The main feature of actively absorbing silencers is the continuous pre
ssure loss along the whole length of its local reacting lining. In con
trast to the principle of hybrid silencers where the wall impedance is
tried to be optimised the feedback resonators described herein decrea
se it. So especially a low frequency noise reduction in HVAC-duct syst
ems is attained. Its high efficiency is strongly coupled to the stabil
ity of the electro-acoustic control loop which depends on the paramete
rs of its elements as well as on the surrounding ductwork. By means of
a model consisting of lumped elements and wave impedances the electro
-acoustic behaviour could be determined and measures for the stabilisa
tion of the system could be developed. On a resulting exemplary silenc
er of 1 m length the theoretically predicted insertion loss was experi
mentally verified. Besides a peak of nearly 40 dB a broad band attenua
tion of minimal 10 dB in the range of 63 Hz up to 200 Hz was achieved.
Additionally the calculation and measurement of the reflection loss o
f about 3 dB underlines the difference to the well known conventional
active noise control with destructive interference. The research repor
ted herein was supported by the Volkswagen-Stiftung and was carried ou
t in co-operation with the Institute of Technical Acoustics and the In
stitute of Telecommunication at the TU Dresden.