Two speakers spaced a distance 1.5 m apart emit coherent sound waves at a frequency of 680

Question:

Two speakers spaced a distance 1.5 m apart emit coherent sound waves at a frequency of 680 Hz in all directions. The waves start out in phase with each other. A listener walks in a circle of radius greater than 1 m centered on the midpoint of the two speakers. At how many points does the listener observe destructive interference? The listener and the speakers are all in the same horizontal plane and the speed of sound is 340 m/s. Experiments like this must be done in a special room so that reflections are negligible.