Experimental investigation of acoustic characteristics of radiation and playing gestures for lip-excited musical instruments
The geometrical characteristics of acoustical radiation are of great importance in instrument design and synthesis, and multiple simplified models have been developed in the past to describe them. In this work two experimental methodologies are proposed and carried out, studying the frequency-dependent radiation in a collection of popular brass instruments with different grades of flaring, and making use of the axis-symmetry of these instruments. The first method uses a scanning linear array and is carefully designed to extract the linear properties of the radiation field. The results of this experimental method are a database of impulse responses distributed in space, and effectively covering a bidimensional on-axis section of the radiation field approximately 0.6 m by 0.9 m. These data can then be used for the validation of a number of simplified physical models used to describe the radiation of these types of instruments. The second method aims at visualising radiation for high amplitude excitation, where shock waves are generated inside the instrument due to non-linear propagation of the plane wave. In this case, the experimental methodology used, taking advantage of the strong density and temperature gradients generated in the air, is an on-axis schlieren optical system. General results of this visualisation show a strong increase in focused directivity at high frequencies and loud playing dynamics, due to the spectral enrichment typical of this family of instruments. The second section of this thesis focuses on the study of playing gestures in the trombone, and could also be applicable to other slide instruments. During glissando playing in the trombone the length of the cylindrical slide section within the bore is altered while waves are propagating. Slide velocities of 2 metres per second are not unusual and result in a (small but measurable) Doppler shift in the wave coming from the mouthpiece before it arrives at the bell. An additional effect is observed in terms of the volume of air within the instrument changing telescopically, leading to a localised change in DC pressure and a resulting flow, which generates infrasound components within the bore. The effects of these playing gestures are investigated in two different setups; one with a high frequency sinusoidal excitation generated by a compression driver, and another one using an artificial mouth to play the instrument. In both experiments the pressures at the mouth or mouthpiece, water key and bell were tracked using microphones and the position of the slide was tracked using a laser distance sensor. Both Doppler shifting and infrasound components were detected for both experimental setups, although the effect on a soft termination such as the artificial lips requires further examination.