Two coherent sources of sound, $$S_1$$ and $$S_2$$, produce sound waves of the same wavelength $$\lambda = 1\,\text{m}$$ are in phase. $$S_1$$ and $$S_2$$ are placed $$1.5\,\text{m}$$ 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 $$2\,\text{m}$$ away from $$S_2$$. The listener moves away from $$S_1$$, keeping the 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: