Development of a frequency-domain filtered-x intensity ANC algorithm

A theoretical model for plane wave active intensity control in a finite duc t is developed and validated experimentally using a frequency domain adapti ve filtered-x control algorithm with a cross-spectrum active intensity erro r measurement. A square duct terminated by a primary acoustic source at one end and open termination to radiate into a free space at the other end is used for plane wave propagation intensity studies. A secondary active contr ol source is positioned nearly midway between the primary source and open t ermination. Results from the model and experiment show that for optimal sou nd cancellation out the open duct end, one has zero intensity everywhere in the duct. However, also predicted by the model and confirmed experimentall y, is that moving the error intensity sensor to a position between the prim ary and secondary source leads to zero intensity between the sources, but a lso radiation out the open end for frequencies where the intensity sensor p osition is not a half-wavelength multiple distance from the secondary sourc e. The duct intensity model and experimental results confirm that for optim al active noise cancellation both sources convert no mechanical energy to a coustical power. Intensity analysis of active noise cancellation systems al so provides a basis for secondary source efficiency optimization through pr oper source positioning in the duct. (C) 1998 Elsevier Science Ltd. All rig hts reserved.