Active control of sound inside a sphere via control of the acoustic pressure at the boundary surface

Here we investigate the practical feasibility of performing soundfield reproduction throughout a three-dimensional area by controlling the acoustic pressure measured at the boundary surface of the volume in question. The main aim is to obtain quantitative data showing what performances a practical implementation of this strategy is likely to yield. In particular, the influence of two main limitations is studied, namely the spatial aliasing and the resonance problems occurring at the eigenfrequencies associated with the internal Dirichlet problem. The strategy studied is first approached by performing numerical simulations, and then in experiments involving active noise cancellation inside a sphere in an anechoic environment. The results show that noise can be efficiently cancelled everywhere inside the sphere in a wide frequency range, in the case of both pure tones and broadband noise, including cases where the wavelength is similar to the diameter of the sphere. Excellent agreement was observed between the results of the simulations and the measurements. This method can be expected to yield similar performances when it is used to reproduce soundfields.Comment: 28 pages de text

Time-domain versus frequency-domain effort weighting in active noise control

Although Active Noise Control aims at reducing the noise at a set of error sensors, it is often designed by minimizing an error index that includes a weightedpenalty on the actuator inputs. In this way, the control tends to be more robust and the effort-weighting parameter allows the maximum voltages applied to the control sources to be monitored. Two similar effort-weighting techniques have been widely implemented in active control studies: optimal control can be computed using Tikhonov regularization in frequency-domain simulations, whereas the leaky Filtered-reference least mean squares algorithm can be implemented for real-time feedforward control. This paper makes explicit the relationship between the two effort-weighting parameters which lead, in the case of a single-tone noise, to exactly the same error index in both the time and frequency domains. The best real-time leakage factor can then be computed from frequency-domain optimization. This paper also discusses numerical simulations of a single-channel set-up, showing that with these two related parameters, the control performances are indeed very similar. One exception occurs in the case of a control flter with a very short impulse response, when the control is more conservative in the time-domain simulations than in those of the frequency-domain

Estimation and global control of noise reflections

International audienceIn theory, active control could be used to reduce the unwanted noise reflections from surfaces such as a submarine hull or the walls of an anechoic room. In the recent years, a real-time algorithm has been developed to this effect at the Laboratoire de Mécanique et d'Acoustique: the noise scattered by the surface is estimated through linear filtering of acoustic pressure signals provided by ordinary microphones and an adaptive feedforward algorithm minimizes the resulting error signals. The paper summarizes the theory underlying the control algorithm, which stems from the integral representation of the scattered pressure, and presents the successive experiments which have been conducted with it: control of terminal reflections in a duct, control of the noise scattered by a parallelepiped in an anechoic room, estimation of the noise reflections on the walls of a small room. It appears that an accurate identification of the linear filters that account for the surface scattering leads to an effective estimation and control of the scattered noise. Facilities allowing such an accurate estimation of the scattered noise are suggested for a future anechoic room where active devices would deal with the wall reflections in the 20-100Hz frequency range

Une introduction au contrôle acoustique actif

Introduction1 - Contrôle actif et acoustique2 - Contrôle actif et automatique3 - Quelques applications du contrôle actifConclusion - RésuméAnnexesA Décomposition en valeurs singulièresB Transferts à phase minimale et factorisation spectraleC Sujets d'examen du DEA d'acoustiqueD Bibliographie sommaireDE

Simulation ARMA de processus stochastiques à partir de leur densité spectrale de puissance

Les méthodes constructives de résolution des problèmes aléatoires font largement appel à la simulation des trajectoires de processus stochastiques. Ce rapport présente plusieurs méthodes pour l'identification des paramètres d'un modèle ARMA discret qui représente au mieux un processus monodimensionnel physique donné, défini par sa seule densité spectrale de puissance. Si le processus cible modélise, par exemple, un "profil type" de route, le modèle ARMA obtenu permettra de simuler à volonté les sollicitations verticales appliquées à un véhicule en mouvement. L'identification du modèle ARMA cherché est liée à la minimisation de critères quadratiques non linéaires et non convexes. On construit en conséquence le modèle par des chemins détournés, en procédant à la factorisation spectrale du processus initial. On considère notamment à cet effet une technique originale basée sur la décomposition en série de Laurent de la densité spectrale, où le recours à la FFT (Fast Fourier Transform) permet une résolution particulièrement rapide et précise. Plusieurs méthodes sont ensuite présentées pour l'obtention du modèle ARMA ; l'utilisation systématique de la FFT permet là-encore un traitement informatique simple et efficace des équations. On présentera les résultats obtenus pour différents spectres de départ et pour finir on s'intéressera aux conditions pratiques d'utilisation du modèle en évoquant en particulier les difficultés qui surgissent lorsque l'on souhaite simuler des trajectoires avec un pas de temps très inférieur aux temps caractéristiques des fluctuations du processus

Active control of scattered acoustic radiation: a real-time implementation for a three-dimensional object

International audienceThis paper presents an active noise control experiment designed to validate a real-time control strategy for reduction of the noise scattered from a three-dimensional body. The control algorithm relies on estimating the scattered noise by linear filtering of the total noise measured around the body; suitable filters are identified from off-line measurements. A modified Filtered-Error Least-Mean-Squares algorithm then leads to the adaptive filters which drive the secondary sources. The paper provides the numerical simulations using a Boundary Element Method which helped in designing a feasible experiment in an anechoic chamber with a limited number of control sources. Eventually a real-time pure-tone implementation with 14 ordinary loudspeakers and a large body is shown to yield on average a 10~dB reduction of the scattered noise at the error sensors, which is close to the optimum reduction predicted by the numerical simulations for the sensor arrangement

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

Infinite non-causality in active cancellation of random noise

Active cancellation of broadband random noise requires the detection of the incoming noise with some time advance. In an duct for example this advance must be larger than the delays in the secondary path from the control source to the error sensor. In this paper it is shown that, in some cases, the advance required for perfect noise cancellation is theoretically infinite because the inverse of the secondary path, which is required for control, can include an infinite non-causal response. This is shown to be the result of two mechanisms: in the single-channel case (one control source and one error sensor), this can arise because of strong echoes in the control path. In the multi-channel case this can arise even in free field simply because of an unfortunate placing of sensors and actuators. In the present paper optimal feedforward control is derived through analytical and numerical computations, in the time and frequency domains. It is shown that, in practice, the advance required for significant noise attenuation can be much larger than the secondary path delays. Practical rules are also suggested in order to prevent infinite non-causality from appearing