Study on a Detection Technique for Scholte Waves at the Seafloor.

Scholte waves at the seafloor have significant potential for underwater detection and communication, so a study about detecting Scholte waves is very meaningful in practice. In this paper, the detection of Scholte waves at the seafloor is researched theoretically and experimentally. Acoustic models with the multilayer elastic bottom are established according to the ocean environment, and a tank experiment is designed and carried out to detect Scholte waves. Different from detecting Scholte waves in the seismic wavefield, a technique for detecting Scholte waves in the sound pressure field is proposed in this paper. The experimental results show that the proposed technique can detect Scholte waves effectively, and there are no problems such as seabed coupling and the effect of wave speeds. Furthermore, the results also show that this detection technique is still effective in conditions with a sediment layer. The existence of sediment layers changes the acoustic field conditions and affects the excitation of Scholte waves.

Robust far-field subwavelength imaging of scatterers by an acoustic superlens.

It is well accepted that the conversion of an evanescent wave into a propagating wave is critical to far-field subwavelength imaging. However, subwavelength resolution can also be achieved using the multiple signal classification (MUSIC) algorithm for the situation of low conversion. In order to explore the difference of imaging performance between these two approaches, an acoustic superlens of length about one wavelength is designed to convert the evanescent wave into a propagating wave, which can be harnessed by the MUSIC algorithm. It is confirmed that the conversion of the evanescent wave into a propagating wave plays a role in improving the imaging resolution against noise, and the imaging resolution is improved by both the MUSIC algorithm and an acoustic superlens.

Flux projection beamforming for monochromatic source localization in enclosed space.

Monochromatic sound source localization becomes difficult in enclosed space. According to the reciprocity theorem, a self-consistent method of source localization in enclosed space, referred to as the flux projection beamforming, is proposed, only using the measurement of the sound pressure and normal velocity on the closed boundary at a single frequency. Its validity is examined both by experiment and simulation.

Three-dimensional ultrathin planar lenses by acoustic metamaterials.

Acoustic lenses find applications in various areas ranging from ultrasound imaging to nondestructive testing. A compact-size and high-efficient planar acoustic lens is crucial to achieving miniaturization and integration, and should have deep implication for the acoustic field. However its realization remains challenging due to the trade-off between high refractive-index and impedance-mismatch. Here we have designed and experimentally realized the first ultrathin planar acoustic lens capable of steering the convergence of acoustic waves in three-dimensional space. A theoretical approach is developed to analytically describe the proposed metamaterial with hybrid labyrinthine units, which reveals the mechanism of coexistence of high refractive index and well-matched impedance. A hyperbolic gradient-index lens design is fabricated and characterized, which can enhance the acoustic energy by 15 dB at the focal point with very high transmission efficiency. Remarkably, the thickness of the lens is only approximately 1/6 of the operating wavelength. The lens can work within a certain frequency band for which the ratio between the bandwidth and the center frequency reaches 0.74. By tailoring the structure of the metamaterials, one can further reduce the thickness of the lens or even realize other acoustic functionalities, opening new opportunity for manipulation of low-frequency sounds with versatile potential.

Acoustic attenuation performance through a constricted duct improved by an annular resonator.

Acoustic attenuation performance through a constricted duct is greatly improved by an annular resonator. A method based on the Green's function for a semi-infinite circular duct is proposed to calculate the transmission loss, where the velocity distributions are assumed to be uniform at the constriction inlet/outlets and at the inlet to the annular resonator. From an analogous acoustical circuit, the effect of higher order evanescent modes on the quarter-wave resonance is described by an extra length. The improved acoustic attenuation performance is also confirmed by an experiment.

Subcritical penetration into rough seafloors due to Bragg scattering.

A tank experiment and theoretical analysis are carried out to study acoustic Bragg scattering by a sinusoidal surface with period 0.3 m and amplitude 2 cm between water and sand sediment. The penetrating field is measured in the frequency range from 20 to 40 kHz at grazing angles 10° to 90° in the tank. A theoretical solution for acoustic scattering by the sinusoidal surface is derived to explain the interference pattern observed in the experiment. The result shows that the minus first order Bragg scattering wave is strong enough to interfere with the refracted wave obeying Snell's law, forming interference patterns that can be detected experimentally.

Subcritical penetration of acoustic waves into inhomogenous seafloors.

A generalized model is applied to estimate the incoherent penetration ratio caused by volume scattering at grazing angles below the critical grazing angle. The factors that affect volume scattering have been discussed using experimental data in literature. A two-layered model that refers to sound scattering in two-layered media is used to evaluate the incoherent penetration ratio for most typical sediments. But for special cases, such as the experiment, SAX04, a three-layered model is necessary to describe scattering features especially for grazing angles θ<30°. It is shown that subcritical penetration is enhanced when the scale of volume fluctuations is comparable with the acoustic wavelength, and the scattered waves into the seafloor dominate over evanescent waves at depths larger than a few wavelengths.

Resonant emission of solitons from impurity-induced localized waves in nonlinear lattices.

We propose a mechanism for soliton creation from resonantly excited localized waves via supratransmission in band gaps of nonlinear lattices. A nonlinear localized wave, which is formed by and vibrates around an impurity with an intrinsic frequency, is found to undergo a local resonance when subject to an external forcing. Under the resonance, an instability develops that leads to the efficient emission of solitons at a much lower rate than that in uniform lattices with no impurity.