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

Some active impedance control experiments in an anechoic chamber are r eported. Further active methods are developed for the design of locall y controlled absorption liners in air, whose basic principles were des cribed by Olson and May and subsequently investigated practically by G uicking and his colleagues at the beginning of the 80's. Two methods a re described and tested. In the first, processing of the acoustic pres sure and velocity information from close to the membrane of a control loudspeaker is used to produce a desired impedance. In the second acti ve and passive means are combined: the impedance of the rear face of a porous layer is actively controlled seas to make the front face norma l impedance take on a prescribed value. The impedance matching perform ance of both systems subject to an incident acoustic field including a single secondary loudspeaker is studied for both normal and oblique i ncidence. The combination of active and passive methods is a pragmatic approach, the aim of which is to simplify the control system for impe dance 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 coating s. (C) 1997 Academic Press Limited.