Nacer Hamzaoui

Noise assessment in a high-speed train

Binaural sound recordings in various positions in a high-speed train have been used as stimuli in listening tests. In a first experiment, sounds were submitted to subjects at their real levels. Loudness revealed to be the most important parameter; it could be very well described by A-weighted levels. In a second experiment, loudness of stimuli were equalised; strong inter-individual differences appeared, which are due to different appreciations of the low-frequency content of sounds. Usual physical or psycho-acoustical parameters failed to correctly described that perceptual parameter; a simple parameter based on the specific loudness calculations was developed and could correctly describe the results.

Reconstruction of Sound Sources of Gear Transmission Mechanism by Planar Near Field Acoustical Holography

This study is to reconstruct the sound sources of the transmission mechanism for spur gear. The inverse technique used is based on the planar near field acoustical holography. The measurements were performed in acoustics and vibration laboratory (LVA) of the INSA of Lyon, in a semi-anechoic room, where the floor is concrete and the walls are covered with glass wool. Accelerometers are positioned on the casing, they serve as reference. The complex acoustic pressures are measured by a rectangular antenna composed of regularly spaced microphones, which is placed above the noisy gear mechanism on a parallel plane. The source reconstruction is an inverse problem, it is classified as a so-called ill-posed problems, hence the necessity of applying regularization. Veronesi filter and Tikhonov method coupled with the L-curve function can reconstructed the image of noise sources on a plane parallel and tangent to the gear mechanism considered open and sometimes closed.

Experimental Validation and Adjustment of the Semi-Active Suspension Numerical Model Incorporating a MR Damping

The purpose of this paper is to experimentally validate the performance of a semi-active suspension incorporating a magneto-rheological damper (MR), where the parameters of the numerical models are often poorly adapted to real responses measured experimentally. To ensure a better representation of a real semi-active suspension, we must consider the internal dynamics of the MR damper in its numerical modeling. By adopting models which demonstrate that dynamic, such as the Bingham and Bouc-Wen models, we can approach the measured responses by adjusting their internal parameters. The law control introduction for feedback control of the semi-active suspension incorporating the internal dynamics of the MR damper allows, through the analysis of its robustness and response time, to better assess its performance. To validate the performance of these models, a comparative analysis was made between the experimentally measured responses by the dSPACE system used as an acquisition and control chain and the calculated or predicted responses. A rapprochement between measured responses and those calculated for the same dynamic characteristics of the test bed is possible by adjusting the most influential parameters of Bouc-Wen model.

Detection of Mechanical Defects by Neural Networks “Experimental Analysis”

Various methods are implemented to identify the nature of a defect on a rotating machine, by using vibratory measures; they differ in their precision, simplicity of implementation and their sensitivity to errors measurement. The identification of several defects combination is still difficult to implement by conventional signal processing, as the vibration signal that emerges is disturbed, thus making any identification so hard. In this study, we proposed a method based on the neural networks to identify one defect or several combinations of mechanical defects. Thus we propose the neuronal method: the Radial Basis Function (RBF). We highlight their capacity to detect the defect and their sensibility with regard to a signal noise characterizing the other independent sources to the defects. This evaluation will be done with measurement will be carried out on a housing bearing and test bench made up of a toothed gearing on two floors, and without lubrication. Some provoked defects will be analyzed in this study.

Auditory Evaluation of Sounds Radiated from a Vibrating Plate Inside a Damped Cavity

This paper deals with the auditory perception of sounds radiated from a vibrating plate inside a damped cavity. It focuses on the adjustment of vibroacoustic computation efforts with the prospect of using vibroacoustic simulations to enhance the quality of the radiated sounds from the design stage of structure. The adjustment issues appear as important when working with synthesized sounds. Also, when sounds are synthesized from computations in frequency under vibroacoustic prediction tools, computational efforts are inherently dependent on a first restrictive parameter: the maximum limit frequency. Thus, as a first step, this frequency parameter is aimed at being adjusted up to a value which would allow a conservation of the perceptual outcomes concerning an optimal structural design, originally obtained from the auditory evaluation of a set of recorded sounds. To reach this aim, recorded sounds are acquired from an experimental cavity-backed plate; they actually correspond to sounds radiated from the vibrating plate inside the air cavity, recorded for different structural configurations of the experimental system. The latter configurations involve the controlled variation of three structural parameters: the plate thickness, the plate tightening conditions and the cavity sound absorption properties. The recorded sounds are first pairwise submitted to a jury of listeners who are asked to give dissimilarity and preference judgments. From the analysis of the collected data, reference perceptual results are established, in terms of perceptual space, preference space and preference ranking. Afterwards, the adjustment process aims at finding out, from a lowpass filtering applied to the recorded sounds, a cutoff frequency within their entire frequency content allowing the reference perceptual results to be preserved.

Dynamic Characterization of MR Damper and Experimental Adjustment of Numerical Model

In this present work, we analyze through the hysteresis loops the modified Bouc-Wen model performance governing the internal dynamics of a Magneto-Rheological (MR) damper. The analysis of the energy dissipated per cycle of vibration is made by a MR damper dynamic characterization on a dynamic testing machine in order to examine the effect of the frequency, the amplitude of the displacement and of the excitation current on the dissipated energy and therefore the equivalent damping coefficient. The dynamic responses of the modified Bouc-Wen numerical model will be compared with experimental responses and then recalibrated on the latter, while performing an adjustment of the influence parameters model.

Localization and Identification of Vibroacoustic Sources of Gear Transmission Mechanism by Inverse Frequency Response Function

Our contribution in this work is to detect, localize and quantify the noise sources radiated by a spur gear transmission mechanism. The imaging technique is used; it is based on the acoustic inverse frequency response function (IFRF). The IFRF is based on the inversion of the transfer matrix built between the source points represented by their complex source strengths and listening points represented by the complex pressures measured by the hologram. The measurements were performed in a semi-anechoic room where the floor is concrete and the walls are covered with glass wool. The complex acoustic pressures are measured by an antenna with microphones regularly spaced; it is placed above the noisy mechanism. The reconstruction problem is therefore an inverse problem and is said ill-posed; thus, regularizations are needed to stabilize and to find the best solutions. As regularization technique, the Tikhonov method is applied and the regularization parameters are chosen according to the L-curve method. The goal is to reconstruct as accurately as possible the acoustic field radiated by the transmission mechanism on a fictive and tangent plane to the noisy mechanism considered open and sometimes closed. The results obtained showed that the sources were located with good approximation. The IFRF method is able to reconstruct the sound sources responsible for the noise radiated by the mechanism without any a priori information of the sources distribution, and the visualization of spatial acoustic fields facilitate the understanding of the complex phenomena of radiation.

Vibroacoustic Sources Identification of Gear Mechanism Transmission

The prediction and the identification of the vibroacoustic sources of the toothed gearings remain rather complex in spite of various carried out work. Often, they are experimental statements on a significant number of transmissions which finish by councils given to the designers in order to limit or to reduce the radiated noise. Our work consists in contributing to treat on a hierarchical basis the vibroacoustic sources on a transmission system defined well, and this, while being based on the method of principal components analysis (PCA). PCA method is based on the statistical analysis, thus partial virtual coherence and multiple virtual coherence are used to quantify the coherent shares with the acquired signals autospectres.