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An acoustic pipe, such as an organ pipe, marimba, or flute resonates at a specific pitch or frequency. Longer pipes resonate at lower frequencies, producing lower-pitched sounds. The details of acoustic resonance are taught in many elementary physics classes. In an ideal tube, the wavelength of the sound produced is directly proportional to the length of the tube. A tube which is open at one end and closed at the other produces sound with a wavelength equal to four times the length of the tube. A tube which is open at both ends produces sound whose wavelength is just twice the length of the tube. Thus, when a Boomwhacker with two open ends is capped at one end, the pitch produced by the tube goes down by one octave.
The analysis above applies only to an ideal tube, of zero diameter. When designing an organ or Boomwhacker, the diameter of the tube must be taken into account. 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. The pitch of a real tube is lower than the pitch predicted by the simple theory. A finite diameter pipe appears to be acoustically somewhat longer than its physical length.
A theoretical basis for computation of the end correction is the radiation acoustic impedance of a circular piston. This impedance represents the ratio of acoustic pressure at the piston, divided by the flow rate induced by it. The air speed is typically assumed to be uniform across the tube end. This is a good approximation, but not exactly true in reality, since air viscosity reduces the flow rate in the boundary layer very close to the tube surface. Thus, air column inside the tube is loaded by the external fluid due to sound energy radiation. This requires an additional length to be added to the regular length for calculating the natural frequency of the pipe system.
It is denoted by and sometimes by . In organ pipes, an antinode is not formed exactly at the open end. Rather, antinode is formed a little distance away from open end outside it.
This is known as end correction, which can be calculated as,
- for a closed pipe (with one opening):
- and for an open pipe (with two openings):
There is no scientifically proven and accepted value for the end correction of a resonant tube, various values ranging from 0.3r to 0.6r have been suggested from numerous disparate experiments. Lord Rayleigh was the first experimenter to publish a figure, in 1871: it was 0.3r.
- End Correction at a Flue Pipe Mouth
- Ruiz, Michael J. "Boomwhackers and end-pipe corrections." The Physics Teacher 52.2 (2014): 73-75. Available online at http://www.mjtruiz.com/publications/2014_boomwhackers.pdf