Consider some physical body that can be set into motion by an impulse, and continue to vibrate for a period of time after the impulse before coming to rest. Examples of such bodies would be a tuning fork or a violin string. If these bodies have a preferred frequency of vibration, then a series of very weak impulses that is, impulses that are too weak to initiate vibration on their own will cause the body to vibrate provided that these impulses are delivered at the preferred frequency (Helmholtz & Ellis, 1954). Furthermore, the vibrations of one body can serve as the set of weak impulses that can set another nearby body into vibration. For example, if one strikes a tuning fork, its vibrations will cause vibrations in a nearby, similarly-tuned tuning fork. This latter phenomenon in which the vibrations in one object produce vibrations in another is called sympathetic resonance.
Sympathetic resonance is an example of physical entrainment, in which periodic behavior of one object can be communicated to another, even when there are no direct physical connections between the two. Christian Huygens was the first to observe entrainment when he noted in 1666 that two nearby pendulum clocks would eventually synchronize their swinging movement. In addition to sympathetic resonance being fundamental to account for musical harmonics, as well as for responses of the cochlea to auditory stimuli, it and entrainment in general is an important concept in accounting for the coordination of actions of groups of musicians (Borgo, 2005), and possibly for the evolution of social coordination and language in humans (Levitan, 2008).