Teresa Bravo

Sound absorption and transmission through flexible micro-perforated panels backed by an air layer and a thin plate

This paper describes theoretical and experimental investigations into the sound absorption and transmission properties of micro-perforated panels (MPP) backed by an air cavity and a thin plate. A fully coupled modal approach is proposed to calculate the absorption coefficient and the transmission loss of finite-sized micro-perforated panels-cavity-panel (MPPCP) partitions with conservative boundary conditions. It is validated against infinite partition models and experimental data. A practical methodology is proposed using collocated pressure-velocity sensors to evaluate in an anechoic environment the transmission and absorption properties of conventional MPPCPs. Results show under which conditions edge scattering effects should be accounted for at low frequencies. Coupled mode analysis is also performed and analytical approximations are derived from the resonance frequencies and mode shapes of a flexible MPPCP. It is found that the Helmholtz-type resonance frequency is deduced from the one associated to the rigidly backed MPPCP absorber shifted up by the mass-air mass resonance of the flexible non-perforated double-panel. Moreover, it is shown analytically and experimentally that the absorption mechanisms at the resonances are governed by a large air-frame relative velocity over the MPP surface, with either in-phase or out-of-phase relationships, depending on the MPPCP parameters.

The experimental synthesis of random pressure fields: Methodology

This paper presents a methodology for the off-line reproduction of random pressure fields with given spatial correlation characteristics. The simulation method is first presented, together with an outline of the signal processing techniques required. The design of an experimental setup is then detailed in relation with the nature of the simulated pressure fields. Of particular interest is the laboratory synthesis of three different types of partially correlated random excitations: an acoustic diffuse field, a grazing incident plane wave, and a turbulent boundary layer. The corresponding excitations are generated in a semianechoic chamber over a test panel using a near-field array of 16 loudspeakers driven by a set of optimal signals. The loudspeakers are positioned at a suitable distance above a sufficiently dense grid of microphones close to the simulation surface. The mutually correlated drive signals are determined from both the target correlation properties and the acoustic transfer functions measured...

A synthesis approach for reproducing the response of aircraft panels to a turbulent boundary layer excitation

Random wall-pressure fluctuations due to the turbulent boundary layer (TBL) are a feature of the air flow over an aircraft fuselage under cruise conditions, creating undesirable effects such as cabin noise annoyance. In order to test potential solutions to reduce the TBL-induced noise, a cost-efficient alternative to in-flight or wind-tunnel measurements involves the laboratory simulation of the response of aircraft sidewalls to high-speed subsonic TBL excitation. Previously published work has shown that TBL simulation using a near-field array of loudspeakers is only feasible in the low frequency range due to the rapid decay of the spanwise correlation length with frequency. This paper demonstrates through theoretical criteria how the wavenumber filtering capabilities of the radiating panel reduces the number of sources required, thus dramatically enlarging the frequency range over which the response of the TBL-excited panel is accurately reproduced. Experimental synthesis of the panel response to high-speed TBL excitation is found to be feasible over the hydrodynamic coincidence frequency range using a reduced set of near-field loudspeakers driven by optimal signals. Effective methodologies are proposed for an accurate reproduction of the TBL-induced sound power radiated by the panel into a free-field and when coupled to a cavity.

The experimental synthesis of random pressure fields: Practical feasibility

A previous paper discussed the methodology for the synthesis of partially correlated random pressure fields using a near-field array of loudspeakers. The acoustic sources are optimally driven so that various random excitations are reproduced over a test surface, namely an acoustic diffuse field, a grazing incident plane wave, and turbulent boundary layer fluctuating loads. This paper shows the physical limitation performances and the practical feasibility of synthesizing these random pressure fields in a series of loudspeakers array simulation experiments. Spatial error criteria are proposed on the number of acoustic sources per unit correlation length. They are more representative than mean-square error criteria to quantify the accuracy with which the assumed correlation structures are experimentally reconstructed. Furthermore, structural and acoustic models are formulated to investigate how sensitive is the panel vibroacoustic response to inaccuracies in the synthesized excitations. It is discussed how the direct reproduction of the panel vibroacoustic response with a limited number of loudspeakers should be feasible within the frequency bandwidth for which the modes at resonance well couple with the excitation.

