bed fused Ti6Al4V by employing an effective bulk modulus elastography technique coupled with ultrasound shear wave velocity measurement at a frequency of 20 MHz inside the material. The combined technique identified significant attenuation of shear velocity from 3322 ± 20.12 to 3240 ± 21.01 m/s at 45 $$^\circ$$ ∘ and 90 $$^\circ$$ ∘ orientations of shear wave plane with respect to the build plane of printed block of Ti6Al4V. Correspondingly, the reduction in shear modulus from 48.46 ± 0.82 to 46.40 ± 0.88 GPa was obtained at these orientations. Such attenuation is rationalized based on the orientations of $$\alpha ^\prime$$ α ′ crystallographic variants within prior columnar $$\beta$$ β grains in additively manufactured Ti6Al4V. 
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