Solve the all parts of given question:
Question: Two loudspeakers are placed on a stage 5.4 m apart and are radiating the same frequency, in phase. An observer walks along a line parallel to the loudspeakers and 12.7 m from them. Starting from the central point of the line (equidistant from the 2 loudspeakers) he observes that as he walks, the sound intensity first decreases and then increase to a maximum again when he has travelled 1.4 m.
Part A: Explain this observation.
Part B: Calculate the frequency of the sound, given that the speed of sound is 343 m/s.
Please show complete calculation thanks!