A more feasible and flexible approach is developed for emission of nonlinear Schrödinger solitons from an impurity-induced localized wave in the β-FPU model. The impurity structure used for inducing the localized wave is proposed to include an additional elasticity impurity α, as well as a mass impurity m. It is argued that the driving threshold γ_{th} for soliton emission can be controlled by regulating α in combination with m, and it can reach its minimum γ_{th}^{(}min) by optimizing the relationship between impurity parameters (α,m). The theoretical results are validated by both numerical simulations and experiments. Further, numerical and experimental investigations show the flexibility and efficiency of the proposed structure in controlling the amplitude of emitted solitons, the rate of emission, and even, the purity of radiated solitonic pulses, either by regulating the impurity parameters or by varying the driving parameters in a broadened range.

Evanescent acoustic waves are usually neglected in plane-wave approximation, but they may have remarkable effects on acoustic fields on occasions where the approximation is not strictly valid. Here, we investigate effects of low-frequency evanescent acoustic waves excited in a cavity of transverse dimensions not much smaller than, or even comparable to, the relevant wavelength. We find that the evanescent acoustic waves excited around a small opening of the cavity collectively behave as an acoustic inertia whose interaction with the acoustic compliance inherent to the cavity gives rise to a L-C-like series resonance at a frequency considerably lower than the cutoff frequency, resulting in vanishing sound pressure on average over the opening interface as if the interface acts as an absolutely soft boundary. Unlike a traditional Helmholtz resonator, the acoustic mass necessary for the resonance is totally provided, without the need for a physical neck, by the transverse evanescent acoustic waves inside the cavity, and thus, the proposed resonance is internal to the cavity. We also find that the interference of evanescent acoustic waves with a propagating sound wave can lead to the formation of a unique nodal curve of the sound field inside the cavity. The theoretically predicted effects of evanescent acoustic waves are well demonstrated both by our experiments and numerical simulations.

Acoustically soft boundaries are of substantial potential in acoustical designs relevant particularly to directional radiation of sound waves. Here an alternative strategy is proposed for designing a much thin metasurface capable of emulating an airborne soft boundary, by utilizing the unusual resonance that relies on excitation of evanescent waves on subwavelength scale. The metasurface is structured by a two-dimensional periodic array of oblate cavities that are open on the same plane to air ambience through apertures of negligible thickness. An analytical theory is developed on the reflection of plane and spherical sound waves incident on the proposed metasurface, and the trade-off between the thickness of the metasurface and working bandwidth are analyzed for adapting the metasurface to different design requirements. As a proof-of-concept demonstration, an airborne metasurface is designed and fabricated in our laboratory, with its thickness roughly 3 times thinner than the thinnest ever reported. The theoretical predictions are well confirmed both by the full-wave numerical simulations and the experimental measurements.

Since the discovery of crispations on a vibrating fluid layer, numerous types of patterns formed on fluid surfaces have been revealed. Here we report the observation of polygonal patterns of Faraday waves in vibrating water containers with parabolic and other concave bottoms. These patterns manifest themselves as simple geometric figures of symmetries ranging from elliptical up to heptagonal, with wavelengths much larger than the capillary length. Hence, they are intrinsically different from the previously studied patterns in vibrating drops or puddles and represent a particular variety of nonlinear shallow-water gravity waves or tidal waves in concave basins. Of specific interest is their resemblance to the collective excitations recently discovered in a driven Bose–Einstein condensate, not only sharing identical square-root scaling dispersion and pattern dynamics but also possessing similar nonlinear features like hard-spring nonlinearity. Based on the close correspondence, we propose an analogue of the patterning dynamics for classical and quantum fluid systems subject to confinement and argue that the analogy is mathematically valid even in the nonlinear regime.

We argue that a collection of evanescent waves inside a short expansion of a waveguide can react as an inertial reactance for an incoming wave and its interplay with the compliant effect inherent to the expansion forms the mechanism of the resonance that results in perfect insulation at a lower frequency than the cutoff of the expansion. We show that this type of resonance is ubiquitous, existing in general classic waves like acoustic and water waves traveling through locally expanded waveguides. The theoretical predictions are demonstrated both by numeric simulations and experiments. (C) 2021 Elsevier B.V. All rights reserved.

We investigate analytically and numerically whether an optimally designed defect structure can significantly reduce the threshold for breather train generation by almost two orders of magnitude in a sine-Gordon chain. By optimizing the parameters of defects based on a variational approach, we show that the decoupling of in-phase and antiphase branches allows us to independently minimize the driving threshold and reach the goal of emission of high-amplitude and well-formed breathers by an ultraweak driving. These results not only provide an optimal way for highly controllable and efficient emission of breathers, but also provide some insights into the mechanism of breather excitations in such processes as the DNA-protein interaction during transcription.

