A hybrid method that combines the calculation of unsteady flow by large eddy simulation (LES) with the evaluation of acoustical performance by a compact Green's function has been proposed to provide detailed information on the flow vortices that contribute to aeroacoustic sound generation. This paper numerically verifies the noise reduction mechanism of pantograph horns with periodic holes and proves that the intermittent flow through holes suppresses the strength of dipole sound sources at the shear layer and collapses the large two-dimensional structure of sound sources in the spanwise direction.