Range 40m overestimated

The sensors communicate with each other and ring simultaneously when test on one of them (I have ordered 2 pairs). However, the scope of 40 meters or so is overestimated measured in free field, that is to say, purely theoretical. I wanted to put the detector in a fourth external dependence, which placed 25 meters from two other sensors: the fourth detector does not communicate with others. We must move forward about ten meters they communicate.
We'll see over time but from the outset I withdraw 2 stars for the staff, which was an important criterion for my choice.