Patch Transfer Function Research Articles

Robotic arm, 3D vision and geometric matching for an efficient implementation of inverse Patch Transfer Function method

The inverse patch transfer function method was developed to reconstruct the acoustic fields (velocity, pressure, intensity) at the surface of a vibrating source with a complex geometry using the concept of a virtual acoustic domain. This method can be used to manage sources with complex shapes in a non-anechoic acoustic environment, even in the presence of masking objects. However, this method is quite demanding experimentally as it requires a large number of measurement positions with good spatial positioning accuracy. This article presents a dedicated experimental platform developed to simplify and automate the entire measurement process. First, a 3D camera placed on a robotic arm constructs a point cloud characterising the source and its environment. Using a geometric matching algorithm, the CAD of the object under test is detected in the cloud of points, enabling the different coordinate systems (CAD, robot and physical part) to be matched. Secondly, the measurement points required to apply iPTF can be defined directly in the CAD and reproduced in near-real time on the test rig, regardless of the type of antenna used, providing an efficient means of applying iPTF and opening the way to many advanced applications.

Acoustic field reconstruction in the presence of masking objects

In acoustics, field reconstruction methods aim at retrieving acoustic fields (pressure, velocity and intensity) from acoustic measurements around a radiating source, which is often a vibrating structure of complex shape (pumps, engines…). If they are extensively used in laboratory conditions, their application to in situ characterization is not straightforward due to the presence of disturbing sources or masking objects, or to non-anechoic environments. The inverse Patch Transfer Function (iPTF) approach, thanks to the concept of virtual acoustic volume modelled by a finite element model, has already demonstrated its ability to deal with sources of complex shape and the presence of disturbing stationary sources in a non-anechoic acoustic environment. The objective of this article is to show how the presence of rigid masking objects can be easily and efficiently taken into account. A numerical experiment consisting of a thin, simply supported rectangular plate radiating noise in a semi-infinite acoustic field and partially masked by a rigid parallelepiped is presented. The acoustic fields identified and the directivity diagrams are compared with the reference and show that iPTF is able to cancel the presence of the masking object even if the latter completely covers the radiating plate. Finally, an industrial example consisting in an electric motor in operation is presented. Two configurations were tested: with and without the presence of a rigid object. Comparison of the results shows that the fields identified are in good agreement, demonstrating the ability of iPTF to cancel out the effect of masking objects.

A hybrid sub-structuring method for prediction of air inlet sound transmission

Air inlets coupled with mechanically controlled ventilation systems are widely used to renew polluted indoor air. They are generally positionned at the top of windows inducing a reduction of the sound insulation. As the harmful consequences of noisy environments on human health are more and more highlighted, it becomes necessary to maximize the sound insulation while ensuring a sufficient fresh air flow. To this end, efficient numerical simulation appears to be a promising way for studying air inlet acoustic behavior: many parameters can be considered without the need to carry out costly and time-consuming laboratory tests. This work aims at developing a reliable numerical model reproducing the acoustic laboratories used by manufacturers for the measurements of air inlet sound reduction index. The low frequency range is studied in the present paper as the measurement uncertainties arising from experimental conditions are the greatest at these frequencies. In order to make the calculations computationally efficient, the proposed model uses a sub-structuring approach called the Patch Transfer Function (PTF) method, and combines analytical or numerical solutions, depending on the subdomain considered. Each subsystem of complex geometries is discretized using finite elements. Conversely, the PTF of simple geometries are analytically computed from a modal decomposition, enriched with a quasi-static correction numerically computed. As empirical improvements pointed out the benefits of the addition of melamine foams, porous material modeling is introduced in the study based on an equivalent fluid model.

