Félix Foucart

Multiload procedure to measure the acoustic scattering matrix of a duct discontinuity for higher order mode propagation conditions

A procedure for measuring the acoustic scattering matrix coefficients of a duct discontinuity for higher order acoustic duct mode propagation conditions is described and tested. The technique requires measurement of pressure waves per mode coming in and out of the discontinuity. Assuming N cut-on modes, the (2N)2 scattering matrix coefficients are determined after repeating the experiment for N linearly independent pressure distributions for at least two load configurations. Experiments were conducted for a straight duct and a reactive chamber. A good agreement was found between experiment and theory except near cut-off frequencies. The overdetermination method based on four loads was shown to improve the results. An analytical simulation of the experiment was developed to compute the influence on the [S] calculation of an error in temperature and total modal pressure assumed to be representative of a real measurement situation. This simulation with a discussion explains the discrepancies between experime...

An indirect method for the characterization of locally reacting liners

An indirect technique for educing the homogenized acoustic impedance of a liner mounted on the wall of a barrel is presented. It is based on measurements and computational simulations of the multimodal scattering matrix of this lined duct. Measurements are performed with a multisource method and the use of an anechoic duct termination. The numerical computation of the scattering matrix relies on a finite element model, and assume that the duct is axisymmetric and uniformly covered by a locally reacting material. The impedance is educed by minimizing the difference between the theoretical and experimental acoustic power dissipations, which are deduced from the scattering matrix. The source is an incoming pressure field generated at one end of the duct only and composed of all cut-on modes. This technique was tested on a cylindrical barrel whose wall was partially lined with a realistic, locally reacting material made of honeycomb cells and a perforated facing sheet. Results for the acoustic impedance are compared with those deduced from semiempirical models and the experimental two microphone method. The best agreement with the indirect method is found with the semiempirical impedance results when the difference between the acoustic power dissipated by the actual lined barrel and the reference barrel is chosen as the cost function of the minimizing procedure.

Experimental setup for measurement of acoustic power dissipation in lined ducts for higher order modes propagation with air mean-flow conditions

A flow duct acoustic facility was developed to measure liner efficiency in attenuating higher order acoustic duct modes propagation conditions with mean air flow. The method is based on measurement, upstream and downstream of a liner, of the acoustic power produced by a periodic source. Directly measured total or modal acoustic powers are deduced from the local measurement, in both cross sections, of acoustic pressure, axial acoustic particle velocity, and axial mean flow velocity which are supplied by a probe made of a microphone and a single hot film. In this paper, the equipment, signal processing, and the data treatment process of this facility are first described. Then, information on the accuracy of the methodology is provided by a validation test performed with a rigid wall duct section. Finally, the results of an experiment carried out with a locally reacting liner and a mean flow velocity of 20 m/s will be presented. Measurements of the main attenuation frequency and of the main total acoustic power dissipated agree with the values for which the liner was designed. These results point out the limitations of the method presented to sources with high-level periodic sounds to provide a sufficient signal-to-noise ratio, the noise being produced by fluctuations of the turbulent flow.

A 3D Multiport Scattering Matrix Based-Method for Educing Wall Impedance of Cylindrical Lined Duct Section: Simulation and Error Evaluation

The first step to achieve the development of an original indirect method to educe the wall normalized acoustic impedance of a cylindrical lined duct section which includes frequency and modal content pressure field dependence is introduced. It is based on the minimization of the difference between numerical and experimental acoustic power dissipations deduced from the 3D numerical and experimental scattering matrices of a lined duct element. The work presented in this paper is a step toward conducting experiments with a flow duct facility developed during the European DUCAT program. To validate this eduction technique, a simulation of the experiment is performed for no flow conditions assuming an axi-symmetric wall lined with a locally reacting material whose impedance was measured with the two microphone method (TMM). The simulation conducted for two incident pressure vectors with a Monte Carlos technique also provides an assessment of the uncertainty in three predominant experimental parameters on the scattering matrix coefficients, the acoustic power dissipation, and the educed impedance whose results will be useful during the experiments being conducted.