Enhancing sound absorption and transmission through flexible multi-layer micro-perforated structures

Theoretical and experimental results are presented into the sound absorption and transmission properties of multi-layer structures made up of thin micro-perforated panels (ML-MPPs). The objective is to improve both the absorption and insulation performances of ML-MPPs through impedance boundary optimization. A fully coupled modal formulation is introduced that predicts the effect of the structural resonances onto the normal incidence absorption coefficient and transmission loss of ML-MPPs. This model is assessed against standing wave tube measurements and simulations based on impedance translation method for two double-layer MPP configurations of relevance in building acoustics and aeronautics. Optimal impedance relationships are proposed that ensure simultaneous maximization of both the absorption and the transmission loss under normal incidence. Exhaustive optimization of the double-layer MPPs is performed to assess the absorption and/or transmission performances with respect to the impedance criterion. It is investigated how the panel volumetric resonances modify the excess dissipation that can be achieved from non-modal optimization of ML-MPPs.

Enhancing the reconstruction of in-duct sound sources using a spectral decomposition method.

The characterization of in-duct acoustic source parameters for their use at the design stage, in virtual prototyping of the noise induced by fans and obstacles, is of great engineering importance. Conventional inverse approaches, such as the equivalent source method, offer accurate reconstruction results, but they require a large number of virtual sources or a suitable location of a few number of them. This paper proposes an alternative method that provides a cross-sectional imaging of the amplitude distribution of ducted sources, which does not depend on a prior adequate placement of equivalent sources, or on the degree of correlation between the real ones. The method is based on a full spectral formulation of the inverse problem, from which a theoretical stopping criterion is derived that provides a stable reconstruction of the unknown source distribution. Accurate and well-resolved imaging results of the axial acoustic velocity generated by wall-mounted drivers are obtained from either in-duct or wall-pressure measurements, acquired respectively without or with flow. The accuracy and the resolution of the retrieved source strength are significantly enhanced from an iterative modal decomposition of the pressure field, when imaging from data acquired respectively at large or small standoff distances to the source plane.

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 low frequency sound transmission measurements using a synthesis method.

The characterization of low frequency sound transmission between two rooms via a flexible panel is investigated experimentally in this work. Previously, the individual effects of the transmission suite on the measured sound reduction index have been studied analytically, and the results have been compared with the ideal case of having free field radiation conditions on both sides of the panel. A new approach is proposed using a near-field array of loudspeakers driven by a set of optimized signals such that a diffuse pressure field is reproduced on the surface of the partition to be tested. The practical effectiveness of this method is assessed when using a set of 16 acoustic sources located in the source reverberant room in close proximity to an aluminium panel. The experimental results obtained confirm the dependence of the characterized sound reduction index on the particular test chamber considered in the low frequency range. They also validate the proposed synthesis method for providing an estimate that only depends on the properties of the partition itself.

In-Duct Acoustic Source Reconstruction from Pressure and Velocity-Based Measurements

The question addressed in this work is the determination of the practical capability of model-based methods to reconstruct a given acoustic source distribution from either pressure-based or velocity-based measurements at field points inside a cylindrical duct. An analytical model has first been implemented for the sound field due to elementary sources in a hard-walled duct of finite length. An interpretation is proposed of in-duct multi-modal propagation in terms of wavenumber-filtering. Due to the modal characteristic impedance terms, spatial low-pass filtering is associated to pressure-based field whereas this filter factor does not appear when considering velocity-based field. The inversion of the pressure-based frequency response functions involves a reciprocal factor which amplifies the high spatial frequency and increases the numerical instability of the inversion. It is found that, under non-ideal conditions, i.e. at low frequency and beyond the near-field zone, velocity-based field measurements are more favourable than pressure-based field measurements for acoustic source reconstruction in terms of both accuracy and spatial resolution.

Extrapolation of Maximum Noise Levels From Near-Field Measurements to Far-Field Positions

The goal of this work is to propose a new strategy for the attenuation of the traffic noise, which constitutes one of the main sources of acoustic pollution in urban and suburban areas. This strategy is based on the measurement of the noise radiated by each individual vehicle using an electro-acoustic system, composed of two microphones for the acquisition of the engine and of the rolling noise. These microphones have been situated inside the engine hood and close to the right back tire respectively. The signals have been recorded for diesel and petrol engines and through typical urban and suburban courses with different persons. Using this procedure, we aim to characterise the drivers responsible of the highest noise levels producing maximum annoyance. The near-field measurements have been then extrapolated to far field positions using an analytical filter that takes into account absorbing properties of the propagation floor. For the internal signal it has been necessary to characterise the acoustic properties of the engine hood experimentally using an array of microphones surrounding the vehicle. The propagated noise is calculated considering the absorption due to the geometrical divergence, the absorption by the air, and the effect of the propagating surface. The signals extrapolated to the receiver position could be compared with the current normative to propose recommendations concerning noise control actions.Copyright