Two coherent sources of sound, $S_1$ ${}_{}$ and $S_2$ ${}_{}$ produce source waves of the same wavelength, $\lambda =1\mathrm{m}$ $$ , in phase. $S_1$ ${}_{}$ and ${\mathrm{S}}_2$ ${}_{}$ are placed $1.5\mathrm{m}$ $$ apart (see fig). A listener located at $\mathrm{L}$ $$ , directly in front of $S_2$ ${}_{}$ finds that the intensity is at a minimum when he is $2\mathrm{m}$ $$ away from ${\mathrm{S}}_2$ ${}_{}$ . 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: