Nicolas Totaro

Non resonant transmission modelling with statistical modal energy distribution analysis

Statistical modal Energy distribution Analysis (SmEdA) can be used as an alternative to Statistical Energy Analysis for describing subsystems with low modal overlap. In its original form, SmEdA predicts the power flow exchanged between the resonant modes of different subsystems. In the case of sound transmission through a thin structure, it is well-known that the non resonant response of the structure plays a significant role in transmission below the critical frequency. In this paper, we present an extension of SmEdA that takes into account the contributions of the non resonant modes of a thin structure. The dual modal formulation (DMF) is used to describe the behaviour of two acoustic cavities separated by a thin structure, with prior knowledge of the modal basis of each subsystem. Condensation in the DMF equations is achieved on the amplitudes of the non resonant modes and a new coupling scheme between the resonant modes of the three subsystems is obtained after several simplifications. We show that the contribution of the non resonant panel mode results in coupling the cavity modes of stiffness type, characterised by the mode shapes of both the cavities and the structure. Comparisons with reference results demonstrate that the present approach can take into account the non resonant contributions of the structure in the evaluation of the transmission loss.

Identification of Boundary Pressure Field Exciting a Plate Under Turbulent Flow

The characterisation of the aeroacoustic wall pressure field generated by turbulent flow is a difficult task that often requires instrumented panels and huge facilities like wind tunnels. In situ and non-intrusive experiments are rather not possible. In addition, the pressure field is dominated by the aerodynamic component and the experimental dynamics are not sufficient to measure correctly spectra in low wavenumbers by microphones. The present chapter deals with such a separation method by using the Force Analysis Technique (FAT). FAT is based on the use of the equation of motion of the structure (here a plate) and on the approximation of the fourth derivatives by a finite difference scheme. In the case of turbulent flow, the force auto-spectrum can be deduced at one point of the structure by measuring the velocity at 13 points synchronously. To this purpose, an array of 13 pU (acoustic pressure/particle velocity) probes has been made up. This array is moved in the near-field of the plate to identify map of the wall pressure level applied on the surface of the plate. In the present application, it is shown that FAT only identifies the component of the excitation with wavenumber lower than the natural flexural wavenumber of the plate, due to filtering effect of the plate and of the finite difference scheme. In most cases, the convective peak is then canceled and only the acoustic part of the turbulent flow is identified. This property can be of great interest for vehicle manufacturers to quantify the part of the wall pressure that is responsible of the radiated noise or to use FAT as a source separation technique.

Identification of source velocities with Inverse Patch Transfer Functions method

The identification of source velocities remains an important problem in noise control. For this purpose, several methods were developed such as Near‐field Acoustic Holography (NAH) or inverse Boundary Elements Method (iBEM). An alternative method, based on the double measurement of pressure and particle velocity fields surrounding the source is presented. This method has been developed in the SILENCE European project framework. In this method, called inverse Patch Transfer Functions method (iPTF), measurement and identification surfaces are divided into elementary areas called patches. Then, source velocities are computed from acoustic field and inversion of impedances matrices obtained by FEM. Theoretically, this method presents two main advantages: it can be applied to sources with complex 3D geometries and measurements can be carried out in a non‐anechoic environment, even in the presence of other sources. In the present paper, theoretical background of iPTF is exposed and results are presented on a so...

Noise source characterisation using patch impedance technique

A novel approach of an environment‐independent sound source characterisation is discussed. The source is defined via a suitable enveloping interface surface by its blocked sound pressure and its surface impedance. Both the blocked pressure and the impedance are discretised using the averaging patch concept. Such a definition avoids singularity of point acoustic impedance and is suitable for numerical as well as experimental implementation. The characterisation of a source by the patch concept allows for the acoustical sub‐structuring, which in turn enables the prediction of the sound field created by the source coupled to an arbitrary environment. Numerical simulations are presented which demonstrate the feasibility of the approach. It is hoped that the proposed approach can serve as a universal tool for noise synthesis of complex equipment incorporating noise sources.

Versatile hybrid sandwich composite combining large stiffness and high damping: spatial patterning of the viscoelastic core layer

With the aim of decreasing CO2 emissions, car producers’ efforts are focused, among others, on reducing the weight of vehicles yet preserving the overall vibrational comfort. To do so, new lightweight materials combining high stiffness and high (passive) damping are sought. For panels essentially loaded in bending, sandwich composites made of two external metallic stiff layers and an inner polymeric (i.e. absorbing) core are broadly used. In the present work, the performances of such sandwich structures are enhanced by optimizing their damping behavior according to their use. More precisely, spatial patterning through selective UV irradiation of the viscoelastic properties of the silicone elastomeric layer is obtained based on a recently published UV irradiation selective technique [1]. Initially developed to modulate the elastic property gradient in Liquid Silicone Rubber (LSR) membranes, the procedure is now generalized to control the viscoelastic behavior of Room Temperature Vulcanization (RTV) silicon...

