The rectangular absorptive attenuators are specifically designed to reduce ductborne noise generated by axial fans or centrifugal fans in ventilation systems, air intakes of high speed compressors and gas turbines. The primary advantage of the attenuators is the effective low frequency attenuation which can be obtained. The units consist of a series of parallel acoustic baffles splitting the airstream into several passages, and thus allowing a larger area of absorption. Combinations of baffle thicknesses, length and passage area provide several possibilities to attenuate specifically desired frequencies.