Application of the patch transfer function method for predicting flow-induced cavity oscillations

The Patch Transfer Function (PTF) method is an efficient approach to deal with complex vibroacoustic problems by decomposing the system into a number of subsystems. Due to inherent theoretical assumptions, the method requires the system to response linearly to the input which greatly constrains its usage. In this work, the PTF method is put under the framework of the describing function method to predict the noise generated by a grazing flow past a cavity, a well-known nonlinear problem due to flow instability. For such type of system, a feedback mechanism with nearly periodic solutions exist so that the system, by analogy to a nonlinear control system, can be interpreted as a feedback loop consisting of a forward gain function and a backward gain function. The former is a nonlinear describing function derived from the vortex sound theory and the latter is a linear describing function derived from the PTF method. The validity of the proposed model and the flexibility of the PTF method in handling systems with complex geometry or boundary conditions are demonstrated by comparing theoretical predictions with results in the open literature and tests of a flexible-walled cavity excited by a grazing flow respectively.

Inverse Patch Transfer Function Method based on Steepest Gradient Descent Algorithm for Sound Field Reconstruction

In the iPTF method with Green’s function, the classical regularization method used to solve the inverse problem can’t provide satisfactory reconstruction results around some eigen frequencies of the cavity in the iPTF method with Green’s function. In this paper, the steepest gradient descent (SGD) algorithm is introduced to solve the inverse problem in the iPTF method. In addition, to make the iPTF method with Green’s function more efficient, Contour-integration is used to split the Fourier series of Green’s function into one single-sum and one double-sum Fourier series. By applying Contour-integration, the computational efficiency of the iPTF method with Green’s function has been improved. The iPTF method with efficient Green’s function is examined in both simulations and application experiments. Experimental validations in the application are carried out in the end. The present results in experiments are also acceptable for field reconstruction.

A sound source reconstruction approach based on the machine vision and inverse patch transfer functions method

The inverse patch transfer functions (iPTF) method can realize local reconstruction of a vibrating structure in non-anechoic environments, and the geometrical shape of the reconstruction area could be irregular as long as it is known. However, the shape of the non-planar reconstruction area is not easy to be obtained in practice, which is adverse to its application in the situ tests. To achieve the sound source reconstruction of the local area with unknown shape in non-anechoic environments, a hybrid method that combines the machine vision and the iPTF method is proposed. The machine vision technology is applied to facilitate the boundary modeling of a target radiation segment. It makes use of the BundleFusion method with an RGB-D camera to accomplish the 3D reconstruction of the target area, as well as the localization of microphones. A virtual cavity consisting of the target area, virtual measurement surface, and the gap between them could be constructed automatically. Consequently, the impedance matrix of the virtual cavity is obtained based on the boundary integral equation with the adoption of the free field Green’s function. A microphone array mounted up with a rigid masker is applied in the iPTF method, which can form the rigid Neumann boundary condition on the masker. A simulation is first carried out to investigate the influence of 3D reconstruction errors of the machine vision technology on the sound source reconstruction. Then, two experiments are performed to validate the feasibility and effectiveness of the reconstruction in noisy environments. One is conducted in the semi-anechoic room with a cylindrical radiator, and a loudspeaker is put aside as the interference source. The other is conducted in the cabin of an aircraft under cruising condition. It is demonstrated that the proposed method can realize the normal velocity reconstruction without prior geometry knowledge of the target area, and the magnitude and distribution of the dominant sound source could be reconstructed accurately.

Acoustic silencing in a flow duct with micro-perforated panel liners

Despite the increasing interest in Micro-perforated panels (MPPs) for various noise control applications, the acoustic behavior of MPP liners in flow ducts has not been fully apprehended. On the top of this is the lack of understanding on the influence of various design arrangements and system parameters on the performance of MPP-based acoustic silencing devices. Incorporating previously developed acoustic impedance formulae within the general framework of the Patch Transfer Function (PTF) framework, these issues are investigated in this paper in the context of a MPP liner, flush-mounted inside a flow duct wall. Numerical analyses reveal the effects of the grazing flow and different partition arrangements in the backing cavity of MPP liners on their silencing performance as well as the underlying physical phenomena. Capitalizing on the efficiency of the modelling approach, a system optimization is carried out. The numerically predicted noise attenuation results are validated through comparisons with experimental measurements under various grazing flow velocities. While shedding light on the underlying sound attenuation mechanism, studies provide guidelines for the design of MPP silencers in flow ducts.

