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Physical model of the sound generation in flue organ pipes

Final Activity Report Summary - PIPESOUND (Physical Model of the Sound Generation in Flue Organ Pipes)

In a flue organ pipe, sound is generated as self-excitation. A feedback loop is made by the following processes:
1) an air jet coming out from a flue is oscillated by sound generated in the pipe;
2) a part of the flow with the jet enters the pipe and excites sound.

Although process 2) is well understood, understanding of process 1) remains in an elementary stage. Only hypothetical models with many conceptual approximations were proposed.

The main objective of this research is to understand process 1) better. To this end, computational fluid dynamical (CFD) simulation of a jet oscillated by sound was first carried out. From the results of the simulation, the amplitude and phase of the jet oscillation were then modelled empirically. To evaluate the model of the jet oscillation, flue pipe organ sound was synthesised in several different conditions using this model in physical modelling technique and was compared with sound generated in an actual flue organ pipe.

Using a new jet oscillation model developed in this project and physical modelling sound synthesis, the following behaviours were simulated in satisfactory manners:

A small change in the flue geometry greatly affects spectrum of the generated sound. This effect was reproduced correctly in the sound synthesis.

When the air pressure supplied to a flue pipe is changed, various oscillation regimes in the first, second and third resonance modes appear. The number of the regimes, sound frequency and the pressure range for each regime were correctly simulated.