Active control of the acoustic impedance of walls in rooms allows fine control of the reverberation in auditoria. Such active materials may use locally reacting cells comprising a transducer connected to an electronic control circuit. In this paper, a simple feedback circuit based on a linear combination of the pressure at the transducer diaphragm and the velocity of the diaphragm is presented. We then discuss the desired characteristics of a transducer dedicated to our application, and show that the isodynamic technology is an interesting candidate, especially if using rubber magnet bars. We present results from simulations involving a finite element model of such transducer, which predict a good control over two frequency decades. Preliminary experimental results obtained with a basic prototype of isodynamic transducer are encouraging, yielding an absorption coefficient approaching 1 ( > 0.7) from 30 up to 500 Hz. We think that far better results can be obtained (specially in the ''super-reflecting'' case) with closer control over the various mechanical parameters. Future work will also address the optimisation of the magnet geometry. #
2014 La dissipation et la dispersion d'ondes planes dans un tuyau cylindrique étant proportionnelle à la racine carrée de la fréquence, la forme analytique de la réponse impulsionnelle existe. Nous étudions la fonction de réflexion à l'entrée d'un tuyau, c'est-à-dire la transformée de Fourier inverse du coefficient de réflexion, et particulièrement les importantes distorsions dues au traitement numérique ainsi que la propriété de stabilité par convolution. De cette propriété, nous déduisons la fonction de réflexion pour plusieurs tubes « en série », et pour un branchement de trois tubes. Une extension de ces formules au cas des tuyaux cylindriques fermés à parois rigides est donnée en Annexe A.
Many performing art centers and multipurpose halls with poor acoustics cannot be improved for economical reasons by architectural correction works or by mechanical moving devices. The principle of virtual acoustic wall [Guicking et al., J. Acoust. Soc. Am. 78 (1985)] has been successfully applied to control the reflectivity and diffusivity of concert halls existing walls. Several independent active cells composed of a microphone, an electronic unit, and a loudspeaker form a virtual wall. By appropriately controlling the open loop gain in the cells of each virtual wall it is possible to create different sound fields fully coherent in space and time with the preexisting one. The first reflections can thus be reinforced and a natural reverberation enhancement effect is obtained by the acoustic exchange between the virtual walls. Computer sound simulation of three virtual walls in a concert hall was performed and experimental results, both objective and subjective, are shown for two concert halls in which a virtual wall system was installed.
Le logiciel MIDAS a été conçu pour la mesure acoustique à toute échelle, mais plus particulièrement à l'échelle 1/50°. Nous présentons les concepts de base de ce logiciel, ainsi que l'analyse des premiers résultats obtenus
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