Two metronomes on a moving tray synchronize their beat

This simple table-top experiment illustrates how mechanical synchronization works. Two metronomes get in tune and beat in a mirror symmetric fashion. Initially, there is a phase difference between both metronomes (left graph), which causes the tray to oscillate (right plot). The rocking motion of the tray imparts small forces on the metronome needles, which subtly change their speed.

Movie credit: Hernan Andres Morales Navarrete

[blakebullwinkel]

How did you monitor the phase difference?