An inverse patch transfer functions method based on a free field Green's function

The inverse patch transfer functions (iPTF) method was proposed to reconstruct the normal velocity on the source surface in noisy environments. iPTF method is based on the measurements of the pressure and velocity on a surface surrounding the sound source and the Green's function of a virtual cavity satisfying Neumann boundary conditions. In the traditional way, the Green's function is obtained by using the modal superposition method, which involves the volume modeling and the modal analysis of the virtual cavity with the finite element method. In this work, an iPTF method based on Green's function in the free field is proposed. Consequently, the impedance matrix is obtained by using the boundary element method, which possesses higher computational efficiency and sufficient accuracy. Double-layer pressure measurements are adopted in this method to approximate the pressure and normal velocity on an intermediate hologram between the two conformal measurement surfaces. Two numerical examples with a sphere and a cubic radiator are given, and plane wave and point source are put aside as the disturbing source, respectively. Analytical and numerical solutions of the pressure and velocity on the hologram are used for these two radiators, respectively. The normal velocity and pressure on the structure radiated by the sound source in the free field could be reconstructed accurately even with a correlated signal to noise ratio (CSNR) at −20 dB. An experiment with a cubic radiator is also performed to confirm the validity of the proposed method.

Inverse Patch Transfer Function With Fast Iterative Shrinkage-Thresholding Algorithm as a Tool for Sparse Source Identification

Inverse patch transfer functions (iPTF) method is an efficient technique for sound field reconstruction and separation of arbitrarily shaped sound sources in noisy acoustic environments. By applying Neumann boundary condition to a closed virtual cavity surrounding the source, the Helmholtz integral equation is simplified and can be solved with a Green's function satisfying the Neumann boundary condition. However, in the identification of sparsely distributed sources, ghost sources appear in the result solved by using the iPTF method with classic Tikhonov regularization methods and influence the accuracy of identification. In the present work, an evanescent Green's function with fast convergence of calculation is utilized, and a technique that combines the iPTF method and Fast Iterative Shrinkage-Thresholding Algorithm aimed at improving the performance for the identification of sparsely distributed source is proposed. Then double layer measurements, instead of using expensive p-u probes, are employed to acquire the normal velocity of the hologram surface. In numerical simulations, the normal velocities of two anti-phased piston sources are well reconstructed with high sparsity while a disturbing source is radiating in the sound field within the frequency band of 50 to 1000 Hz. Finally, an experiment on two baffled loudspeakers has been carried out. The results of simulations and experiments indicate that the proposed technique has obviously improved the accuracy of the iPTF method in the identification of sparsely distributed sources for the frequency band from 50 to 1000 Hz.

Comments on the validity of transfer matrix based models for the prediction of the effect of curved sound packages

In this paper, the validity of using the Transfer Matrix Method (TMM) to account for the effect of practical curved sound packages (i.e. spring-mass treatment) in vibroacoustic systems is assessed. For this purpose, a sub-structuring method that employs a Patch Transfer Functions (PTF) approach is used to efficiently couple standard Finite Element Method (FEM) of the structure and the acoustic cavity with the sound package model. Three models of the noise control treatment are compared, namely (i) a locally reacting model, (ii) a non-locally reacting (NLR) model based on the TMM, and (iii) a FEM. The FEM model considers the actual curved geometry of the SP in order to estimate its dynamic behavior while the simplified TMM based models assume the SP to be flat (i.e. unwrapped if curvature is present), homogeneous and of infinite lateral extent. In addition, these results are systematically compared to a full Finite Element/Boundary Element Methodology. The accuracy and the limitations of adopting the unwrapped approximation in TMM based models are shown through two parametric studies. The first considers the radius of curvature of the system as a parameter while the SP thickness is constant. The second considers the thickness of the SP as a parameter while the radius of curvature of the system is held fixed. For the considered configurations, the first parametric study allows to conclude that the curvature of the SP does not affect the dynamic behavior of the system due to its large dissipation and softness and it can be practically assumed flat if the NLR model is used to model the SP. However, the second parametric study shows that the thickness of the curved SP may affect the efficiency of the NLR model. Moreover, the limitations of locally reacting models are confirmed.

