SURFACE IMPEDANCE CONTROL FOR SOUND ABSORPTION: DIRECT AND HYBRID PASSIVE/ACTIVE STRATEGIES

Some active impedance control experiments in an anechoic chamber are reported. Further active methods are developed for the design of locally controlled absorption liners in air, whose basic principles were described by Olson and May and subsequently investigated practically by Guicking and his colleagues at the beginning of the 80's. Two methods are described and tested. In the first, processing of the acoustic pressure and velocity information from close to the membrane of a control loudspeaker is used to produce a desired impedance. In the second active and passive means are combined: the impedance of the rear face of a porous layer is actively controlled so as to make the front face normal impedance take on a prescribed value. The impedance matching performance of both systems subject to an incident acoustic field including a single secondary loudspeaker is studied for both normal and oblique incidence. The combination of active and passive methods is a pragmatic approach, the aim of which is to simplify the control system for impedance control over extensive areas of wall. Indeed, the association of active control with a porous material allows the active system to be reduced in complexity to a simple active pressure release. Even though somewhat sub-optimal for sound absorption, the hybrid passive/active systems support feedback methods and lead to highly absorptive coatings.