In certain cases, a wave may be reflected back along the path it was originally traveling. Examples of such cases are a rope fixed at one end where a wave is initiated at the free end or the air in a tube closed at the other end being excited by a noise at the open end. In these cases, the reflected wave interferes with the original wave. At certain frequencies, the wave itself appears to remain stationary while the amplitude changes. These waves are known as standing waves. For example, starting at its maximum, the amplitude at a given point appears to decrease, becomes 0, reachs its minimum, and increases again until it reaches its maximum. Points where the amplitude always appears to be 0, i.e. where the amplitude does not change, are called nodes. Halfway between nodes are points called antinodes. The maximum and minimum amplitudes are achieved at the antinodes.
