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In midair haptics, ultrasound phased arrays are mainly used due to their high spatiotemporal controllability. The constraint on the presentation distance of phased arrays to form a focus can be mitigated by utilizing concave reflectors. This paper numerically examines the convergence of a surface approximated by multiple planes serving as a reflector, substituted for an ideal concave surface. A mirrored phased array produced by the planar segments forms a focus and concurrently creates interference among imaginary sources. A single-point convergence condition is derived that constrains the accuracy of the approximated reflector and the phased array size. As long as it satisfies the convergence condition, the approximated reflector can form a single focal point. Numerical simulations were conducted to confirm the validity of the convergence equation and the 5% tolerance of the segment size for the reflector deformation. An experimental campaign was also conducted and confirmed that the polyhedral reflector was able to form a single small focus by controlling the phase shift of the sound source. [ABSTRACT FROM AUTHOR]

Frequency hopping (FH) signals have been widely used to improve performance against frequency selective fading phenomenon of underwater channels. However, the channel is slowly varying in regard to changes in weather conditions, and thus the conventional FH detection transmitting signals with fixed frequency cannot guarantee good detection performance in the dynamic underwater environment. To overcome the performance degradation in slowly-varying fading dispersive channels, this paper proposes an adaptive frequency-hopping (AFH) target detection method. Compared with conventional FH detection methods, the AFH can adaptively select the optimal detection frequency based on premeasured background noise and channel frequency response measured from previous experiments. Numerical simulations and lake trials are conducted to verify the effectiveness of the AFH. The simulation results show that the AFH has better detection performance than the conventional FH. The lake trial results have also verified the validity and feasibility of AFH. Importantly, AFH also achieves a better output signal-to-noise ratio under actual noise interference. [ABSTRACT FROM AUTHOR]

Gigahertz acoustic streaming enables the synthesis of localized microjets reaching speeds of up to meters per second, offering tremendous potential for precision micromanipulation. However, theoretical and numerical investigations of acoustic streaming at these frequencies remain so far relatively scarce due to significant challenges including: (i) the inappropriateness of classical approaches, rooted in asymptotic development, for addressing high-speed streaming with flow velocities comparable to the acoustic velocity; and (ii) the numerical cost of direct numerical simulations generally considered as prohibitive. In this paper, we investigate high-frequency bulk streaming using high-order finite difference direct numerical simulations. First, we demonstrate that high-speed micrometric jets of several meters per second can only be obtained at high frequencies, due to diffraction limits. Second, we establish that the maximum jet streaming speed at a given actuation power scales with the frequency to the power of 3/2 in the low attenuation limit and linearly with the frequency for strongly attenuated waves. Last, our analysis of transient regimes reveals a dramatic reduction in the time required to reach the maximum velocity as the frequency increases (power law in –5/2), leading to characteristic time on the order of μs at gigahertz frequencies, and hence accelerations within the Mega-g range. [ABSTRACT FROM AUTHOR]

Plate-type acoustic metamaterials (PAMs) are thin structures that exhibit antiresonances with high sound transmission loss (STL) values, making PAMs a promising new technology for controlling tonal noise in the challenging low-frequency regime. A PAM consists of rigid masses periodically attached to a thin baseplate. The periodicity of PAM can be exploited in simulations, allowing to model only a single unit cell using periodic boundary conditions. This approach essentially represents the PAM as an infinite structure, but real PAM implementations will always be finite and influenced by boundary conditions. In this paper, extensive numerical simulations of different PAM configurations have been performed to study the performance of finite PAM compared to infinite PAM. The results indicate that as the number of unit cells in a finite PAM increase, the STL converges toward that of an infinite PAM. The impact of the finite PAM edge boundary conditions becomes negligible at some point. Based on the numerical results, a simple criterion is proposed to determine a priori how many unit cells are required in a finite PAM design to consider it quasi-infinite. This criterion aids in justifying unit cell models with periodic boundary conditions for efficient design optimizations in practical PAM applications. [ABSTRACT FROM AUTHOR]

This document is a retraction notice from AIP Advances regarding four manuscripts that have been retracted due to evidence of manipulation in the publication and peer review processes. The editors of AIP Advances have determined that critical steps in the peer review process were manipulated, including the return of identical reviewer reports and author-suggested reviewers requesting revisions to add citations to unrelated works by the authors. As a result, the validity of these papers has been called into question and they are considered potentially unreliable. The authors maintain that the data support the validity of the papers, but the editors believe that the issues with the peer review process undermine confidence in their validity. [Extracted from the article]