Two speakers emit sound waves with frequency 4.71 kHz. They are driven by the same oscillator so that they are in phase with each other. I place the speakers side-by-side, and I stand across the room from them. If someone moves ones of the speakers toward me, I hear the total intensity drop and then rise again. How far (cm) did he move the speaker to the point where I heard a minimum intensity due to destructive interference of the sound waves from the two speakers? The speed of sound is 343 m/s.