A system with multiple freedom of movement exhibits different modes in which each part of the system will move in the same frequency. By proper choosing the initial condition, these different modes can add up into a superposition state and make the movement of the system show phenomenon of beat. Both of modes and beat phenomena are crucial to the understanding of oscillating and periodical properties of the system. This design of experiment clearly demonstrates these phenomena by using two pendulums coupled through an elastic band. With digital sensors and data collectors, the detail movement of the system are recorded by computer software which is available for further analysis by students. The coupling strength can be easily adjusted to show the dependence of beat on it.