Two coherent sources of sound,  S1 and  S2, produce sound waves of the same wavelength,  λ=1m, in phase.  S1 and S2  are placed 1.5 m apart (see fig). A listener, located at L, directly in front of  S2 finds that the intensity is at a minimum when he is 2 m away from  S2. The listener moves away from  S1, keeping his distance from  S2 fixed. The adjacent maximum of intensity is observed when the listener is at a distance d from  S1. Then, d is :