A preliminary study of an isodynamic transducer for use in active acoustic materials

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.