What Radius Produces Intensity Minimum?

[tg22542]

Homework Statement

In the figure, sound with a 38.0 cm wavelength travels rightward from a source and through a tube that consists of a straight portion and a semicircle. Part of the sound wave travels through the semicircle and then rejoins the wave that goes directly through the straight portion. This rejoining results in interference. What is the smallest radius r that produces an intensity minimum at the detector?

Also a picture: http://gyazo.com/68576b343c662325703528445328da23

Variables:

d (cm)
r (cm) - needed

Homework Equations

None (that I know of)

The Attempt at a Solution

I honestly have no idea how to solve this, I feel like it is very simple and I am over thinking it. I cannot find any relevant equations, because I feel like I don't have enough information to even need an equation.

Can anyone point me in a better direction here please?

 

[lightgrav]

an intensity minimum means that one path is ½ wavelength longer than the other path, so the 2 waves cancel.

 

[tg22542]

Nvm, figured it out.