Marie-Annick Galland

A finite-element study of a piezoelectric/poroelastic sound package concept

This paper presents a complete finite-element description of a hybrid passive/active sound package concept for acoustic insulation. The sandwich created includes a poroelastic core and piezoelectric patches to ensure high panel performance over the medium/high and low frequencies, respectively. All layers are modelled thanks to a Comsol environment. The piezoelectric/elastic and poroelastic/elastic coupling are fully considered. The study highlights the reliability of the model by comparing results with those obtained from the Ansys finite-element software and with analytical developments. The chosen shape functions and mesh convergence rate for each layer are discussed in terms of dynamic behaviour. Several layer configurations are then tested, with the aim of designing the panel and its hybrid functionality in an optimal manner. The differences in frequency responses are discussed from a physical perspective. Lastly, an initial experimental test shows the concept to be promising.

Active enhancement of the absorbent properties of a porous material

We present in this paper an active anechoidal termination composed of a porous material, whose absorbent properties are improved by a secondary source. A simple model of acoustic propagation in a porous medium leads to optimal absorption criteria, which are verified by absorption coefficient measurements under normal incidence at the end of a Kundt duct. A secondary source is placed at the backward interface of the material, and imposes zero pressure by destructive interferences, which is a necessary condition for the maximum absorption. Several control set-ups are tested and provide the anechoism for broadband excitations.

Design of an adaptive hybrid liner for flow duct applications

This paper introduces a multiple-channel adaptive digital feedback algorithm for active noise control applications(Internal Model Control Multiple-input/multiple-output Diagonalized Filtered-x Least Mean Squares algorithm, IMC-MDFXLMS). This algorithm is devised to outfit a new type of acoustic liner, the hybrid cell, which is realized thanks to a passive material backed by an active noise control system. Its objective is to yield a wall impedance which provides a maximum of insertion loss throughout a wide frequency range. Theoretical and experimental studies show that the best solution is achieved when active control is on at low frequencies and off at high frequencies. The control algorithm allows the design of large absorbent surfaces because it rests on a parallel operation cell by cell. The IMC-MDFXLMS is performed by using an adaptive digital feedback controller with the FXLMS algorithm applied to a diagonalized IMC architecture. Special attention is focused here to the development of MIMO systems for evolutionary primary signals. Indeed, acoustic coupling occurring between the cells can produce a global unstable behaviour even if the optimal filter is obtained in each cell. An adaptive bandpass filter is therefore used to prevent the development of instabilities. An automatic frequency detection loop is then introduced in the control algorithm, allowing the continuous adaptation of the bandpass filtering. Thus, the control system is able to reduce a non-stationary tone as produced by a change in the engine speed. This multi-cell algorithm is experimentally validated for a 4-cell system located on a duct wall in presence of flow.

Passive layer optimization for active absorbers in flow duct applications

A new kind of acoustic liners developed for broadband noise reduction in flow duct applications is considered in this paper. Noise control is achieved by hybrid cells combining absorbent properties of a porous layer and active control. The main purpose is to realize an optimal impedance theoretically determined for a specific flow duct, in order to reach maximal sound attenuations over a wide frequency bandwidth. Passive and active optimization proceedings are carried out separately. This article deals with the complete design process of the hybrid liner passive part. Some experimental comparisons between active and passive functionning of the cells, obtained in our laboratory facility, are presented.

Identification of the Characteristic Parameters of Porous Media Using Active Control

A method for determining the characteristic parameters of a porous material by means of acoustic measurements on a unique experimental set up, the standing wave tube, is presented in this paper. The principle of the identification procedure is based on the modification of the boundary conditions at the rear face of the material, especially rigid wall, air cavities of distinct depths, and soft impedance achieved thanks to an active control system. A set of about ten different porous materials was tested. The acoustic parameters are obtained from Lafarge-Allard’s model considering the frame of the material is rigid. Whether all the experimental and atmospheric constraints are strictly respected, the parameter identification method is either rapid and reliable. Very good agreement is observed between predictions and measurements for different experimental configurations.

