Active control in an anechoic room : Theory and first simulations

International audienceNoise control and source design require the measurement of sound radiation at low frequencies. Anechoic rooms, which are designed for this purpose, allow echo-free measurements at medium or high frequency but passive wall treatment is less effective at low frequency and in practice no facility provides anechoicity below 50Hz. This paper discusses the applicability of an active control algorithm which has been previously introduced to minimize the echoes from a scattering object to the cancellation of the low frequency wall echoes in an anechoic room including wall-embedded secondary sources. At first the paper discusses, in the general case then for a free half-space as a model case, the algorithm key which consists in estimating the scattered acoustic pressure from total pressure measurements. Boundary Element Method computations are secondly used to simulate estimation and active control of error signals accounting for the low-frequency scattered pressure in an anechoic room. The simulations show that control with a few dozen microphones and noise sources allows a large reduction of the noise scattered from the walls at low-frequency

Towards an active anechoic room

International audienceThis is a presentation of some works in active control. Most of them were conducted by members of the LMA. We will show how these previous works have led to the project which is called ''Active anechoic room'' and is now carried out in the laboratory. The final aim of this project is to develop an active control system in order to increase sound absorption in the LMA anechoic room at low frequencies. Indeed, the characteristics of an anechoic room is to reduce the echoes coming from the walls in a very large frequency range. In the middle and high frequency range, this is very well achieved by covering the walls with absorbing materials. At very low frequencies (below 100Hz for example), this is more difficult but the active control systems are quite efficient at these frequencies and can be used as an additional tool to improve the acoustic performances of the passive system that is the coating on the walls. Apart from its practical applications, the study addresses more general questions related to sound synthesis and representations of sound fields and sources

RAFAL: RANDOM FACED ACOUSTIC LENS USED TO MODEL INTERNAL WAVES EFFECTS ON UNDERWATER ACOUSTIC PROPAGATION

International audienceWe present here an experimental protocol to reproduce the effects of linear internal waves (LIW) on acoustic wave propagation in a very controlled and reproducible manner. In fact, the experiment consists in propagating an ultrasonic wave through an acoustic lens presenting a plane input face and a randomly rough output face. The so-called RAFAL (Random Faced Acoustic Lens) was designed so that the roughness of the output face induce resulting acoustic pressure field featuring typical characteristics of propagation though LIW.To ensure representativeness of our model, we conducted analytical calculations leading to dimensionless parameters equivalent to the ones developed by Flatté (strength parameter Φ and diffraction parameter Λ). In our case, the strength parameter was calculated after evaluation of the phase of the average acoustic field propagated through the RAFALS, whereas our diffraction parameter was evaluated using the phase sensitivity kernel. On the other hand, we calculated the ratio of correlation length of the acoustic field to wavelength. Measurements were conducted on several RAFALS, corresponding to various realistic configurations. The regimes of saturation (full and partial) and unsaturation were explored. The results are presented in terms of order 2 (coherence function) and order 4 (intensity) statistics and demonstrate the accuracy of our experimental scheme with respect to real scale simulations and simplified theory. Other representations, such as phasors, also show a very meaningful behavior

INFLUENCE OF DE-COHERENCE EFFECTS ON SONAR ARRAY GAIN: SCLAED EXPERIMENT, SIMULATIONS AND SIMPLIFIED THEORY COMPARISON

International audienceOur study focuses on the subject of acoustic wave propagation through spatially fluctuating ocean. The fluctuations are here linear internal waves (LIW) and we developed an experimental protocol in water tank in order to reproduce the effects of LIW on ultrasound propagation. The present paper gathers the results obtained in terms of coherence function (second-order moment) for various configurations. Typical regimes of the ΛΦ plane developed by Flatté were explored, resulting into coherence function becoming narrower as the saturation increases. We also relate the coherence function to an array gain degradation parameter, δAG, which accounts for how the system performance will be mitigated in a given configuration. δAG was calculated for various sizes of vertical linear array (VLA) and showed an important dependence on the VLA's length. Typically, in any case (scaled experiment, computer simulations and simplified theory), we note that the longer the VLA, the greater the corresponding δAG. Moreover, as the saturation induced by medium fluctuations increases, δAG increases as well. This highlights the need for corrective signal processing techniques when large VLAs are used in a fluctuating environment. Signal processing techniques from various domains (e.g. adaptive optics, radio) are also studied

Dimensional analysis adapted to scaled experiments

The authors focus here on the study of a scaled experiment. The intrinsic objective is to reproduce the effects of medium fluctuations on underwater acoustic propagation. To do so, an adaptation of the derivation of the dimensionless parameters generally used to define the regimes of fluctuations is proposed. The aim of the present paper is to present of the calculations leading to the evaluation of these parameters. The procedure is based on the analytical calculation of the sound field propagated through an acoustic lens presenting a plane input face and a randomly rough output face. Statistics on the sound field (first and second-order moments) and sensitivity kernels lead to the evaluation of the so-called strength and diffraction parameters, as well as the ratio of acoustic correlation length to the wavelength. Continuity between our scaled experimental protocol and realistic oceanic configurations is therefore ensured

Psychomécanique appliquée aux Trains à Grande Vitesse

La psychomécanique est l'étude des interactions entre les paramètres mécaniques d'une structure vibrante, les paramètres acoustiques des signaux rayonnés et leur perception par un auditeur. Nous réalisons cette étude parallèlement sur deux systèmes : une voiture de TGV et sur un système plus académique constitué d'une cavité rigide fermée par une plaque mince vibrante. Ce système peut être représentatif d'un habitacle de véhicule. Nous cherchons alors à évaluer l'influence des paramètres de modélisation sur la perception. Le comportement vibroacoustique du système plaque/cavité est décrit par une méthode modale analytique et celui du TGV est décrit par une méthode modale expérimentale. Nous présentons les résultats de ces deux méthodes ainsi que ceux des premiers tests perceptifs

A rare and exclusive endoperoxide photoproduct derived from thiacalix[4]arene crown-shaped derivative bearing 9,10-substituted anthracene moiety

A rare and exclusive endoperoxide photoproduct was quantitatively obtained from a thiacalix[4]arene crown-shaped derivative upon irradiation at λ=365 nm; the structure was unambiguously confirmed by 1H/13C NMR spectroscopy and X-ray crystallography. The prerequisites for the formation of the endoperoxide photoproduct have also been discussed. Furthermore, the photochemical reaction rate could be greatly enhanced in the presence of the thiacalix[4]arene platform because it served as a host to capture oxygen

An unprecedented photochemical reaction for anthracene-containing derivatives

A series of anthracene-containing derivatives have been synthesised and characterised. The photochemical behaviour of these derivatives have been investigated by 1 H NMR spectroscopy. An unprecedented photolysis reaction for anthracene-containing derivatives was observed in the case of anthracenes directly armed with a -CH 2 O-R group upon UV irradiation. The photolysis reaction process has been demonstrated to occur in three steps. Firstly, the anthracene-containing derivatives are converted into the corresponding endoperoxide intermediate upon UV irradiation in the presence of air; then, the endoperoxide intermediate is decomposed to the corresponding starting compound and 9-anthraldehyde; finally, 9-anthraldehyde is further oxidised to anthraquinone. Additionally, the photolysis reaction of anthracene-containing derivatives is significantly promoted in the presence of a thiacalix[4]arene platform