Hybrid passive-active absorption of a microperforated panel in free field conditions

Although hybrid passive-active absorption in impedance tubes has been widely analyzed, its study in more realistic conditions remains to be accomplished. This letter describes the practical measurement of the passive-active absorption coefficient in a free field. The passive absorber consists of a microperforated panel backed by an air cavity and a wooden panel. A multichannel active system controls the sound pressure in a reduced cell behind the absorber. Measured absorption coefficients between 200 and 400Hz account for 0.2–0.4 and 0.72–0.75, in the passive and active cases, respectively, and above 500Hz both are comparable.