The efficiency of proposed multi - layered waveguide-below-cutoff array (WBCA) is demonstrated with respect to air flow and shielding effectiveness (SE) in duct. By comparing to the conventional unit rectangular WBCA, the entrance and exit air velocities of the multi-layered WBCA are improved at most 11 % and 32 %, respectively, when the inlet velocity of duct is 1 m/s. Also, it is confirmed that the SE increases from 70 dB to 125 dB in the increase with the number of layers from 2 to 64 at 1 GHz, which is similar to the unit rectangular WBCA. Therefore, such a multi-layered WBCA can improve the air flow while it compensates for the SE by increasing the number of layers. In addition, the result of proposed modified SE equation well corresponds to the full-wave simulation results.