Paper detail

A proposal for a minimal model of free reed

In this paper we propose a minimal model for free reeds taking into account the significant phenomena. This free reed model may be used to build models of free reed instruments which permit numerical simulations. Several definitions for the section by which the airflow passes through the reed are reviewed and a new one is proposed which takes into account the entire escape area under the reed and the reed thickness. To derive this section, it is necessary to distinguish the neutral section (the only section of the reed which always keeps its length constant while moving) from the upstream or downstream sections. A minimal configuration is chosen to permit the instabilities of both (-,+) and (+,-) reeds on the basis of a linear analysis of instabilities conditions. This configuration is used to illustrate, with temporal simulations, the minimal model for both kinds of reeds and to discuss the model assumptions. Some clues are given about the influence, on the playing frequency and on the dynamic of the sound, of two main parameters of the geometrical model: the size of the volume and the level of the excitation. It is shown that the playing frequency of a (+,-) reed can vary in a large range according to the size of the volume upstream of the reed; that the playing frequency is nearly independent of the excitation but that the dynamic of the sound increases with the excitation level. Some clues are also proposed to determine the nature of the bifurcation for free reeds: it seems that free reeds may present inverse bifurcations. The influence of the reed thickness is also studied for configurations where the reed length or the reed width vary to keep the mass constant. This study shows that the reed thickness can have a great influence on the sound magnitude, the playing frequency and the magnitude of the reed displacement which justifies its introduction in the reed model.This article has been published in Acta Acustica united with Acustica, Vol. 93 (2007), p. 122-144.