The influence of side hole undercutting on vorticity and frequency content in wind instruments

In this work, we conducted an experimental campaign to observe the oscillating flow velocity spectra and the vorticity inside a wind instrument. Time-resolved particle image velocimetry (PIV) measures the internal flow velocity within the transparent bore of a square bass recorder. Although vortex formation at the side holes of wind instruments has been identified in numerical simulations, limited experimental investigations have been carried out. While frequency content analysis is an unexplored approach to investigate the interior flow of a wind instrument with side holes, it has the potential to provide new insights. Flow velocity measurements at a 3.5 kHz sampling rate resolve at least five harmonics of the acoustic flow and reveal vortex formation. Measurements across the full length of the bore reveal vertical standing waves towards the open end. Concurrent air pressure measurements confirm the locations of flow velocity frequency peaks. The experiments were conducted under realistic input conditions, where a commercial recorder mouthpiece and a constant pressure source were used for excitation. We compare flow velocity oscillation spectra for varying side hole geometries. Undercutting was observed to concentrate more energy at the fundamental frequency and to reduce vortex formation at the side hole.