Hybrid passive-active absorption using microperforated panels

A theoretical and experimental study of a proposed hybrid passive-active plane-wave system to provide broadband acoustic absorption is presented. The passive absorber consists of a microperforated panel (MPP), used in place of conventional fibrous materials, in front of an air layer. The active system uses an active transducer (a loudspeaker), an error sensor (a microphone), and an adaptive controller. MPPs are thinner than fibrous materials and provide a better solution in hygienic environments. For two such systems, the dependence of performance on MPP parameters is studied for two control strategies: impedance matching and pressure release. The first condition is found to be better for cases where the acoustic impedance of the system approaches that of air. Otherwise, the pressure-release condition is better, and a wideband solution is the implementation of the active control system up to a frequency where the natural band of the passive system provides higher absorption. Therefore, the use of a low-pass filter in the error signal is implemented to afford hybrid passive-active wideband absorption. The control effort of active system is also considered. Experimental verification shows good agreement with theory, and an average absorption coefficient of 0.82 in the frequency bandwidth 100 to 1600 Hz was achieved. (C) 2004 Acoustical Society of America.