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Acoustic detection of machinery defaults from in-duct measurements is of practical importance in many areas, such as the health assessment of turbines in ventilation systems or engine testing in the surface and air transport sectors. This approach is, however, impeded by the low signal-to-noise ratio (SNR) observed in such environments. In this study, it is proposed to exploit the slow sound effect of Sonic Black Hole (SBH) ducted silencers to enhance the sensing of incident pulse acoustic signals with low SNR. It is found from transfer matrix and finite element modelling that fully opened SBH silencers with perforated skin interfaces are able to substantially enhance an incident pulse amplitude while channeling an air flow. We demonstrate that the graded depths of the SBH cavities provide rainbow spectral decomposition and amplification of the incident pulse frequency components, provided that impedance matching, slow sound, and critically coupled conditions are met. In-duct experiments showed the ability of a 3D printed SBH silencer to simultaneously enhance acoustic sensing and fully trap the pulse spectral components in the SBH cavities in the presence of a low-speed flow. This study opens up new avenues for the development of dual-purpose silencers designed for acoustic monitoring and noise control in duct systems without obstructing the air flow.

A challenging issue is currently the design of non-fibrous ultra-thin acoustic absorbers that are able to provide broadband performance in demanding environments. The objective of this study is to compare using simulations and measurements the broadband absorption performance of highly porous micro-capillary plates (MCPs) to that of micro-perforated panels (MPPs) under normal incidence while considering unbacked or backed configurations. MCPs are unusual materials used for sound absorption with micron-sized channels and a high perforation ratio. Impedance-based modeling and Kundt tube experiments show that MCPs with suitable channel diameters have a pure constant resistance that outperforms the acoustic efficiency of MPP absorbers. Unbacked MCPs exhibit a controllable amount of high absorption that can exceed 0.8 over more than five octaves starting from 80 Hz, thereby achieving a highly sub-wavelength absorber. MCPs still provide broadband high absorption when backed by a rigid cavity. Their bandwidth-to-thickness ratio increases toward its causal limit when the cavity depth decreases. A parallel MCP resonant absorber partly backed by closed and open cavities is proposed. Such MCP-based absorbers could serve as short anechoic terminations for the characterization of acoustic materials at low frequencies.

Broadband noise reduction over the low–mid frequency range in the building and transportation sectors requires compact lightweight sound absorbers of a typical subwavelength size. The use of multi-layered, closely spaced (micro-)perforated membranes or panels, if suitably optimized, contributes to these objectives. However, their elasticity or modal behaviors often impede the final acoustical performance of the partition. The objective of this study is to obtain insights into the vibrational effects induced by elastic limp membranes or panel volumetric modes on the optimized sound absorption properties of acoustic fishnets and functionally graded partitions (FGP). The cost-efficient global optimization of the partitions’ frequency-averaged dissipation is achieved using the simulated annealing optimization method, while vibrational effects are included through an impedance translation method. A critical coupling analysis reveals how the membranes or panel vibrations redistribute the locations of the Hole-Cavity resonances, as well as their cross-coupling with the panels’ first volumetric mode. It is found that elastic limp micro-perforated membranes broaden the pass-band of acoustic fishnets, while smoothing out the dissipation ripples over the FGP optimization bandwidth. Moreover, the resonance frequency of the first panels mode sets an upper limit to the broadband optimization of FGPs, up to which a high dissipation, high absorption, and low transmission can be achieved.

Wideband reduction of both sound reflection and transmission using compact non-intrusive liners remains a challenging issue for duct noise control applications. This work focuses on the acoustical performance limitations of cylindrical silencers made up of annular ring resonators with axial gradient of their cavity depths. It is shown from theoretical, numerical and experimental studies that a sub-wavelength silencer with optimized acoustic black hole (ABH) properties can be designed through which incident sound waves are retarded and fully dissipated within the activated resonant cavities. Such ABH-type silencer requires a suitable interplay between the wall impedance axial variations and the visco-thermal losses within the cavities. Key parameters that influence this balance are the axial growth of cavity depths and the wall porosity. A causal-based criterion has been proposed that maximizes the total integrated dissipated power, thereby leading to the optimal value of the wall porosity and to the ultimate bandwidth-to-length ratio that can be achieved by an ABH-type silencer given a target dissipation value. A space-frequency region has been numerically and experimentally identified over which the ABH effect is prominent. The causal-based criterion has been extended to account for the low-frequency ultimate performance of ABH silencers with coiled cavities and for the effects of a low-speed grazing flow.

