Suppose that the separation between two speakers A and B is 4.40 m and the speakers are vibrating in phase. They are playing identical 148-Hz tones and the speed of sound is 343 m/s. An observer is seated at a position directly facing speaker B in such a way that his line of sight extending to B is perpendicular to the imaginary line between A and B. What is the largest possible distance between speaker B and the observer, such that he observes destructive interference?