M. Furstoss et al., SURFACE IMPEDANCE CONTROL FOR SOUND-ABSORPTION - DIRECT AND HYBRID PASSIVE ACTIVE STRATEGIES/, Journal of sound and vibration, 203(2), 1997, pp. 219-236
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.