Passive and active acoustic properties of a diaphragm at low Mach number: Experimental procedure and numerical simulation

In this paper a low noise flow duct facility that performs a multimodal characterization of the passive and active acoustic properties of obstacles in the aero-acoustic conditions of an automotive air conditioning system is presented. The experimental procedure is made of two steps. In the first one the passive data, i.e. the multimodal scattering matrix, is measured with a multi-sources method. In the second step the outgoing modal pressure spectra radiated upstream and downstream by the flow obstacle interaction source is achieved. A numerical simulation of the experiment based on a 3D-Finite Element Method is developed. A very good agreement between the theoretical and experimental results of the no flow scattering matrix of a diaphragm is found. The effect of the flow on the scattering matrix is shown before performing the measurement of the aero-acoustic source characteristics of the diaphragm.

A transverse isotropic equivalent fluid model combining both limp and rigid frame behaviors for fibrous materials

Due to the manufacturing process, some fibrous materials like glasswool may be transversely isotropic (TI): fibers are mostly parallel to a plane of isotropy within which material properties are identical in all directions whereas properties are different along the transverse direction. The behavior of TI fibrous material is well described by the TI Biots model, but it requires one to measure several mechanical parameters and to solve the TI Biots equations. This paper presents an equivalent fluid model that can be suitable for TI materials under certain assumptions. It takes the form of a classical wave equation for the pressure involving an effective density tensor combining both limp and rigid frame behaviors of the material. This scalar wave equation is easily amenable to analytical and numerical treatments with a finite element method. Numerical results, based on the proposed model, are compared with experimental results obtained for two configurations with a fibrous material. The first concerns the absorption of an incident plane wave impinging on a fibrous slab and the second corresponds to the transmission loss of a splitter-type silencer in a duct. Both configurations highlight the effect of the sample orientation and give an illustration of the unusual TI behavior for fluids.

How reproducible are methods to measure the dynamic viscoelastic properties of poroelastic media

There is a considerable number of research publications on the acoustical properties of porous media with an elastic frame. A simple search through the Web of Science™ (last accessed 21 March 2018) suggests that there are at least 819 publications which deal with the acoustics of poroelastic media. A majority of these researches require accurate knowledge of the elastic properties over a broad frequency range. However, the accuracy of the measurement of the dynamic elastic properties of poroelastic media has been a contentious issue. The novelty of this paper is that it studies the reproducibility of some popular experimental methods which are used routinely to measure the key elastic properties such as the dynamic Youngs modulus, loss factor and Poisson ratio of poroelastic media. In this paper, fourteen independent sets of laboratory measurements were performed on specimens of the same porous materials. The results from these measurements suggest that the reproducibility of this type of experimental method is poor. This work can be helpful to suggest improvements which can be developed to harmonize the way the elastic properties of poroelastic media are measured worldwide.

Elaboration of a sacttering matrix measurement procedure using the p‐v probe

The scattering matrix which relates traveling waves amplitudes as state variables has been shown to be more attractive than transfer or mobility matrices since it reflects the fundamental duct nature: it gives a more complete description of the transmission, reflection, and conversion properties of the duct. In the University of Technology of Compiegne, an experimental procedure was developed to measure this matrix: a p‐p probe mounted on a setup designed during DUCAT project is used to measure pressures at two cross sections on the both side of the test lined duct, then by using a modal decomposition and separation techniques, the scattering matrix is deduced. In this paper, a method to measure the multimodal scattering matrix based on the use of a p‐v probe getting simultaneously the acoustic pressure and velocity at one section on the both side of the test duct is developed. A comparison of some acoustics values (scattering matrix coefficients, acoustic powersœ) of a hard wall duct straight duct obtained by each technique with the theory is presented to evaluate its advantages and limitations.

Effect of curvature on the scattering coefficients of Herschel‐Quincke tubes

The concept of Herschel‐Quincke (HQ) tubes has been known since the beginning of the 20th century and was shown to be of interest to reduce tonal and broadband noise from turbofan engine. To help to understand the physical phenomena underlying the HQ concept, Hallez & Burdisso have developed a 3D theoretical model. The 3D modelling technique considers the tubes‐inlet interfaces as finite piston sources that couple the acoustic field inside a hard‐walled duct with the acoustic field within the HQ tubes. This model makes a geometrical approximation by considering the tube as a straight duct whereas in reality it corresponds to a curved duct. In this paper, a model is presented which takes into account the curvature of the bend by integrating two differential equations for the pressure and velocity in the bend, projected on the local transverse modes. Results of some typical coefficients of the scattering matrix [S] deduced from the curved HQ model are compared with the 3D straight tube model, the experiment...