A study of the wall vibrations excited during the playing of lip-reed instruments

Abstract

The question of whether vibrations induced in the wall of a played instrument affect the tone produced has been the subject of numerous studies. While instrument manufacturers and musicians regularly claim the ability to distinguish between wind instruments of different materials, researchers today still debate the legitimacy of this claim and the mechanism by which it could occur. In this thesis, a series of experiments is presented in which a simplified wind instrument, consisting of a trombone mouthpiece coupled to a section of brass pipe, is blown using an artificial mouth and the induced wall vibrations measured using a Laser Doppler Vibrometer. The brass pipe’s natural resonance frequencies and bending mode shapes are established and compared with the velocity amplitude variation along the pipe induced by an artificially blown note. The induced wall vibrations are shown to occur at the harmonics of the played note and to match the shapes of the bending modes of the pipe at those frequencies. Adapting this technique, the excitation mechanism by which the induced wall vibrations are set in motion is investigated by decoupling the section of brass pipe, in turn, from the air column and the mouthpiece. Results indicate that it is the motion of the lips against the mouthpiece, rather than air column pressure changes within the pipe, that is the dominant mechanism in exciting wall resonances. Five pipes of identical dimensions but manufactured from different metals and three pipes of the same brass alloy but drawn to a differing wall thickness are then tested in a series of comparison measurements. The pipes are measured under identical conditions and their structural and induced vibrational velocity measurements compared. Results reveal that the magnitude of the induced wall vibration depends on the material from which the instrument is made and its wall thickness.