My suspicion was right about the phase of some transducers being out of phase. 

To measure the output phase, a senspr is placed at a fixed distance from the sensor. It so happened he most easily reproduced distance is to have the two metal cans touching. 

The following scope traces shows the two phases.

Turns out 3 of the 7 transducers had reversed phases. A quick solder job later they are all in phase. Time to check in sensor phases.