Cédric Maury

Analytic solutions to the acoustic source reconstruction problem

In this paper, we generalize the recently developed analytical solutions of the radiation modes problem to the determination of closed-form expressions for the singular value expansion of a number of integral operators that map the boundary velocity of a baffled planar structure onto the acoustic pressure radiated in far-field or intermediate regions. Exact solutions to this problem involve prolate spheroidal wave functions that correspond to a set of independent distributions with finite spatial support and maximal energy concentration in a given bandwidth of the transform domain. A stable solution to the inverse source reconstruction problem is obtained by decomposing the unknown boundary velocity into a number of efficiently radiating singular velocity patterns that correspond to the number of degrees of freedom of the radiated field. It is found that the degree of ill-posedness of the inverse problem is significantly reduced, when considering a hemi-circular observation arc with respect to a linear array of sensors, by a factor scaling on the small angular aperture subtended by the observation line. Estimates are derived from the spatial resolution limits that can be achieved in the source reconstruction problem from the dimension of the efficiently radiating subspace.

Enhancing the Characterisation of Ducted Noise Sources Using a Spectral Formulation

The characterization of ducted sound sources is very useful to the aeronautical and automotive industries. It provides inputs to models used for predicting flow-generated noise at the design stage. Conventional inverse approaches, such as the Equivalent Source Method, offer accurate reconstruction results. But they require either a large number of in-duct field pressure measurements or a suitable location of a few number of equivalent sources. This research proposes an alternative approach formulated in the spectral domain: the Spectral Decomposition Method. It directly provides a cross-sectional imaging of the source velocity distribution from pressure data measured at small or large stand-off distance to the source. Well-resolved imaging results of the axial acoustic velocity generated by wall-mounted drivers have been obtained from either induct or wall-pressure measurements acquired respectively without or with flow.

Absorption of anisotropic fibrous materials partitions shielded by thin micro-perforated panels

Recent trends in broadband noise reduction have considered the design of bio-inspired multi-layer sound absorbers. In particular, the silent flight of certain species of night bird predators can offer an inspiration for the design of acoustic partitions for the control of the airframe noise in terrestrial and aeronautic transport systems. The layout usually comprises a suitable combination of thin micro-perforates, fibrous materials and airspaces with a view to mimic the remarkably low acoustic emissions of the owl flight. One of the main features is the anisotropic texture of the highly porous material beneath the micro-perforate. This study examines how the absorption properties of rigidly-backed micro-perforated panels are modified when lined with anisotropic fibrous materials with specified inclination of the parallel fibers within the material thickness. A fully anisotropic model has been proposed that accounts for anisotropy and frequency dependence in complex densities and bulk modulus. This model ...

Reducing discrete-tones generation by micro-perforating the base of an open shallow cavity under a subsonic flow

Experimental and numerical studies have been carried out to investigate the effect of micro-perforating the base of an open shallow cavity under a low-speed air flow in order to reduce the amplitudes of the discrete tones due to the shear layer-cavity interaction. An open shallow transitional cavity with a length-to-depth ratio of 10.6 has been considered flush-mounted on the test section of a low-speed wind-tunnel with mean flow velocity 30.7 m/s. The bottom wall of the cavity was either a plain or an unbacked micro-perforated panel (MPP). A set of wall-pressure measurements over the base showed that the MPP was able to reduce by up to 8 dB the amplitude of the discrete tones, mostly noticeable beneath the upstream edge of the cavity within the recirculation bubble where the acoustic components dominate over the broadband components. The streamwise and spanwise cavity resonance frequencies were identified from a simple model. These results were assessed against aeroacoustic numerical simulations performe...

Micro-Perforated materials for the reduction of flow-induced noise

Turbulent Boundary Layer (TBL) induced noise is one of the dominant noise sources in modern aircraft. The aim of this work is to study the behavior of insulating partitions composed of Micro-Perforated Panels (MPPs) exposed to a TBL excitation. We have carried out a set of simulations to establish a comparison between the performance of the MPP control devices when varying the physical configurations of the partition and the nature of the primary noise excitation. It can be shown that when exciting the partition with less correlated random loads, the corresponding TL increases progressively. Experiments have been performed in the low-speed wind tunnel in order to determine the acoustic and aerodynamic TL performance of a number of MPP multilayer partitions. The effect of inserting a micro-perforated panel within the cavity at unequal distances from the front and back panels dampens more efficiently the Mass-Air-Mass controlled resonances of the Panel-Cavity-Panel system with respect to those of the MPP-Ca...

Parsimonious approaches for the laboratory synthesis of wall-pressure excitations

The experimental synthesis of acoustic or aerodynamic wall-pressure excitations over industrial structures is a cost-efficient approach to optimise the insulating performance of these structures under real-life forcing generated in a controlled environment. Several strategies like inverse filtering in the spatial or wavenumber domains, holophonic or holographic reproductions required a prohibitive number of control sources for the real-time synthesis of sub-wavelength scales such as those associated with the wall-pressures induced by a Turbulent Boundary Layer (TBL). The present study compares the performance of several strategies for a direct reproduction of TBL excitations, independently of the test panel physical properties and with a reasonable number of sources. A first approach, the focused synthesis, uses spatial windowing to reduce the surface over which the TBL is reproduced, thereby enlarging the range of synthesized wavenumbers in the subsonic domain, if possible beyond the convective ridge. Th...