Underwater acoustic stealth is of significance for hiding targets. Since perfect cloaking achieved by carpet cloaks or metasurface carpet cloaks may only be suitable for static targets, here we propose a broadband 2-bit coding metasurface to reduce the scattering cross section (SCS) of an object by the destructive interferences among waves scattered by different coding elements. A simple and easy fabricated design is developed by drilling square holes into a steel plate. Utilizing the antiresonance induced by couplings among four square holes of the unit cell, we achieve an approximate $95.62\mathrm{%}$ fractional bandwidth of acoustic stealth with the SCS reduction below $\ensuremath{-}10\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$, which has been confirmed by both the theoretical prediction and experimental measurement. Compared with the previously proposed underwater acoustic carpet cloaks by the transformation acoustics, the bandwidth of SCS reduction is greatly expanded by the destructive interferences. In addition, simulations have demonstrated that the proposed metasurface may also be suitable for an object with a nonplanar surface.

Simultaneous temporal and spatial focusing of a pulse is of significance for detection and imaging. Here, an achromatic reflected metalens is designed using hybrid resonance and anti-resonance. The theoretical result demonstrates that the anti-resonance provides an extra degree of freedom to control local phases of reflected waves, yielding an achromatic lens of thickness equal to one half of central wavelength. To overcome the shortcoming of traditional approach to design lenses (neglecting the intercell coupling), a boundary integral method is proposed to alleviate the focus deviation over a broadband. The achromatic feature of designed lens is then verified in the frequency range from 2800 to 5600 Hz by an experiment. Owing to a very weak frequency dependence of focal point and a high reflected focusing efficiency over a broadband, a highly directional and long-distance acoustic probing scheme (the mainlobe width about 8(0)) is proposed with the aid of achromatic reflected metalens and being confirmed by another experiment, where a signal processing method using triple sensors separated by a subwavelength interval is adopted to eliminate the interferences between incident waves and reflected waves. Our result may find its application in a long-distance underwater acoustic probing.

We demonstrate the emission of solitons from a resonantly excited localized standing wave in a nonlinear chain of spring-coupled masses. The localized wave in this system is induced by a properly designed "impurity" and vibrates around the "impurity" with an intrinsic frequency. We observe that, when subjected to an external forcing, it is amplified to a large amplitude under the nonlinear resonance, and, then, its wave envelope splits apart leading to the release of most of its energy in the form of a large-amplitude traveling soliton. The experiment also shows that the rate of the emission can be controlled by finely tuning the driving parameters, thereby providing a feasible and controllable way for creation of solitons.

By combining Helmholtz resonances and Bragg scatterings, perfect acoustic blazing, a type of acoustic Wood’s anomaly, is achieved at the Bragg blazing points and non-Bragg blazing points. Owing to couplings among these blazing points, we experimentally observe a broadband and wide-angle acoustic negative reflection by an acoustic grating based on multiple coupled Helmholtz resonators per cell with a subwavelength thickness of 25.7 mm, where the frequency range is 3430 to 5145 Hz.

High-quality broadband ultrasound transducers yield superior imaging performance in biomedical ultrasonography. However, proper design to perfectly bridge the energy between the active piezoelectric material and the target medium over the operating spectrum is still lacking. Here, we demonstrate a new anisotropic cone-structured acoustic metamaterial matching layer that acts as an inhomogeneous material with gradient acoustic impedance along the ultrasound propagation direction. When sandwiched between the piezoelectric material unit and the target medium, the acoustic metamaterial matching layer provides a broadband window to support extraordinary transmission of ultrasound over a wide frequency range. We fabricated the matching layer by etching the peeled silica optical fibre bundles with hydrofluoric acid solution. The experimental measurement of an ultrasound transducer equipped with this acoustic metamaterial matching layer shows that the corresponding −6 dB bandwidth is able to reach over 100%. This new material fully enables new high-end piezoelectric materials in the construction of high-performance ultrasound transducers and probes, leading to considerably improved resolutions in biomedical ultrasonography and compact harmonic imaging systems.

The mysterious “Maoshan Bugle”phenomenon,the clearly audible bugle-like sounds after a launched firecracker blast in the air about 39 m high in the square in front of the Southern Jiangsu Anti-Japanese War Victory Monument at Mount Maoshan in Jiangsu,China,has drawn attentions of architects and acousticians in the past two decades.In this report,we attempted to reveal the physical mechanism of bugle calls and proposed the concept of acoustic negative reflection to explain the phenomenon.First,a model of plane sound wave reflection from an infinite periodic rigid surface is built and rigorously solved using an exact method of variation of boundaries,especially in the resonance region where the sound wavelength is of the same order as the periodic spacing.Then,numerical calculation is performed for the model with its periodic unit being taken as regular steps.Attention is then paid on the singular phenomenon that most of the incident sound power is reflected to the side of incidence,rather than to the side of specular reflection.This is what we call the negative reflection of sound,which is similar to the effects of blazed grating in optics.We then show the mysterious bugle calls at Mount Maoshan are actually the negative reflections,which are most clearly heard on the central axis in front of the square.The six notes of the bugle call are a result of sound reflection from the six groups of steps in front of the monument,which serve as acoustic gratings. According to the characteristics of the grating reflection,only certain frequencies in the explosion spectrum can be re-ceived.We further proved that these frequencies are integer multiples of the fundamental.Thus people hear the echo much like the rhythmic sound of a bugle,instead of the explosive sound.The spectrum characteristic of the reflected sound is theoretically analyzed by using the grating equation,with the calculated fundamental and harmonic frequencies well fitting with the audio samples recorded on site.Finally,we discussed the possibilities of finding similar phenomenon in other step-like structures.

