A listener is standing in front of two speakers that are producing sound of the same frequency and amplitude, except that they are vibrating out of phase. Initially, the distance among the listener and each speaker is the same (see the drawing). As the listener moves sideways, the sound intensity gradually increases. When the distance x in the drawing is 0.98 m, the change reaches the maximum amount (from soft to loud). Using the data shown in the drawing and 343 m/s for the speed of sound, decide the frequency of the sound coming from the speakers.