Two loudspeakers are placed side by side at a distance d = 4.00 m apart. A listener observes maximum constructive interference while standing in front of the loudspeakers, equidistant from both of them. The distance from the listener to the point halfway between the speakers is l = 5.00 m.

One of the loudspeakers is then moved directly away from the other. Once the speaker is moved at a distance r = 60.0 cm from its original position, the listener, who is not moving, observes destructive interference for the first time.

Find the speed of sound v in the air if both speakers emit a tone of frequency 700 Hz.

Required

What is the phase difference Δϕ0 between the sound waves reaching the listener's ear initially, before the upper speaker is moved?