From SKYbrary Wiki
A refinement of the propeller synchroniser is the propeller synchrophaser. Like the synchroniser, the synchrophaser precisely matches the RPM of all of the propellers. However, the synchrophaser also compares the propeller to propeller position of the propeller blades within their individual arcs and then adjusts them to their optimum relative positions. This refined adjustment results in a significant further reduction in propeller noise and vibration as compared to those achieved by the more basic synchronisation system.