Active Acoustic Treatment (AAT) is proposed as a method to absorb noise propagating in a duct with the objective of reducing the total noise power radiated from the duct exit. The effectiveness of AAT concept in attenuating both planar and higher-order acoustic duct modes is investigated experimentally in a rectangular duct. Decentralized feedback control is proposed as the control solution and is shown to perform satisfactorily. The performance of AAT is measured using 1) acoustic mode decomposition with microphone arrays located in the duct, upstream and downstream of the AAT sheet, and 2) a single microphone located external to the duct exit. Experimental results are included to demonstrate the attenuation of the total acoustic power propagating in the duct, and the reduction of the noise radiated from the duct.