In Figure S1 and S2 are two isotropic point sources of sound. They emit waves in phase

Question:

In Figure S1 and S2 are two isotropic point sources of sound. They emit waves in phase at wavelength 0.50 m; they are separated by D = 1.60 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?