Fully Coupled Finite-Element Modeling of Active Sandwich Panels With Poroelastic Core

Active sandwich panels are an example of smart noise attenuators and a realization of hybrid active-passive approach for the problem of broadband noise reduction. The panels are composed of thin elastic faceplates linked by the core of a lightweight absorbent material of high porosity. Moreover, they are active, so piezoelectric actuators in the form of thin patches are fixed to their faceplates. Therefore, the passive absorbent properties of porous core, effective at high and medium frequencies, can be combined with the active vibroacoustic reduction necessary in a low frequency range. Important convergence issues for fully coupled finite-element modeling of such panels are investigated on a model of a disk-shaped panel under a uniform acoustic load by plane harmonic waves, with respect to the important parameter of the total reduction of acoustic transmission. Various physical phenomena are considered, namely, the wave propagation in a porous medium, the vibrations of elastic plate and the piezoelectric behavior of actuators, the acoustics-structure interaction and the wave propagation in a fluid. The modeling of porous core requires the usage of the advanced biphasic model of poroelasticity, because the vibrations of the skeleton of porous core cannot be neglected; they are in fact induced by the vibrations of the faceplates. Finally, optimal voltage amplitudes for the electric signals used in active reduction, with respect to the relative size of the piezoelectric actuator, are computed in some lower-to-medium frequency range. [DOI: 10.1115/1.4005026]

Experimental investigation of noise reduction in a flow duct through hybrid passive/active liner*

This paper deals with the design and prototyping of a new type of acoustic liner, combining absorbent properties of a porous layer and active control. The final objective is to achieve, for a broadband noise, a targeted impedance, which was theoretically predetermined to produce the best noise reduction when applied to real aircraft engine nacelles. The basic principle of the hybrid liner is as follows : the active control system ensures a low pressure at the back face of a resistive layer having well suited characteristics, so that its resistance approaches the desired impedance. Using high resistive materials and a piezoelectric actuator as secondary source allowed us to achieve a thin active liner (thickness below 20 mm) , which is efficient over a wide frequency range. Standing wave tube measurements validated the concept and promising

Multi-cell digital feedback control for noise reduction through hybrid absorbers

This paper deals with the control aspect of a new type of hybrid acoustic absorbent developed in the LMFA Center for Acoustics. This hybrid cell aims to achieve large noise reduction over a wide frequency range by combining passive and active control. Active control is performed by using an adaptive digital feedback controller with the FXLMS algorithm applied to the IMC architecture. For extended liner surfaces, that is to say for MIMO structures, the main conclusion of the theoretical study is that acoustic coupling occurring between the hybrid cells can produce a global unstable behavior even if the optimal filter is obtained in each cell. An adaptive bandpass filter is therefore used to prevent the development of instabilities. This multi-cell algorithm has been experimentally validated for a 4-cell system located on a duct wall in the presence of flow.

Noise Reduction in a Flow Duct by Active Control of Wall Impedance

This paper deals with the complete design procedure of a new type of acoustic liner, combining passive absorbent properties of a porous layer and active control. The final objective is to achieve, for a wide frequency range, a targeted impedance, which was theoretically predetermined to produce the best noise reduction when applied to a flow duct. The optimization phase is carried out on the passive and active part of the system, and first results are presented concerning the complete design process initiated for our laboratory facility.

Performance in wind tunnel of hybrid active/passive absorbent panels

We present the results of a new measurement campaign, carried out in an anechoic wind tunnel, to evaluate the performance of hybrid active/passive panels for reducing sound in a flow duct. Each panel features 27 individual cells, which use an active control system at the rear face of a resistive layer in order to enhance the absorber capabilities to attenuate low frequencies. The tests were conducted with pure sine waves from 850 Hz to 2 kHz and with a flow velocity up to Mach 0.3. The architecture of the control system allows a rapid and robust convergence in all cases. Hybrid liners allow the development of large absorbent surfaces and active control proved its eciency at a representative Mach number. According to predictions, active functioning is generally more eective up to 1 kHz and the hybrid liner provides a global noise reduction rather independent of the frequency.