Two coherent sources of sound, $S_1$ and $S_2$, produce sound waves of the same wavelength, $\lambda=1 \mathrm{~m}$, in phase. $S_1$ and $S_2$ are placed 1.5 m apart (see fig). A listener, located at L , directly in front of $\mathrm{S}_2$ finds that the intensity is at a minimum when he is 2 m away from $\mathrm{S}_2$. The listener moves away from $\mathrm{S}_1$, keeping his distance from $S_2$ fixed. The adjacent maximum of intensity is observed when the listener is at a distance $d$ from $S_1$. Then, $d$ is