title: Low-frequency absorption using a two-layer system with active control of input impedance.
abstract: Broadband noise absorption, including low frequencies, may be obtained by a hybrid passive-active two-layer system. A porous layer in front of an air layer provides passive absorption, at medium and high frequencies. Active control of the input impedance of the two-layer system yields absorption at low frequencies. The active control system can implement either pressure-release or impedance-matching conditions. A simple analytical model based upon plane waves propagating in a tube permits the comparison of both control strategies. The results of this simple model show that the pressure-release condition affords higher absorption than the impedance-matching condition for some combinations of geometrical and material parameters. Experimental results corroborate the good performance of the pressure-release condition under the prescribed geometrical setup.
authors.affiliation: Instituto de Acústica, CSIC, Serrano 144, 28006 Madrid, Spain. iacpc24@ia.cetef.csic.es
authors.forename: Pedro
authors.initials: P
authors.lastname: Cobo
authors.forename: Alejandro
authors.initials: A
authors.lastname: Fernández
authors.forename: Olivier
authors.initials: O
authors.lastname: Doutres