Enhancing the low-frequency performance of compact acoustical micro-perforated treatments is a challenging task that requires global optimization studies performed on numerical or theoretical models. However, for a given geometry, there always exists a lower frequency bound below which the treatment will exhibit poor dissipation performance. In this study, it is shown how this lower frequency bound is related to the total integrated dissipated power through a causal-based criterion. Examples are shown for the cases of a rigidly-backed micro-perforate under plane wave incidence and for a locally-reacting micro-perforated resonant silencer mounted in a duct. Such criterion provides the ultimate bandwidth-to-length ratio that can be achieved by the acoustical treatments for a given target dissipation value. One can also deduce the optimal values of individual parameters such as the hole diameters or the wall porosity that leads to a suitable balance between leakage and dissipation effects. The causal-based optimized treatments are validated against simulated or measured impedance tube data.

Improvement of acoustic absorption and transmission in the low frequency range when facing constraints on total volume and weight constitutes a constant challenge in the field of noise control. Emergence of acoustic metamaterials has allowed the development of compact sub-wavelength absorbers that can provide solutions in terms of the required wideband low-frequency sound dissipation. In a lined waveguide, the effective velocity of the incoming sound can be progressively reduced to zero leading to the Acoustic Black Hole (ABH) effect. In this work, this concept of sound trapping has been mostly explored for widely-opened ABH silencer for the reduction of the transmission properties without obstructing flow circulation. Attention has been paid to the attenuation of the acoustic backreflections that can be of interest for improving engine combustion efficiency for instance. Parametric studies are carried out to study the influence of the physical ABH factors on the acoustic performance. We have also performed a causality-constrained optimization to determine the minimum bandwidth-to-length ratio required for a fixed dissipation requirement. Results have been validated with experimental results.

Attenuation of acoustic disturbance with compact and low-weight acoustical devices is a demanding requisite in industries such as vehicle exhaust mufflers, turbofan liners or ventilation systems. Acoustic Black Holes (ABH) is a concept that has been studied originally in the field of vibration control for the design of a structure based on a power-law thickness profiles along its edges. The idea of wave trapping and dissipation has been extended to the field of duct acoustics using compact devices that avoid blocking the air flow. Based on this idea, in this work we propose the study of a micro-perforated silencer that should prevent reflection and transmission of the sound waves over a wide range of audible frequencies. The use of micro-perforations over the facing cavities should help to extend performance towards the low frequency range. The acoustic behavior of the ABH is assessed from theoretical and numerical modeling. The performance can be greatly improved when the physical constituting parameters are optimized simultaneously. Particle swarm optimization is a computational method inspired by the behavior of natural systems that is able to provide a satisfactory solution with enhanced total dissipation.

Micro-perforated liners can be of interest for the attenuation of the low-frequency noise in ducted geometries that can appear for instance due to the presence of a Turbulent Boundary Layer (TBL) excitation that contributes to the noise between 500 Hz and 2 kHz. Classical acoustic liners are locally-reacting devices constituted of perforates rigidlybacked by a honeycomb filled cavity. An alternative approach for reducing wall-friction in aero-acoustic applications is the use of facesheets with sub-millimetric diameter perforations. In this study, we perform a costefficient experimental characterization for comparing the performance of micro-perforated partitions with those of fibrous non-resonant absorbers for the attenuation of acoustic waves and flow-induced noise in a low-speed wind tunnel. Two types of excitations have been studied, a fullydeveloped TBL and a source-induced noise using flushmounted compression drivers. We have used a nosecone microphone system for the determination of velocity and intensity profiles above the micro-perforated and fibrous test sections. The results obtained showed that the sound pressure levels and intensity fluxes in the proximity of the wall-treatments present important modifications when considering micro-perforated panels at the cavity resonance.

Attenuation of low frequency noise constitutes a long-stating problem in the field of noise control. The use of Micro-Perforated Panels has been proved to be efficient if the primary noise source spectrum is confined in a narrow frequency band. To get rid of the limitations due to the backing cavity depth, unbacked configurations have also been considered. They are able to supply good attenuation results if the size of the hole diameters is reduced and the perforation ratio is increased. These devices are denoted Micro-Capillary Plates. These ultra-perforated membranes consist of highly porous panels made up of leaded glass, of high porosity with millions of micro-channels per centimeter square, the holes or square channel dimensions varying between 5µm and 50µm. They are 1-2 millimeters thick and can be produced in various shapes using drawn tubes of fused, chemically etched fibers, working on both backed or unbacked configurations. These characteristics provide them with exceptional acoustical properties compared to other classical absorbers or MPPs. If rigidly backed by an air cavity of 24 mm, a MCP device provides a half-bandwidth absorption over 12 octaves whereas a MPP device works only over one-two octaves. Their performance will be examined in ducted propagation conditions.

