Two small speakers A and B are driven in step at 720Hz by the same audio oscilla

Question

Two small speakers A and B are driven in step at 720Hz by the same audio oscillator. These speakers both start out 4.50m from the listener, but speaker A is slowly moved away. (See the figure

At what distance d will the sound from the speakers first produce destructive interference at the location of the listener?

If A keeps moving, at what distance d will the speakers next produce destructive interference at the listener?

After A starts moving away, at what distance will the speakers first produce constructive interference at the listener?

Explanation / Answer

For destructive interference the distance should be (2n-1)?/2where n = 1,2,3 etc.
For the first destructive interference the distance d = ?/2

Part A -
?/2 =c/2n = 344.16 / 2*720 = 0.239m

Part B
3*0.236 = 0.717

Part C
The distance is ? = 2*0.239 = 0.478m

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