806 where the flexuous cord 110 is helically wound around the duct 102 at the determined pitch 112 this may be performed by a method similar to that described above in regard to fig7 as further described above the noise attenuation characteristics of the noise reducing duct 102 depend upon the pitch 112 of the helical winding of the flexuous cord 110 as well as the material from which the cord 110 is constructed if it is determined at operation 808 that the noise reducing duct 102 wound with a flexuous cord 110 at the determined optimal pitch 112 does not adequately dampen noise across the entire target range of acoustic frequencies then the routine 800 continues to operation 810 at operation 810 an additional helically wound flexuous cord 110 or an additional duct segment is added to the noise reducing duct 102 targeting the portion of the range of acoustic frequencies not adequately dampened by the first flexuous cord 110 or duct segment from operation 810 the routine 800 returns to operation 804 where an optimum pitch for the helical winding for the additional flexuous cord 110 or duct segment is determined according to the portion of the target range of frequencies once it is determined at operation 808 that sufficient noise reduction is achieved across the entire target range by the combination of flexuous cord s 110 and duct segment s the routine 800 ends based on the foregoing it.