2 at its aft part downstream from the fan 1 not shown thus the duct at its aft end part downstream from the constricting part 7 is of reduced diameter relative to the forward inlet part therefore the air flow velocity becomes large in this reduced diameter part whereby the shear flow layer becomes thinner at the same time the air flow is straightened and the generation of vortexes 6 is suppressed accordingly noise within the duct 2 due to the vortexes 6 is reduced another feature of this invention is the use of a fan with wide chord blades as shown in fig2 and 3 the chord of each wide chord blade 8 increases from root to tip in contrast the conventional blade 9 has a constant chord as indicated by the two dot chain line or has a chord which decreases from root to tip for the same rotational speed of the fan the quantity of work output of the wide chord blade 8 is larger than that of the conventional blade 9 and accordingly the thrust produced by the wide chord blade 8 is greater than that produced by the conventional blade 9 therefore with respect to the same thrust the fan rotating speed of the wide chord blade 8 is slower than that of the conventional blade 9 the predominant factor of the power level of noise is the factor which is proportional to the 6th power of the tip speed of the blades as indicated by the following equation accordingly reducing the fan rotating speed has the effect of reducing the noise generating therefrom equ1 where yn o is specific power level 68 u00b1 3 db referring to fig4 and 6 the duct 2 encompassing the fan 1 constitutes a sound absorbing structure 13 which.