A new hybrid passive/active noise absorption system

A new hybrid passive/active system is developed for sound absorption over a wide frequency range. The system is comprised of a layer of absorbing material positioned at a distance from an active wall, leaving an air space. The motion of the active wall is based on a new control approach which consists,of the minimization of the reflected wave within the airspace which modifies the layer's back surface impedance so as to match the characteristic impedance of air. This technique is referred here as inducing an impedance-matching condition. Both numerical and experimental results of such a system are presented for normally incident planar waves. The hybrid passive/active system results in a high absorption coefficient of .8-1.0 over the frequency range 100-2000 Hz and is insensitive to system parameters such as air space depth and absorbing layer thickness. (C) 1997 Acoustical Society of America.