Find Min Radius for Sound Wave in Tube

[OopsIDied]

Homework Statement

"A sound wave of 40.0 cm wavelength enters the tube shown below at the source end. What must be the smallest radius r such that a minimum will be heard at the detector end?" (figure attached)

Homework Equations

v =$$\lambda$$$$f$$

The Attempt at a Solution

I don't have any idea how to do this problem. My first idea was to use the formula for the wavelength of a closed or open tube, but whether the tube is closed or open isn't given so I'm guessing that's not right. Also, I don't see how the radius of the half circle has anything to do with the tube through which the sound is traveling.

Thanks in advance

 

[ehild]

The sound travels along both the straight tube and the circular one. The sound wave enters in both tubes with the same phase, but the lengths the waves travel along the different segments will be different. What is the minimum difference between the lengths traveled so as destructive interference occur at the other end of the circular tube?

ehild