Two speakers are driven in phase by a common oscillator at 800 Hz and face each other at a distance of 1.25 m. Locate the points along a line joining the two speakers where relative minima of sound pressure amplitude would be expected (Use v = 343 m/s). This problem ended up screwing my mind until I found out that the waves are supposed to be, as the problem said, in phase. I was able to get the answer, which are .518 m, .303 m, .0891 m, .732 m, .947 m, and 1.16 m from either speaker. However, in the solutions book, the way they did it was by stating that the point halfway between the speakers (.625 m) is an antinode of pressure, and since the distance between adjacent nodes is .214 m and adjacent node-antinodes is .214/2 m, they used .625 - .214/2 = .518 m to find a pressure node. Then they added/subtracted .214 m to get the other values. The statement about the midpoint being a pressure antinode began to screw my mind. They didn't prove anything for it, they stated it as seemingly a postulation out of nowhere...how do they know the midpoint is an antinode?