Influence of wall vibration on the sound of wind instrumentsJean-Pierre Dalmont
Wall vibrations of a musical wind instrument can be clearly noticed and measured in playing configuration. However, the problem of quantifying its effect on the emitted sound remains a subject of debate. Wall vibrations can be generated by two mechanisms: first mechanically by the impacts of a reed or musicians lips on the mouthpiece and second acoustically by the sound field inside the instrument. In this paper, we present an investigation of the second mechanism using an experimental approach and a theoretical model of a generic simplified instrument i.e. a cylindrical vibrating shell, with a slightly distorted circular cross section. Analysis leads to the conclusion that in most situations vibroacoustic couplings are small and do not induce any audible contribution. However, the wall vibration can play a significant role for some particular choices of material and geometry, which lead to coincidences between structural and acoustical modes. In these configurations, model and experiments show that the input acoustic impedance is significantly perturbed by wall vibrations. Using a blowing machine, it is shown that these perturbations can induce changes in timber or even unstable oscillations. The vibrations of the pipe also radiate directly outside which may induce an audible contribution to the sound field. Experiments on a trombone bell suggest that this effect is very limited and probably not audible in most situations.