Week 12 Problem 3

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A riverside warehouse has several small doors facing the river. Two of these doors are open as shown in the figure below. The walls of the warehouse are lined with sound-absorbing material. Two people stand at a distance L = m from the wall with the open
doors. Person A stands along a line passing through the midpoint between the open doors, and person B stands a distance y = m to his side. A boat on the r iver sounds its horn. To person A, the sound is loud and clear. To person B, the sound is barely audible.
The princi pal wavelength of the sound waves is m . Assuming person B is at the position of the first minimum, determine the distanced between the doors, center to center.

[yjsong]

Could you elaborate why it would be 40m instead of 20m?