Selective structural source identification

In the field of acoustic source reconstruction, the inverse Patch Transfer Function (iPTF) has been recently proposed and has shown satisfactory results whatever the shape of the vibrating surface and whatever the acoustic environment. These two interesting features are due to the virtual acoustic volume concept underlying the iPTF methods.The aim of the present article is to show how this concept of virtual subsystem can be used in structures to reconstruct the applied force distribution. Some virtual boundary conditions can be applied on a part of the structure, called virtual testing structure, to identify the force distribution applied in that zone regardless of the presence of other sources outside the zone under consideration. In the present article, the applicability of the method is only demonstrated on planar structures. However, the final example show how the method can be applied to a complex shape planar structure with point welded stiffeners even in the tested zone. In that case, if the virtual testing structure includes the stiffeners the identified force distribution only exhibits the positions of external applied forces. If the virtual testing structure does not include the stiffeners, the identified force distribution permits to localize the forces due to the coupling between the structure and the stiffeners through the welded points as well as the ones due to the external forces. This is why this approach is considered here as a selective structural source identification method.It is demonstrated that this approach clearly falls in the same framework as the Force Analysis Technique, the Virtual Fields Method or the 2D spatial Fourier transform. Even if this approach has a lot in common with these latters, it has some interesting particularities like its low sensitivity to measurement noise.

Efficient prediction of the transmission loss of curved systems with attached noise control treatment

This paper discusses the modeling of the transmission loss of curved panels with attached sound absorbing materials (foam or fiber). Two approaches are compared. The first is a classical coupled FEM/BEM (Finite element/Boundary element) approach wherein the panel and its cavity are modeled using the FEM and the blocked pressure on the excitation side and radiation from the transmission hole are modeled using the BEM. The second is an approximate approach using a Patch Transfer Function (PTF) method to describe the coupling between the panel, the sound package, the cavity and the transmission hole. To compute the PTF relations, the panel and the cavity are described by FEM while the effect of the sound package is described by a Green’s function methodology using the Transfer Matrix Method (TMM) to speed up the process. It is shown that this latter approach is quick and accurate enough to estimate the TL of the system. Moreover, it allows for a quick comparison of the performance of various sound packages f...

Application of Inverse Patch Transfer Functions Method With Wideband Holography Algorithm to Sparsely Distributed Sources Identification

Inverse patch transfer functions (iPTF) method has been developed to reconstruct the sound field of irregularly shaped sources in a noisy environment. The iPTF method, which uses classic regularization methods to solve the ill-posed problems generally, would incur some sidelobes ghosting in the process of identifying sparse sources. In view of the fact that the algorithm in wideband holography (WBH) can promote sparsity of results, a technique combining iPTF method with WBH algorithm is proposed to identify sparsely distributed sources in the present work. In the proposed technique, double layer pressure measurements are used to replace the measurements of the pressure and normal velocity which uses costly p-u probes. A gradient descent algorithm and a filtering process are applied to solve the minimization problem of identifying the normal velocities of target sources, which can suppress ghosting sources rapidly by an iterative process. In simulations, the field reconstruction results of two antiphase square piston sources show good sparsity and accuracy by employing the technique, nearly without ghosting sources. At different distances and frequencies of the two sources, the technique still performs well. Experimental validations at 200 Hz and 400 Hz are carried out in the end. The results of experiments are also coinciding with those of simulations.