Ultrasonic transducers have widely been used in industrial non-destructive detection,medical diagnosis,and other fields of applications.With the development of technology and broadening and deepening applications,there is a constant need of higher performance transducers,e.g.,for higher resolution imagining,etc.To break through the bandwidth limit of traditional ultrasonic transducers that are based on the quarter-wavelength matching,novel impedance matching technology has to be developed.Following the acoustic wave equation in inhomogeneous media, we study the transmission spectrum of continuous media of gradiently varying acoustic properties,and then,explore the possibility of using tubes of variable cross-section and multi-layered media to realize the equivalent gradient media with their matching performance well agreeing with those of the gradient media.Numerical simulations of the performance of transducers with two,three,and four matching layers are conducted by using the finite element method to attain optimized parameters of matching layers.

We show that sound waves can resonantly transmit through Bragg bandgaps in an acoustical duct periodically attached with an array of Helmholtz resonators, forming within the normally forbidden band a transparency window with group velocity smaller than the normal speed of sound. The transparency occurs for the locally resonant frequency so much close to the Bragg one that both the local-resonance-induced bandgap and the Bragg one heavily overlap with each other. The phenomenon seems an acoustical analog of the well-known electromagnetically induced transparency by quantum interference. Different from the Fano-like interference explanation, we also provide a mechanism for the transparency window phenomenon which makes it possible to extend the phenomenon in general.

It is revealed that the Fano-like interference leads to the extraordinary acoustic transmission through a slab metamaterial of thickness much smaller than the wavelength, with each unit cell consisting of a Helmholtz resonator and a narrow subwavelength slit. More importantly, both the theoretical analysis and experimental measurement show that the angle-independent acoustical transparency can be realized by grafting a Helmholtz resonator and a quarter-wave resonator to the wall of a narrow subwavelength slit in each unit cell of a slit array. The observed phenomenon results from the interferences between the waves propagating in the slit, those re-radiated by the Helmholtz resonator, and those re-radiated by the quarter-wave resonator. The proposed design may find its applications in designing angle-independent acoustical filters and controlling the phase of the transmitted waves.

A method based on ensemble empirical mode decomposition (EEMD) is proposed for accurately detecting the time varying pitch of speech in tonal languages. Unlike frame-, event-, or subspace-based pitch detectors, the time varying information of pitch within the short duration, which is of crucial importance in speech processing of tonal languages, can be accurately extracted. The Chinese Linguistic Data Consortium (CLDC) database for Mandarin Chinese was employed as standard speech data for the evaluation of the effectiveness of the method. It is shown that the proposed method provides more accurate and reliable results, particularly in estimating the tones of non-monotonically varying pitches like the third one in Mandarin Chinese. Also, it is shown that the new method has strong resistance to noise disturbance.

A method based on ensemble empirical mode decomposition (EEMD) is proposed for accurately detecting the time varying pitch of speech in tonal languages. Unlike frame-, event-, or subspace-based pitch detectors, the time varying information of pitch within the short duration, which is of crucial importance in speech processing of tonal languages, can be accurately extracted. The Chinese Linguistic Data Consortium (CLDC) database for Mandarin Chinese was employed as standard speech data for the evaluation of the effectiveness of the method. It is shown that the proposed method provides more accurate and reliable results, particularly in estimating the tones of non-monotonically varying pitches like the third one in Mandarin Chinese. Also, it is shown that the new method has strong resistance to noise disturbance.

Reflection of an obliquely incident plane acoustic wave by a rigid boundary that is periodically embedded with Helmholtz resonators is investigated.The resonator spacing d and the incident wavelengthλare assumed of the same order,both longer than the dimension of each Helmholtz resonator.Both analysis and computation show that for an incident acoustic wave with a frequency satisfying both conditions for the Bragg resonance and the Helmholtz resonance,the reflected wave disappears in the usual direction of reflection,but appears in the direction opposite to the incidence.The phenomenon is referred to in this work as back reflection,a new type of reflection completely different from the usual one.

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.

Tuning the extraordinary acoustical transmission is shown to be practically feasible simply by controlling acoustical impedances induced by surface evanescent waves. We demonstrate this idea with an example of making a sound tunnel in an acoustical waveguide with a subwavelength short throat and a catenoid horn working below its cutoff frequency. The throat acting as a resonant aperture assists sound waves effectively tunneling through the normally barred horn, leading to resonant transmission of sound waves within an adjustable narrow band. The example may find its applications for highly efficient acoustical filters and transmitters.