Active muffler

In the active muffler disclosed herein, a porous flow resistive member defines a wall of a duct carrying a noisy flow, e.g., a jet exhaust. The flow resistive member is characterized by having a flow resistance substantially less than rho C, the characteristic impedance of the acoustic medium. Loudspeakers in a chamber on the side of said porous member opposite the duct are actuated to reduce acoustic pressure in the region between the porous member and the loudspeaker, thereby increasing the acoustic flow through the porous member which in turn increases the dissipation of acoustic energy in the duct. The apparatus is particularly effective at low frequencies where a passive dissipative muffler would be of impractical size.