Resonance is a special case of forced oscillation in a system, wherein the first object's driving frequency matches the second object's natural frequency, forcing the second object to vibrate at the same frequency with a significantly higher amplitude. Consider a tuning fork and a hollow cylindrical tube closed at one end. When the tines of the tuning fork vibrate at their natural frequency, a sound wave is generated, which impinges upon the entrance of the tube, forcing the air inside the tube to vibrate at the same frequency. If the tuning fork's natural frequency matches the air column's normal modes, resonance results in a louder sound. A similar thing happens for a tube open at both ends. The air columns in the tube have maximum air displacements at both ends, and thus, the reflections of the sound at each end produce a large-amplitude wave for particular frequencies. The resonant frequencies for a tube open at both ends follow the same equation as that of a standing wave on a string fixed at both ends.