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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 feasibility is considered of synthesizing a spatially correlated random pressure field having specified statistical properties. Of particular interest is the use of a near-field array of acoustic sources to synthesize a pressure field whose statistical properties are similar to either a diffuse acoustic sound field or to that generated by a turbulent boundary layer (TBL). A formulation based on least-squares filter design is presented. Initially, the more fundamental question is addressed of how many uncorrelated signal components are required to approximate the pressure field. A one-dimensional analysis suggests that two uncorrelated components per acoustic wavelength are required to approximate a diffuse pressure field. Similarly, for a TBL pressure field, about one uncorrelated component per correlation length is required in the spanwise direction and about two uncorrelated components per correlation length are required in the streamwise direction. These estimates are in good agreement with theoretical predictions for an infinite array, based on the Fourier transform of the spatial correlation function. When a full simulation is performed, including the acoustic effect of an appropriately positioned array of monopole sources, it is found that the number of acoustic sources required to reasonably approximate the diffuse or TBL pressure field is only slightly greater than the lower bound on this number, set by the number of uncorrelated components required

This paper presents theoretical and experimental results on the influence of panel vibrations on the sound absorption properties of thin micro-perforated panel absorbers (MPPA). Measurements show that the absorption performance of thin MPPAs generates extra absorption peaks or dips that cannot be understood assuming a rigid MPPA. A theoretical model is established that accounts for structural-acoustic interaction between the micro-perforated panel and the backing cavity, assuming uniform conservative boundary conditions for the panel and separable coordinates for the cavity cross-section. This model is verified experimentally against impedance tube measurements and laser vibrometric scans of the cavity-backed panel response. It is shown analytically and experimentally that the air-frame relative velocity is a key factor that alters the input acoustic impedance of thin MPPAs. Coupled mode analysis reveals that the two first resonances of an elastic MPPA are either panel-cavity, hole-cavity, or panel-controlled resonances, depending on whether the effective air mass of the perforations is greater or lower than the first panel modal mass. A critical value of the perforation ratio is found through which the MPPA resonances experience a frequency "jump" and that determines two absorption mechanisms operating out of the transitional region.

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 between the loudspeakers and the microphones. This approach could provide a cost-effective method of reducing the variation of low frequency sound transmission measurements as well as simulating the propeller-induced noise and the boundary layer noise transmitted through aircraft fuselage structures.

The feasibility is discussed of simulating a random pressure field having the same spatial correlation function as a turbulent boundary-layer pressure field using an array of loudspeakers. This approach could provide a cost-effective laboratory method of measuring the boundary-layer noise transmitted through aircraft fuselage structures. Initially, a theoretical model is used to predict the vibroacoustic response of randomly excited panels. A method of generating a pressure field with predefined statistical properties using an array of loudspeakers is then introduced. Results are obtained in a typical test case for the simulation of boundary-layer-induced noise. It is shown how the number of loudspeakers required to achieve a reasonable approximation of the boundary-layer excitation scales with frequency. It is found that a coarse reproduction of the boundary-layer excitation, using a reduced set of loudspeakers, can still give a good approximation of the panel vibroacoustic response, thus suggesting that direct simulation of the panel response to a boundary-layer excitation using loudspeakers could be feasible. NomenclatureA = area of each panel element c = sound speed D P = rigidity of the panel d = height between the panel and the microphones below the panel d = (N x N y ) vector of turbulent boundary-layer (TBL) wall pressures at the microphones positions d(M, M ) = distance between two points M and M E P = Young's modulus of the panel E[x]= mathematical expectation of a random variable x e = (N x N y ) vector of error signals at the microphones outputs G = (N x N y × NL x NL y ) plant response matrix between the microphones and the loudspeakers G p = panel acoustic response to a unit point force excitation G v = panel velocity response to a unit point force excitationtransfer mobility matrix and the (PM x PM y × N x N y ) transfer impedance matrix, respectively h = height between the loudspeakers and the microphones above the panel h P = panel thickness J e , J ep , J ev = normalized mean-square error signals respectively for the approximate TBL field, panel acoustic response, and panel velocity response k = acoustic wave number L {y,x} = correlation lengths of the TBL along the y and x axes l {y,x} = panel dimensions along the y and x axes m r = modal generalized mass term N col,{y,x} = number of correlation lengths to reproduce along the y and x axes N L = total number of loudspeakers N {y,x} = number of discrete elements on the panel along the y and x axes NL {y,x} = number of loudspeakers along the y and x axes NM {y,x} = number of microphones above the panel along the y and x axes P = nondimensional metric for the sound power radiated by the panel P = (N x N y × N x N y ) TBL-generating matrix PM {y,x} = number of microphones below the panel along the y and x axes p b = "blocked" TBL pressure field ℘ M ω = integral operator to describe the pressure field radiated at point M R = number of panel modes accounted for in the modal series R = (N x N y × N x N y ) radiation resistance matrix r {y,x} = separati...

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.