Two coherent sources of sound,  $S_1$ and  $S_2$, produce sound waves of the same wavelength,  $\lambda =1$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  $S_2$ finds that the intensity is at a minimum when he is 2 m away from  $S_2$. The listener moves away from  $S_1$, 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 :