Loss-induced modal selection by a resistive wiremesh.

This work examines the impact of local losses produced by a resistive wiremesh on the modes of an acoustic cavity. In the one-dimensional case, we demonstrate the ability to selectively affect the modes, ranging from being completely unaffected by the wiremesh to being fully absorbed by it. This effect can be used to filter the cavity modes.

Higher-order mode filtering by a resistive layer.

A method for filtering higher-order acoustic modes using a resistive layer is proposed and applied to a two-dimensional rectangular waveguide with a quiescent fluid. An analogue of Cremer's criterion is discussed and used to obtain the optimal modal attenuation of the non-planar waves while the plane wave is preserved. Numerical validation of the concept is performed for a straight waveguide and an abrupt expansion in a waveguide.

Frozen sound: An ultra-low frequency and ultra-broadband non-reciprocal acoustic absorber.

The absorption of airborne sound is still a subject of active research, and even more since the emergence of acoustic metamaterials. Although being subwavelength, the screen barriers developed so far cannot absorb more than 50% of an incident wave at very low frequencies (<100 Hz). Here, we explore the design of a subwavelength and broadband absorbing screen based on thermoacoustic energy conversion.

Subwavelength Su-Schrieffer-Heeger topological modes in acoustic waveguides.

Topological systems furnish a powerful way of localizing wave energy at edges of a structured material. Usually, this relies on Bragg scattering to obtain bandgaps with nontrivial topological structures. However, this limits their applicability to low frequencies because that would require very large structures.

Exceptional Points of PT-Symmetric Reflectionless States in Complex Scattering Systems.

We investigate experimentally and analytically the coalescence of reflectionless (RL) states in symmetric complex wave-scattering systems. We observe RL exceptional points (EPs), first with a conventional Fabry-Perot system for which the scattering strength within the system is tuned symmetrically and then with single- and multichannel symmetric disordered systems. We confirm that an EP of the parity-time (PT)-symmetric RL operator is obtained for two isolated quasinormal modes when the spacing between central frequencies is equal to the decay rate into incoming and outgoing channels.

Sensitivity to losses and defects of the symmetry-induced transmission enhancement through diffusive slabs.

We inspect the robustness to absorption and to symmetry defects of the symmetry-induced broadband enhancement through opaque barriers in disordered slabs. The sensitivity of this phenomenon to symmetry defects is found to be strongly related to the distance from to barrier to the nearest defect, and, following, we propose a probabilistic model to estimate the conductance of a medium with an arbitrary number of randomly distributed defects. Also, the conductance enhancement is shown to be robust to absorption in the disordered medium, though being of course weakened.

Anomalous transmission through periodic resistive sheets.

This work investigates anomalous transmission effects in periodic dissipative media, which is identified as an acoustic analogue of the Borrmann effect. For this, the scattering of acoustic waves on a set of equidistant resistive sheets is considered. It is shown both theoretically and experimentally that at the Bragg frequency of the system, the transmission coefficient is significantly higher than at other frequencies.

Slow sound laser in lined flow ducts.

This work considers the propagation of sound in a waveguide with an impedance wall. In the low frequency regime, the first effect of the impedance is to decrease the propagation speed of acoustic waves. Therefore, a flow in the duct can exceed the wave propagation speed at low Mach numbers, making it effectively supersonic.

Non-reciprocal scattering in shear flow.

This work presents a study of scattering phenomena in shear flows and its application to impedance walls. These flows are described by a dimensionless shear layer thickness and a mean Mach number. Both transmission through a given shear layer and reflection on an acoustic treatment are studied.

Acoustic radiation pressure for nonreciprocal transmission and switch effects.

Systems capable of breaking wave transmission reciprocity have recently led to tremendous developments in wave physics. We report herein on a concept that enables one-way transmission of ultrasounds, an acoustic diode, by relying on the radiation pressure effect. This effect makes it possible to reconfigure a multilayer system by significantly deforming a water-air interface.

Using liner surface modes in acoustic ducts to make obstacles reflectionless.

Acoustic cloaking for the suppression of backscattering inside ducts is proposed in the audible range where plane waves are curved around the object using the surface modes of the liner. It is numerically shown that a slowly varying resonant liner (e.g.

Broadband-Enhanced Transmission through Symmetric Diffusive Slabs.

We report on a significant and broadband enhancement of the transmission through an opaque barrier, when placed between symmetric diffusive disordered slabs. The transmission enhancement is accompanied by a bimodal distribution of the transmission eigenvalues, and, for a given transmittance of the barrier, it finds an optimal value for a particular length of the disordered slabs. A simple model allows us to quantify the scalings between the parameters that show that the stronger the barrier, the stronger the maximum possible enhancement.

Explicit approximation of the wavenumber for lined ducts.

For acoustic waves in lined ducts, at given frequencies, the dispersion relation leads to a transcendental equation for the wavenumber that has to be solved by numerical methods. Based on an Eckart explicit expression initially derived for water waves, accurate explicit approximations are proposed for the wavenumber of the fundamental mode in lined ducts. While the Eckart expression is 5% accurate, some improved approximations can reach maximum relative errors of less than 10.

Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels for transmission problems.

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying crosssection waveguides, each of them being loaded by Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission.

PT-Symmetric Scattering in Flow Duct Acoustics.

We show theoretically and experimentally that the propagation of an acoustic wave in an airflow duct going through a pair of diaphragms, with equivalent amounts of mean-flow-induced effective gain and loss, displays all the features of a parity-time (PT) symmetric system. Using a scattering matrix formalism, we observe, experimentally, the properties which reflect the PT symmetry of the scattering acoustical system: the existence of spontaneous symmetry breaking with symmetry-broken pairs of scattering eigenstates showing amplification and reduction, and the existence of points with unidirectional invisibility.

Asymmetric Acoustic Propagation of Wave Packets Via the Self-Demodulation Effect.

This Letter presents the experimental characterization of nonreciprocal elastic wave transmission in a single-mode elastic waveguide. This asymmetric system is obtained by coupling a selection layer with a conversion layer: the selection component is provided by a phononic crystal, while the conversion is achieved by a nonlinear self-demodulation effect in a 3D unconsolidated granular medium. A quantitative experimental study of this acoustic rectifier indicates a high rectifying ratio, up to 10^{6}, with wide band (10 kHz) and an audible effect.

Empirical mode decomposition profilometry: small-scale capabilities and comparison to Fourier transform profilometry.

We present the empirical mode decomposition profilometry (EMDP) for the analysis of fringe projection profilometry (FPP) images. It is based on an iterative filter, using empirical mode decomposition, which is free of spatial filtering and adapted for surfaces characterized by a broadband spectrum of deformation. Its performances are compared to Fourier transform profilometry, the benchmark of FPP.

Slow sound in lined flow ducts.

The acoustic propagation in lined flow duct with purely reactive impedance at the wall is considered. This reacting liner has the capability to reduce the speed of sound, and thus to enhance the interaction between the acoustic propagation and the low Mach number flow ( M≃0.3).

Superadiabatic evolution of acoustic and vorticity perturbations in Couette flow.

Nonadiabatic transitions between the acoustic and the vorticity modes perturbing a plane Couette flow are examined in the context of higher-order WKB asymptotics. In the case of the Schrödinger equation, it is known that looking at the solution expressed in the superadiabatic base, composed of higher-order asymptotic solutions, smoothes quantum state transitions. Then, increasing the order of the superadiabatic base causes these transitions to tend to the Gauss error function, and, once an optimal order is reached, the asymptotic process starts to diverge.

Improved multimodal admittance method in varying cross section waveguides.

An improved version of the multimodal admittance method in acoustic waveguides with varying cross sections is presented. This method aims at a better convergence with respect to the number of transverse modes that are taken into account. It is based on an enriched modal expansion of the pressure: the first modes are the local transverse modes and a supplementary (+1)th mode, called boundary mode, is a well-chosen transverse function orthogonal to the first modes.

Low frequency acoustic resonances in urban courtyards.

Urban courtyards can be regarded as open cavities in the urban area, in which resonances can be excited by waves generated in the neighboring streets. The aim of the present work is to experimentally and numerically investigate low frequency resonance phenomena in these configurations. Experiments are carried out in a scale model and a numerical study is performed with a coupled modal-finite elements method.

Tunneling of electromagnetic energy in multiple connected leads using ε-near-zero materials.

A realization of a reflectionless power splitter is proposed by use of a metamaterial junction. To design the junction, the electromagnetic wave transmission in multiple connected leads is investigated theoretically and numerically. A closed analytical form is derived for the scattering matrix of any geometry of the interconnected leads.

Diffusive transport of waves in a periodic waveguide.

We study the propagation of waves in quasi-one-dimensional finite periodic systems whose classical (ray) dynamics is diffusive. By considering a random matrix model for a chain of L identical chaotic cavities, we show that its average conductance as a function of L displays an ohmic behavior even though the system has no disorder. This behavior, with an average conductance decay N/L, where N is the number of propagating modes in the leads that connect the cavities, holds for 1≪L≲√N.

A coupled modal-finite element method for the wave propagation modeling in irregular open waveguides.

In modeling the wave propagation within a street canyon, particular attention must be paid to the description of both the multiple reflections of the wave on the building facades and the radiation in the free space above the street. The street canyon being considered as an open waveguide with a discontinuously varying cross-section, a coupled modal-finite element formulation is proposed to solve the three-dimensional wave equation within. The originally open configuration-the street canyon open in the sky above-is artificially turned into a close waveguiding structure by using perfectly matched layers that truncate the infinite sky without introducing numerical reflection.

Flower patterns in drop impact on thin liquid films.

We describe experimentally the formation of a pattern for drop impacts on thin liquid films for a large range of impact parameters. Using the shallow-water approximation, we are able to explain the main mechanisms leading to these patterns: it consists in the linear instability of the self-similar axisymmetric radial solution of the equations. Agreement between the experiments and the theory is remarkably good, leading, in particular, to the prediction that the most unstable fold number scales like (We/h∞)2/7.