To improve the noise proof performance of a cover with apertures, the occurrence of the inner acoustic mode at lower frequencies must be prevented. To do this, the authors propose an interference wave inducing aperture. To overcome the noise propagation from the outer source to inside the cover that has an aperture on the top end, the location and the size of another aperture (interference wave inducing aperture) were optimized by a calculation using boundary element method and validated by experimental measurement. The calculation results show that the first inner acoustic mode to be prevented is the second in the height direction when the noise source is set at the outer cover and the sound pressure evaluation point is set at the cover center. In this mode, the point at the cover center along the height direction has a reverse phase of the location at the original aperture, so the interference wave inducing aperture seems to be set at the cover center along the height direction. The location and the size of the interference wave inducing aperture were optimized by analysis. The results show that if the interference wave inducing aperture is set at the center of the cover along the height direction, both the second inner acoustic mode in the height direction and the second in the width direction are the most prevented. The reduction effect is approximately 9 dB at the peak level. The effect of the interference wave inducing aperture that is caused by these calculations was validated by an experimental measurement. The results show that the effect is approximately 11dB at peak level reduction and approximately 4 dB overall between 220 Hz and 600 Hz.