Halliday, Resnick & Walker — Waves-II: Problem 49

49 Figure 17-31 shows two isotropic point sources of sound, $S_1$ and $S_2$. The sources emit waves in phase at wavelength $0.50 \text{ m}$; they are separated by $D = 2.20 \text{ m}$. If we move a sound detector along a large circle centered at the midpoint between the sources, at how many points do waves arrive at the detector (a) exactly in phase and (b) exactly out of phase?