A point source is emitting sound waves of intensity 16 × 10 ^ - 8 W...

Intensity for a point source varies as $I ∝ \dfrac{1}{{r}^{2}}$.

Intensity at a distance of $2m$, ${I}_{2} = 16 \times {10}^{- 8} \times \dfrac{1}{{2}^{2}} = 4 \times {10}^{- 8}W{m}^{- 2}$

Similarly, intensity at a distance of $4m$, ${I}_{4} = 16 \times {10}^{- 8} \times \dfrac{1}{{4}^{2}} = 1 \times {10}^{- 8}W{m}^{- 2}$

Required difference, ${I}_{2} - {I}_{4} = 3 \times {10}^{- 8}W{m}^{- 2}$