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 (the distance between the speakers is 2.99m). Initially, the distance between the listener and each speaker is the same: 4.23m. As the listener moves sideways, the sound intensity gradually increases. When the distance x is 0.91 m, the change reaches the maximum amount (from soft to loud). Using the data given and 343 m/s for the speed of sound, determine the frequency of the sound coming from the speakers.