End correction

In acoustics, end correction is a short distance applied or added to the actual length of a resonance pipe, in order to calculate the precise resonance frequency of the pipe.

A simple notion is that the fundamental resonance of a pipe occurs when the resonator length is half or a quarter of the sound wavelength. It is however well recognized that the practical frequency comes out lower than this, you have to apply an end correction, the pipe appears to be acoustically somewhat longer than its physical length.