Chapter 14 - Sound - Learning Path Questions and Exercises - Exercises - Page 524: 28

$1.1\times10^{-5} W/m^{2}$

$beta=10log\frac{I}{I_{0}}$ $I=I_{0}10^{\frac{beta}{10}}$ For intensity level 60dB, $I=I_{0}10^{\frac{60}{10}}=10^{-12}\times10^{6}=10^{-6}$ For intensity level 70dB, $I=I_{0}10^{\frac{70}{10}}=10^{-12}\times10^{7}=10^{-5}$ So, combined intensity =$10^{-6}+10^{-5}=1.1\times10^{-5} W/m^{2}$