A broadband passive-active sound absorption system

In recent years, hybrid absorption systems have been implemented which achi eve high sound absorption over a broad frequency range. This work is an exp erimental study of a broadband hybrid absorption system which is comprised of a layer of sound-absorbing material (the passive component) positioned a t a distance from a movable wall (the active component) inside an impedance tube. The movable wall is used to impose desired boundary conditions in th e cavity behind the passive layer, thereby increasing the absorption of the system at frequencies where the passive material is not independently effe ctive. Both pressure-release (i.e., minimizing the pressure at the back sur face of the layer) and impedance-matching (i.e., minimizing the reflected w ave from the layer) boundary conditions are studied. The performance of the hybrid system for these two boundary conditions is compared for broadband disturbances over a frequency range of 100-1000 Hz. The unmodified passive system showed absorption coefficients greater than 0.7 only above 500 Hz, w hile the impedance-matching condition yielded absorption coefficients of 0. 8 to 1.0 over the 100-1000-Hz range. The impedance-matching condition provi ded significantly better absorption than that achieved with the pressure-re lease condition. The sensitivity of these control approaches to system para meters is also investigated. (C) 1999 Acoustical Society of America. [S0001 -4966(99)00711-0].