Sound with a 41.1 cm wavelength travels rightward from a sou…

Sound with a 41.1 cm wavelength travels rightward from a source and through a tube that consists of a straight 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 r (in cm) that results in an intensity minimum at the detector? (Take the speed of sound to be 343 m/s.)