Glass break placement- can it be too close?

Is it possible to place the glass break too close to the windows monitored? I have a child's room with heavy black out curtains where I placed the sensor within the window frame inside the curtains. Alternately I could move it about 5 inches away on a wall 45 degrees adjacent to the window outside the curtains. I also have a sensor on a rarely used bathroom door that is in the door frame about 2 inches from the glass. It seemed to me if they work on sound so the closer the better- or is there a reason I shouldn't do this? (They obviously easily pass the clap test!) Thanks for your input!