Harmonics is the generalized term used to describe the distortion of a sinusoidal waveform by waveforms of different frequencies. A closed pipe can have only odd-numbered harmonics. That’s because closed pipes or tubes have a node at one end and an antinode at the other. As shown in the diagram, a closed tube or pipe is one where one end is open and the other is closed, and like open pipes, these can form a standing wave with the sound of an appropriate frequency. In a closed pipe harmonic, there can be a standing wave whenever the wavelength allows an antidote at the open end of the pipe.