Investigating the impact of tone hole geometry on the flow in wind instruments via particle image velocimetry

An experimental study is conducted to investigate the effects of both the diameter and shape of wind instrument tone holes on the flow behaviour in an artificial, simplified scenario. Time-resolved particle image velocimetry is used to measure flow velocity over the entire length of a square recorder bore. Flow velocity measurements enable various approaches to analyse the flow under different tone hole configurations. Simultaneous to the flow measurements, the air pressure at the input (blowing pressure) and in the interior of the tube (at five locations along the bore) is measured. Realistic excitation conditions are emulated, incorporating a commercial bass recorder mouthpiece. The flow for three different tone hole diameters is examined, with two of the tone holes also varying in height and undercutting. The amplitude of the flow oscillations is examined and compared for all tone hole configurations. The effect of tone hole undercutting is further analysed in the frequency domain, where it is observed that undercutting concentrates more energy at the fundamental frequency. Finally, it is experimentally shown that undercutting lowers the vorticity of the flow.