11.3: Impedance

But why do sound waves reflect from the open end of a tube or when the size of tube changes abruptly? The resistance to the movement of a wave crossing a boundary from one medium into another is called impedance and occurs for waves on a string, sound waves in air and electronic signals in a circuit. When a wave tries to travel from a medium with one impedance to a region where the impedance is different, there will be a partial reflection.