Wave Optics Part 1

Two coherent sources of sound, S1 and S2, produce sound waves of the same wavelength, λ=1m, in phase. S1 and S2 are placed 1.5m apart (see fig.) A listener, located at L, directly in front of S2 finds that the intensity is at a minimum when he is 2m 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 :