Source fields reconstruction with 3D mapping by means of the virtual acoustic volume concept

This paper presents the theoretical framework of the virtual acoustic volume concept and two related inverse Patch Transfer Functions (iPTF) identification methods (called u-iPTF and m-iPTF depending on the chosen boundary conditions for the virtual volume). They are based on the application of Green׳s identity on an arbitrary closed virtual volume defined around the source. The reconstruction of sound source fields combines discrete acoustic measurements performed at accessible positions around the source with the modal behavior of the chosen virtual acoustic volume. The mode shapes of the virtual volume can be computed by a Finite Element solver to handle the geometrical complexity of the source. As a result, it is possible to identify all the acoustic source fields at the real surface of an irregularly shaped structure and irrespective of its acoustic environment. The m-iPTF method is introduced for the first time in this paper. Conversely to the already published u-iPTF method, the m-iPTF method needs only acoustic pressure and avoids particle velocity measurements. This paper is focused on its validation, both with numerical computations and by experiments on a baffled oil pan.

Prediction of the vibro-acoustic response of a structure-liner-fluid system based on a patch transfer function approach and direct experimental subsystem characterisation

The vibro-acoustic response of a structure-liner-fluid system is predicted by application of a patch impedance coupling methodology. In contrast to existing numerical approaches, impedance matrices of structure and liner are determined by a direct experimental approach, avoiding the requirement of material parameters. Emphasis is placed on poroelastic lining materials. The method accounts for surface input and transfer terms and for cross and cross-transfer terms through the specimen. A single test-rig characterisation procedure for layered poroelastic media is proposed. The specimen is considered as a single component -- no separation of layers is necessary. Problem specific boundary conditions for skeleton and fluid, which may cause non-reciprocal cross terms, are dealt with by the procedure. Methods of measurement for the assessment of impedance matrices are presented and their accuracy and limitations are discussed. An air gap correction method for surface impedance measurements is presented.

Sound source identification in a noisy environment based on inverse patch transfer functions with evanescent Green's functions

A modified inverse patch transfer function (iPTF) method is used to reconstruct the normal velocities of the target source in a noisy environment. The iPTF method simplifies the Helmholtz integral equation to one term by constructing a Green's function satisfying Neumann boundary conditions for an enclosure, which is generally constructed by slowly convergent modal expansions. The main objective of the present work is to provide an evanescent Green's function to improve the convergence of calculations. A brief description of the iPTF method and two sets of Green's functions for a rectangular cavity are presented firstly. In simulations, both the Green's functions are used to calculate the condition numbers of impedance matrices describing the relation between source and measurement patches, and the time cost of calculation based on the two sets of Green's functions at 450Hz is compared. Double pressure measurements are then employed as the input data instead of pressure and velocity measurements. The normal velocities of two baffled loudspeakers are reconstructed by the combination of a measurement method and a Green's function in the presence of a disturbing source in the frequency range of 50–1000Hz. In addition, the double pressure measurements are examined by an experiment. The precise identification of the sources indicates that the double pressure measurements are capable of localizing sources in a noisy environment. It is also found that the reconstruction with the evanescent Green's functions is slightly better than that with the modal expansions.

Nearfield acoustic holography in noisy environment based on the measurement using an acoustic mask with microphone array

Nearfield acoustic holography (NAH) has been widely used for identifying acoustic noise sources. However, conventional NAH is theoretically available only in the free field; therefore, the NAH may meet difficulties in many application in-situ environments. An NAH based on the measurement using an acoustic mask with microphone array is proposed in this presentation to reconstruct sound pressure and velocity of target sources in the environment with reflections and disturbing acoustic noise sources. The microphone array is flush-mounted at the bottom plane of an open rectangular mask. Four side surfaces are designed to acquire sound pressure in Neumann’s boundary condition. In an actual measurement, the mask faces to near field of a target source. Although the sound wave from disturbing sources may propagate into the mask by passing through the gap between the mask and source surface, the disturbing waves can be decomposed by the proposed NAH using inverse patch transfer functions. Numerical simulations and experiments indicate that this novel NAH using mask with microphone array is valid for reconstructing sound source in noisy environment. The influence of different distances between the source surface and the mask is also investigated.

