Scientists may have finally discovered why pendulum clocks synchronise when placed nearby. After 350 years two Portuguese researchers believe that pendulum clocks eventually swing together because of sound pulses that travel through the wall from clock to clock.

Henrique Oliveira, a mathematician at the University of Lisbon and co-author of the new study published in the Scientific Report, staged an experiment with study co-author and Lisbon university physicist Luis Melo.

In it, the pair placed two clocks on an immobile wall – opposed to the movable beam as had been tested previously – and examined how they interacted. What they discovered was that the speed of the clocks synchronisation coincided with the cycles of the recorded sound pulses.

“We could verify that the energy transfer is through a sound pulse,” Melo told the Guardian.

Not only does their discovery solve a problem scientists have puzzled over since Huygens – who is credited with creating the first pendulum clock – noticed it in 1665, the research has also pushed forward their understand of other types of oscillators.

Mr Oliveira told LiveScience that the idea came to the two over coffee, because “nobody had ever tested properly the idea of clock hanging on the same wall”.