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.

In this paper we discuss the characterization of low frequency sound transmission between two rooms via a flexible panel. A fully-coupled modal model is used to investigate the individual effect of the source room and the receiving room on the measured sound reduction index, and the results are compared with the ideal case of having a free field on both sides of the panel. The effect of the source room on the measured sound reduction index at low frequencies can be reduced by using a number of suitable-driven loudspeakers close to the panel to simulate a diffuse incident field. However, the effect of the receiving room was found not to be reduced by calculating the transmitted acoustic power from a dense array of acoustic intensity measurements, instead of an array of microphones in the receiving room.

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.

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.

This paper will focus on the ne bis in idem principle, in the AFSJ as a defence right and argues that emancipating this principle from the conflict of jurisdictions debate will increase the status of defence rights in the EU and enhance its dimension as a procedural safeguard. Article 82(2), b) of the TFEU paves the way to harmonisation of procedural safeguards, on an EU level, and can also be understood as a means to create a “common legal grammar” between Member States. Consequently, European legislation regarding ne bis in idem would not only bring legal certainty but it also contributes to diminishing the grounds of refusal as far as mutual recognition legal instruments are concerned. The need for a deontic European model regarding defence rights and procedural safeguards is also in discussion as it would serve as a counterweight between the two dimensions of European integration, in the AFSJ: freedom and security.

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.

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.