Structure/cavity coupling using Statistical Energy Analysis: Coupling loss factors and energy maps into subsystems

Prediction of interior noise is one of the most concerning issues of vehicle industry. Statistical Energy Analysis theoretically allows to determine energy spread over a structure divided into subsystems when one subsystem is submitted to a rain‐on‐the‐roof excitation. Subsystems can be either a part of the structure or a cavity. Recently, a method (SmEdA) based on modal bases of uncoupled subsystems have been derived. This method permits to compute CLF using standard FEM software. This method has been successfully applied on structure/structure coupling and is extended to estimation of CLF between a structure and a cavity in the present article. In addition, in the case of interior noise, SEA can only provide a global energy into the cavity on frequency bands. No information on energy repartition into the subsystem is given. In the present article, an extension of SmEdA method is proposed to quickly estimate energy repartition into subsystems (structure or cavity).

Spatial Patterning of the Viscoelastic Core Layer of a Hybrid Sandwich Composite Material to Trigger Its Vibro-Acoustic Performances

With the aim of decreasing CO2 emissions, car producers’ efforts are focused, among others, on reducing the weight of vehicles yet preserving the overall vibrational comfort. To do so, new lightweight materials combining high stiffness and high (passive) damping are sought. For panels essentially loaded in bending, sandwich composites made of two external metallic stiff layers (skins) and an inner polymeric (i.e. absorbing) core are broadly used. Now aiming at creating materials by design with a better control of the final performance of the part, the tuning of the local material properties is pursued. To this end, the present work focuses on controlling the spatial in-plane viscoelastic properties of the polymeric core of such sandwich structures. The spatial patterning is achieved using a recently developed UV irradiation selective technique of Room Temperature Vulcanization (RTV) silicone elastomeric membrane, in which the ultraviolet (UV) irradiation dose, curing time and temperature are the process parameters controlling the viscoelastic properties of the polymeric membrane. Finally, a protocol for the realization of architected aluminum - silicone - aluminum composite sandwich panels is proposed. The influence of UV irradiation selective technique is demonstrated by Dynamic Mechanical Analysis (DMA) measurements on the silicone core itself and by the Corrected Force Analysis Technique (CFAT) to measure the equivalent Young’s modulus and damping of the sandwich structure over a large frequency band. As a first demonstration application, sandwich beams with different core patterns (homogeneous and heterogeneous) are designed and tested. Furthermore, the analytical formalism developed by Guyader et al. is used to model the vibro-acoustic performances of the homogenous sandwich beams and fair model-experiments comparisons are obtained. The spatial patterning of the polymer layer is found to successfully affect the local properties of the composite heterogeneous beam as evidenced by the CFAT method. Finally, this work permits the enunciation of guidelines for designing complex architectured systems with further control of the vibro-acoustics performances.

Experimental characterization of short-glass-fiber polymer composite for vibroacoustic applications

In order to design vehicles with diminished CO2/km emissions level, car manufacturers aim at reducing the weight of their vehicles. One of the solutions advocated by the automotive engineers consists in the replacement of metallic parts by lighter systems made of polymer composites. Unfortunately, the numerical simulations set to evaluate the vibratory and acoustic performances of systems made of this kind of materials are often not sufficiently effective and robust so that convincing test/simulation correlations are rarely met. Indeed, for polymer-based materials, numerous parameters affect the vibroacoustic behavior of the system. For the present study, focusing on Polyamide 6 reinforced glass fiber plates (PA6-GF35), it will be demonstrated using DMA (Dynamic Mechanical Analysis) and FAT (Force Analysis Technique) analysis that the viscoelastic properties depend on the temperature and frequency but also on the humidity content. We will compare the FAT method which permits to identify the equivalent com...

Extension of SmEdA to non-resonant transmission

Statistical modal Energy distribution Analysis (SmEdA) may be used as alternative to Statistical Energy Analysis for describing subsystems with low modal overlap. In its original form, SmEdA predicts the power flow exchanged between the resonant modes of different subsystems. In the case of the sound transmission through a thin light structure, it is well-known than the non-resonant response of the structure may have a significant role on the transmission below the critical frequency. In this paper, one presents an extension of SmEdA taking into account the contributions of the non resonant modes of the thin structure. The dual modal formulation (DMF) is used for describing the behavior of two acoustic cavities separated by a thin structure knowing their subsystem modes. A condensation in the DMF equations is achieved on the amplitudes of the non-resonant modes. Using some simplifications, a new coupling scheme between the resonant modes of the three subsystems is obtained. It shows direct couplings of th...