In the figure, sound with a 36.0 cm wavelength travels from a source and through a tube that consists of a str

aight portion and a half-circle. Part of the sound wave travels through the half-circle and then rejoins the rest of the wave, which goes directly through the straight portion. This rejoining results in interference. What is the smallest radius the source end. What must be the smallest radius r results in an intensity minimum at the detector?
Not really sure where to start I think maybe getting the 1/2 the circumference then doing something with an interference formula that makes it fully destructive, but just not sure how to go about it.

The path lengths have to differ by 1/2 wavelength or 18 cm. The straight portion cuts across the diameter, traveling 2*r, while the half-circle portion travels pi*r. Then (pi-2)*r = 18 cm.