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Passive materials provide sound absorption in a frequency band whose low frequency limit depends on its thickness. In general, sound absorption in the lower frequency range requires such bulky materials that the passive solution becomes impractical, except for special installations, such as anechoic chambers. On the other hand, active systems allow control of the input impedance of multilayer absorbers, affording low frequency absorption. When the properties of the passive system are properly combined with the active controller, it is possible to design broadband sound absorbers with an absorption coefficient close to unity from 100 Hz. These systems are named hybrid passive-active absorbers. This paper describes the implementation of such a hybrid absorber, using a micro perforated panel (MPP) as the passive material.

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