Experimental investigation of Doppler shift and infrasound generation during wave propagation within the bore of the Trombone during slide movement

During glissando playing in the trombone the length of the approximately cylindrical slide section within the bore is altered while waves are propagating. Slide movements of 2 metres per second are not unusual. The simplest way to visualise the effect is in terms of the slide being represented by a moving reflector, resulting in a (small but measurable) Doppler shift in the wave coming from the mouthpiece before it arrives at the bell for instance. An additional effect is to be 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 and also impacts on the sound velocities for forward and backward going waves. Lastly there will be sections of bore with moving walls which could introduce additional mean flow effects and excitation of (mostly evanescent) transverse modes of vibration.

In this study experimental data is presented showing the pressures measured by microphones mounted in the mouthpiece, in the water key (in the slide section) and at the bell of a trombone while slide movements are performed. Some measurements were performed using a fixed excitation frequency provided by a loudspeaker mounted onto the mouthpiece. Moving the slide results in changes in both the amplitude and frequency of the signal being measured by microphones (in spite of the input signal being produced by the loudspeaker being fixed in frequency). Infrasound components were also detected inside the instrument bore. Frequency tracking of audio was combined with optical tracking of slide movement to provide evidence concerning the nature of the physics of wave propagation within the dynamically changing trombone bore and conclusions drawn concerning any implications for perception and synthesis.