Pitch and level changes in organ pipes due to wall resonances

The resonating air column in a thin-walled metal organ pipe was observed to interact with a wall resonance. Effects became audible when a wall resonance frequency was nearly the same as that of the air column. Level changes of 6 dB and frequency shifts of 20 cents were found. Instabilities similar to the wolf-tone on string instruments could occur: the air column oscillations switched between closely spaced frequencies. It was observed that the cross-section of the organ pipe was slightly elliptical and could deform under influence of the pressure inside the tube. Based on this mechanism, a phenomenological model is proposed. It qualitatively explains the observed changes in resonance behaviour of the air column. It allows identification and verification of the parameters governing the interaction. The results suggest that similar effects might occur in other wind instruments such as saxophones, bassoons and trumpets.