Sound source A is located at x = 0, y = 0, and sound source B is placed at x = 0, y = 2.4 m. The two sources radiate coherently in phase. An observer at x = 40 m, y = 0 notes that as she walks along in either the positive or negative y direction away from y = 0, the sound intensity diminishes. What is the lowest and the next higher frequency of the sources that can account for that observation?