A point source is emitting sound waves of intensity $16 \times 10^{-8} \mathrm{~Wm}^{-2}$ at the origin. The difference in intensity (magnitude only) at two points located at a distances of $2 m$ and $4 m$ from the origin respectively will be _________ $\times 10^{-8} \mathrm{~Wm}^{-2}$.