On the excitation mechanism in reed wind instruments

Abstract

The production of self-sustained oscillations in reed woodwinds in based mainly on two physical mechanisms: the coupling mechanism—which makes use of the reflections coming from bore discontinuities—and the Bernoulli mechanism—which consists of a Venturi effect acting on the reed. As far as woodwinds with striking inward reeds are concerned, the only widely studied instrument has been the clarinet—a roundly cylindrical bore driven by a single reed—in which the coupling mechanism plays an essential role while the Bernoulli mechanism can be neglected in a first approach. However, little attention has been paid in the literature to the wider set of double-reed conical woodwinds (oboe, bassoon, shawms, etc.), in which sound production is based on the cooperation between both coupling and Bernoulli mechanisms. In this paper, both mechanisms are assessed and a comparative analysis is made of cylindrical and conical bores' input impulse responses, whose shape and strength are related to the intensity of the coupling mechanism. Due to the low intensity of the impulse response reflections in conical bores, the coupling mechanism in this case behaves mainly as a trigger of the intense Bernoulli mechanism, which is the one that strongly acts on the reed. Experimental results are also presented showing the bore input pressure for several instruments when producing a note attack, and illustrating the collaboration between both mechanisms.