To investigate the noise reduction mechanism of plenum windows, the paper experimentally studied the influence of the air-gap spacing between two glass panes, the panel thickness, and the opening size on the sound transmission loss (TL). The results show that when the half wavelength of impinged sound is smaller than the air-gap spacing, the TL can be improved by the decrement of the opening sizes and the increment of the glass thickness but varies little with changes in air-gap spacing. When the half wavelength of impinged sound is larger, the modification of the three factors makes little impact on the TL.