Inverse Patch Transfer Functions Method as a Tool for Source Field Identification

Many methods to detect, quantify, or reconstruct acoustic sources exist in the literature and are widely used in industry (near-field acoustic holography, inverse boundary element method, etc.). However, the source identification in a reverberant or nonanechoic environment on an irregularly shaped structure is still an open issue. In this context, the inverse patch transfer functions (iPTF) method first introduced by Aucejo et al. (2010, “Identification of Source Velocities on 3D Structures in Non-Anechoic Environments: Theoretical Background and Experimental Validation of the Inverse Patch Transfer Functions Method,” J. Sound Vib., 329(18), pp. 3691–3708) can be a suitable method. Indeed, the iPTF method has been developed to identify source velocity on complex geometries and in a nonanechoic environment. However, to obtain good results, the application of the method must follow rigorous criteria that were not fully investigated yet. In addition, as it was first defined, the iPTF method only provides source velocity while wall pressure or intensity should also give useful information to engineers. In the present article, a procedure to identify wall pressure and intensity of the source without any additional measurement is proposed. This procedure only needs simple numerical postprocessing. Using this new intensity identification, the influence of background noise, evanescent waves, and mesh discretization are illustrated on numerical examples. Finally, an experiment on a vibrating plate is shown to illustrate the iPTF procedure.

Sound fields separation and reconstruction of irregularly shaped sources

Nowadays, the need of source identification methods is still growing and application cases are more and more complex. As a consequence, it is necessary to develop methods allowing us to reconstruct sound fields on irregularly shaped sources in reverberant or confined acoustic environment.The inverse Patch Transfer Functions (iPTF) method is suitable to achieve these objectives. Indeed, as the iPTF method is based on Green׳s identity and double measurements of pressure and particle velocity on a surface surrounding the source, it is independent of the acoustic environment. In addition, the finite element solver used to compute the patch transfer functions permits us to handle sources with 3D irregular shapes.In the present paper, two experimental applications on a flat plate and an oil pan have been carried out to show the performances of the method on real applications. As for all ill-posed problem, it is shown that the crucial point of this method is the choice of the parameter of the Tikhonov regularization, one of the most widely used in the literature. The classical L-curve strategy sometimes fails to choose the best solution. This issue is clearly explained and an adapted strategy combining L-curve and acoustic power conservation is proposed. The efficiency of this strategy is demonstrated on both applications and compared to results obtained with Generalized Cross Validation (GCV) technique.

Modeling vibroacoustic systems involving cascade open cavities and micro-perforated panels.

While the structural-acoustic coupling between flexible structures and closed acoustic cavities has been extensively studied in the literature, the modeling of structures coupled through open cavities, especially connected in cascade, is still a challenging task for most of the existing methods. The possible presence of micro-perforated panels (MPPs) in such systems adds additional difficulties in terms of both modeling and physical understanding. In this study, a sub-structuring methodology based on the Patch Transfer Function (PTF) approach with a Compound Interface treatment technique, referred to as CI-PTF method, is proposed, for dealing with complex systems involving cascade open/closed acoustic cavities and MPPs. The co-existence of apertures and solid/flexible/micro-perforated panels over a mixed separation interface is characterized using a compound panel subsystem, which enhances the systematic coupling feature of the PTF framework. Using several typical configurations, the versatility and efficiency of the proposed method is illustrated. Numerical studies highlight the physical understanding on the behavior of MPP inside a complex vibroacoustic environment, thus providing guidance for the practical design of such systems.