Chapter 14 - Sound - Learning Path Questions and Exercises - Exercises - Page 524: 21

a). $(4)$ b). increase 1.4 times.

a). Intensity is inversely proportional to square of distance. So, $\frac{I_{1}}{I_{2}}=\frac{R_{2}^{2}}{R_{1}^{2}}$ $\frac{I_{1}}{I_{2}}=\frac{(3R)^{2}}{R^{2}}=9$ Thus, $I_{2}=\frac{1}{9}I_{1}$ So, (4) is the answer. b). $\frac{I_{1}}{I_{2}}=\frac{R_{2}^{2}}{R_{1}^{2}}=2$ $R_{2}=\sqrt 2\times R_{1}$ So, the distance is increased 1.4 times.