Not over yet
Sometimes I just can't get things out of my head like this problem with the minimas in sound intensity around the room, they are definitely there, I heard them. They are about 20 or 30cm apart so if you move your head from side to side you hear a couple but according to the theory they shouldn't be there or at least if they are (with the lower notes) they should be further apart. What I normally do at times like this is look for a simulation so I can see what's going on. Paul Falstad's ripple tank is a good place to start. So I set up two sources about 1/2 a wavelength apart and got the result on the left, a bit of cancellation in the line of the sources but not complete, certainly no minima and maxima.
I tried adding some walls to see if I'd get any cancellation but all got was this. Hmm, maybe there are some areas. I think I'm going to have to try it again in the class room.