The shifting of natural frequencies due to porous treatment.

I created this animation to illustrate the back-and-forth movement of air molecules when a source with specific directivity emits sound. The area on the right, delineated by a vertical line, represents porous material.

Sound waves penetrate the porous material with varying ease, depending on its air flow resistivity value. This penetration allows for multiple viscous interactions. Additionally, the characteristic impedance and wavenumber of the sound wave are complex, resulting in a phase change.

This phenomenon has interesting consequences, such as causing a room to behave as though it were larger than its actual size, as its natural frequencies are shifted downward.