Sound field separation with sound pressure and particle velocity measurements

International audienceIn conventional near-field acoustic holography (NAH) it is not possible to distinguish between sound from the two sides of the array, thus, it is a requirement that all the sources are confined to only one side and radiate into a free field. When this requirement cannot be fulfilled, sound field separation techniques make it possible to distinguish between outgoing and incoming waves from the two sides, and thus NAH can be applied. In this paper, a separation method based on the measurement of the particle velocity in two layers and another method based on the measurement of the pressure and the velocity in a single layer are proposed. The two methods use an equivalent source formulation with separate transfer matrices for the outgoing and incoming waves, so that the sound from the two sides of the array can be modeled independently. A weighting scheme is proposed to account for the distance between the equivalent sources and measurement surfaces and for the difference in magnitude between pressure and velocity. Experimental and numerical studies have been conducted to examine the methods. The double layer velocity method seems to be more robust to noise and flanking sound than the combined pressure-velocity method, although it requires an additional measurement surface. On the whole, the separation methods can be useful when the disturbance of the incoming field is significant. Otherwise the direct reconstruction is more accurate and straightforward

Modèle monocouche équivalent de plaque multicouche orthotrope

La caractérisation de matériaux complexes comme les multicouches a déjà fait preuve de nombreux travaux. Différentes approches existent, et en particulier, celles basées sur la recherche d'une plaque équivalente homogène (Equivalent Single Layer Model). Ces dernières comparent le multicouche à une plaque mince dont les paramètres simulent le comportement dynamique du multicouche à une fréquence donnée. La caractérisation du multicouche se fait donc par l'obtention de ces paramètres « équivalents ». En particulier, le modèle développé par J.L. Guyader [Internoise 2007] est de type plaque mince, se limitant à la caractérisation d'un multicouche isotrope, indépendant du nombre de couche et permettant d'obtenir comme paramètres équivalents, un module d'Young et un amortissement dynamique. A une fréquence donnée, la plaque équivalente présente le même nombre d'onde naturel que le multicouche. En reprenant une méthodologie similaire, nous proposons d'étendre le modèle de Guyader au cas des multicouches orthotropes, par le calcul des différentes rigidités de flexion D1, D2, D3 et D4 solutions de l'équation de dispersion de la plaque mince équivalente orthotrope. Cette méthode permet d'obtenir une représentation des caractéristiques du multicouche sur le plan d'onde kx ? ky. Des comparaisons expérimentales (vibrométrie laser) / analytiques seront présentées entre 2 et 8 kHz pour différents cas d'orthotropie sur des plaques en fibre de carbone afin d'illustrer les performances de la méthode

Calibration in-situ d'une antenne de capteurs de pression acoustique en conduit

Nous présentons dans cette contribution une méthode de calibration in-situ appliquée à une antenne de capteurs en paroi. Le protocole expérimental nécessaire est assez simple et consiste à placer une source acoustique à différentes positions dans le conduit, à condition que celle-ci soit éloignée des microphones. Des estimations a posteriori des paramètres physiques tels que la célérité acoustique, la vitesse de convection et éventuellement des coefficients de réflexion du banc d'essais sont également données comme sous-produits de l'approche proposée

Direct formulation of the supersonic acoustic intensity in space domain

International audienceThis paper proposes and examines a direct formulation in space domain of the so-called supersonic acoustic intensity. This quantity differs from the usual (active) intensity by excluding the circulating energy in the near-field of the source, providing a map of the acoustic energy that is radiated into the far field. To date, its calculation has been formulated in the wave number domain, filtering out the evanescent waves outside the radiation circle and reconstructing the acoustic field with only the propagating waves. In this study, the supersonic intensity is calculated directly in space domain by means of a two-dimensional convolution between the acoustic field and a spatial filter mask that corresponds to the space domain representation of the radiation circle. Therefore, the acoustic field that propagates effectively to the far field is calculated via direct filtering in space domain. This paper presents the theory, as well as a numerical example to illustrate some fundamental principles. An experimental study on planar radiators was conducted to verify the validity of the technique. The experimental results are presented, and serve to illustrate the usefulness of the analysis, its strengths and limitations

Multi-harmonic phase demodulation method for instantaneous angular speed estimation using harmonic weighting

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Benefits of Mann–Kendall trend analysis for vibration-based condition monitoring

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Experimental investigation of the supersonic jet noise from aircraft engines using acoustic imaging

International audienceThis paper focuses on the study of broadband shock-associated noise (BBSAN) emittedby the jet of dual-stream aircraft engines in cruise flight conditions. This work builds onpast experience of BBSAN-oriented experimental campaigns conducted within Airbus, and onrecent progress made on signal processing techniques and source modeling in the literature inorder to gain understanding on the source generation mechanism. A dataset from a flight testcampaign is considered and compared to a prediction of BBSAN following the methodologyproposed by Wong et al. [1]. In this test, pressure on the rear part of the aircraft is measuredon a phased array of flush-mounted microphones placed on the fuselage. On the numericalside of this study, parabolized stability equations are used to model the coherent turbulentstructures that develop on the jet mean flow. These wavepackets are combined with shockcellquantities to build the Lighthill’s stress tensor, which is propagated to the nearfield of thesource. Computations show that two instability modes per frequency coexist, correspondingto turbulence developing mainly in the inner and in the outer shear layer of the dual-streamjet at dierent convection velocities. Results show very similar wavenumber patterns betweenflight test and synthesized data. The identified turbulence convection velocities are in goodagreement. It is shown that the BBSAN signature is dominated by turbulence developing inthe outer shear layer, between the secondary stream of the jet and the ambient flow

Separation of vibratory components in complex systems for condition monitoring

International audienceMechanical signals are often a mixture of multiple components produced by different sources within multiple subsystems. The separation of these components is beneficial for differential diagnosis, fault severity assessment and prognosis. The synchronous average (SA) is a powerful and widely used technique for this purpose, providing optimal estimation performance and simplicity. However, the complexity of such systems often implies the existence of several spectral interferences, which jeopardize the straightforward application of the SA. This paper addresses this issue by proposing a general strategy based on the synchronous average and the set theory, to isolate each vibratory contribution related to a source of interest, while accounting for spectral interferences. This approach enables a comprehensive understanding of the system’s health status and the ability to locate specific issues within a subsystem. The proposed methodology is demonstrated through an example of an accessory gearbox vibration signal acquired during a ground test campaign on a CFM56 turbojet engine

A probability model with Variational Bayesian Inference for the complex interference suppression in the acoustic array measurement

International audienceThe microphone array is widely used in acoustics as a non-contact measurement tool, whichcan obtain multi-dimensional information about the sound source, such as spatial, time, andfrequency. The microphone array is not always used in an ideal anechoic chamber environment,making the sound source signal contaminated with the background interference. The separationof the sound source signal from the complex background interference is very challenging,especially when arrays are used in wind tunnel measurements. A probability model on the time–frequency matrix is constructed in this paper to address this issue. The background interferenceis constructed by the Gaussian mixture model to fit its complex probability distributionsadaptively. The sound source signal is constructed as a low-rank model according to itscorrelation characteristics on the microphones. The distributions of parameters involved in thelow-rank and Gaussian mixture model are estimated through variational Bayesian inference,which can realize the separation of the sound source signal from the complex backgroundinterference. The performance of the proposed method is evaluated by the numerical simulationand the DLR closed wind tunnel experimental. The robustness and the effectiveness of extractingthe sound source signal from the complex background interference are also verified