Improving Absorption of Sound Using Active Control

Absorption of sound is a common problem especially at low frequencies. Absorbing materials available today perform well at medium and high frequencies but are much less performing at low frequencies at least when considering layers of realistic thickness. By contrast active control of sound is the most powerful at low frequencies where the sound field that is to be controlled is rather simple. Hence a combination of passive materials and active control seems to be a promising way to improve the efficiency of sound absorbing acoustic linings. The paper reflects upon two main directions. First, it studies the elimination by active control of sound of a sound field reflected by an absorbing layer. Such a procedure may be applied to improve the quality of acoustic testing facilities like anechoic chambers around or below its cut-off frequency. Secondly, the paper considers the design of hybrid absorbing materials consisting of a passive materials whose sound absorption is improved using either acoustic or mechanic actuators. Both studies are characterized by a strong link of numerical studies and experimental verification.