High noise levels at low frequencies and no space for sound absorbing components - this is the dilemma of numerous air-conditioning and exhaust systems. Active resonance silencers represent an alternative noise control technique based on compact cassettes which contain several acoustic and electroacoustic elements, e.g. loudspeaker, microphone(s), amplifier. The system elements are covered by protective layers (fabric, plastic foils) against abrasive air flow and temperature. With at least one microphone very near to the loudspeaker membrane a control loop is built up where the measured microphone voltage is amplified and fed back to the loudspeaker. The loudspeaker (membrane mass) together with the rear volume of the box (air spring) form an acoustic resonator. In ducts the independently working active cassettes are assembled to splitter silencers. At air inlets a very short silencer of only one active cassette length is used in connection with a thin absorptive layer on the opposite gap wall. Furthermore, the active cassette can be modified by an acoustic waveguide between loudspeaker membrane and interface to the duct wall. The theoretical models of the activated mass-spring resonator and other coupled active systems will be presented and illustrated by practical results.
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