Helmholtz-type resonance absorbers constitute noise control devices widely used in many areas that have attracted attention in the last years due to the advancement of acoustic metamaterials. Micro-perforated panels placed over a backing cavity work on the same principle at low frequencies. They can provide important absorption values but confined in a narrow frequency band. To overcome this limitation, unbacked configurations have been considered, but care has to be taken for a proper selection of their constitutive parameters. In this work, freestanding micro-perforated plates with holes diameters down to 10 micrometers and high perforation ratio are shown to be good candidates as wideband low-frequency sound absorbers. Several micro-capillary plates, classified as a function of the Knudsen number, are studied analytically and experimentally. Most of the porous microsystems that use gases work in slip-flow regime whose properties differ considerably from the classical continuum regime. Results showed that unbacked micro-capillary plates can achieve absorption values greater than 0.7 up to 7 kHz with an absorption plateau above 0.85 up to 4 kHz under normal incidence. Dependence of their performance to the backing load is analyzed. They could be used as low-frequency noise dissipation devices with applications as calibrated anechoic terminations.

Mitigating the propagation of low frequency noise in ducted flows represents a challenging task since wall treatments have often limited dimensions. Micro-perforated panels have been successfully used backed by a cavity constituting a bulk-reacting resonator or in combination with honeycomb for a locally-reacting system. In this work, a cost-efficient methodology for the study and characterization of the aero-acoustic properties of flush-mounted micro-perforated resonators has been developed. Although Laser Doppler Ve- locimetry is a non-intrusive technique, it requires delicate instrumentation to obtain an estimation of the acoustic velocity and pressure fluctuations at several points over a wall liner. Here, the attenuation has been estimated from sound pressure level measurements performed with two nosecone microphones positioned along a vertical line centered on the resonator axis. Experiments have been performed in a wind tunnel in presence of a low-speed turbulent boundary layer of air fully developed over different samples made up of micro-perforated sheets or porous materials with surface roughness. The micro-perforates have been flush- mounted over a cylindrical cavity of depth 30 mm situated on the floor of the test section. The ability of bulk-reacting resonators for reducing the acoustic or flow-induced noise has been assessed in comparison with locally-reacting treatments

A causal-based optimization criterion has been theoretically derived in an earlier study for a single-layer micro-perforated absorber (MPA) under normal incidence. In this study, it is extended to multiple and parallel MPA systems as well as to general incidence. It is shown that the ultimate wideband performance of multiple (resp. parallel) MPAs scale on their overall (resp. effective) cavity depths, with adverse effect of the incidence angle. The multi-variate constrained optimization process requires maximising the sensitivity of the total reflected intensity, integrated over all wavelengths, with respect to the parameters constitutive of the micro-perforated panels. It leads to perfect absorption and broad bandwidth efficiency at one or several resonant states of the MPAs. As for serial MPA systems, it is a suitable sub-optimal alternative to total absorption and inverse-frequency weighted absorption maximizations. Concerning MPA arrays, the causal-based optimal design brings near-unit absorption over a wide bandwidth in the low-frequency domain due to efficient grouping and merging of cross-coupled resonant states. It brings similar performance to direct maximization of the total inverse-frequency weighted absorption that puts weight on the low frequencies. These optimal performances have been verified through standing wave tube absorption measurements for single and double-layer MPAs and through finite element simulation for MPA arrays.

This theoretical study adapts results obtained in electromagnetism on the optimal performance of radar slab absorbers to the field of acoustics to enhance the broadband dissipation of single-layer rigidly-backed micro-perforated absorbers (MPAs) under normal incidence. Based on the causality principle, an integral identity is derived. It shows that the MPA ultimate wideband performance, that integrates contributions of the intensity reflection coefficient over all the positive wavelengths, is upper bounded by the absorber cavity depth. A sensitivity analysis led to the proposal of a causal-based optimization criterion in order to find optimal MPAs that achieve maximum wideband performance while reaching perfect absorption at their Helmholtz resonance frequency. This simple criterion maximizes the sensitivity of the total reflected intensity with respect to the micro-perforated panel constitutive parameters. It can be readily implemented using numerical quadrature and optimization solvers. It is shown to be a suitable alternative to maximization of the total absorption and inverse-frequency weighted absorption for single-layer MPA broadband optimization.