In 1666 the Dutch physicist Christian Huygens, the inventor of the pendulum clock discovered the principal of 'entrainment' when he noticed that two pendulum clocks mounted on a board had begun to swing with the same rhythm.
Entrainment is found in all areas of science and in the natural world. Entrainment is one of the main principals behind the effectiveness of sound healing.
http://www.simonheather.co.uk/pages/articles/entrainment.pdf
