Modifying the Resonances of a Xylophone Bar Using Active Control
Résumé
A simple active control method is described which extends the possibilities of a xylophone bar. It allows the performer to modify the vibration of its structure, unlike post-processing effects involving loudspeakers. These variations change the characteristics of the partials radiated by the bar. The xylophone bar, made of composite material, is equipped with two actuators and one sensor in PVDF (polyvinylidene fluoride), the mass and stiffness of which do not modify the mechanical characteristics. A controller in a feedback loop is executed on a midrange digital signal processor. It is composed of a sum of second order band-pass filters. The selection of the controller coefficients relies on the measured transfer function between the input of the controller and the output of the sensor. First the active control method is designed to modify the resonance peaks of a simple model of a xylophone bar, whose transfer function is a superposition of three eigenmodes. Then it is applied to the real system. It is illustrated by increasing and reducing the amplitudes and/or frequencies of the first resonances, and by modifying the tuning of the xylophone bar.