Walker, Halliday & Resnick — Diffraction: Problem 26

26 Sound waves with frequency 2500 Hz and speed 343 m/s diffract through the rectangular opening of a speaker cabinet and into a large auditorium of length d = 100 m. The opening, which has a horizontal width of 30.0 cm, faces a wall 100 m away (Fig. 36-35). Along that wall, how far from the central axis will a listener be at the first diffraction minimum and thus have difficulty hearing the sound? (Neglect reflections.)