$\mathrm{dB}=10 \log \frac{I}{\mathrm{I}_{0}}$ ($I$ is intensity of sound, $I_0$ is reference intensity of sound)

$120=10 \log \left(\frac{I}{I_{0}}\right)$

$\Rightarrow I=1 \mathrm{W} / \mathrm{m}^{2}$

Also, $\mathrm{I}=\frac{\mathrm{P}}{4 \pi \mathrm{r}^{2}}=\frac{2}{4 \pi \mathrm{r}^{2}}$

$\therefore r=\sqrt{\frac{2}{4 \pi}}=\sqrt{\frac{I}{2 \pi}} \mathrm{m}=0.399 \mathrm{m}$

$\approx 40 \mathrm{cm}$