In-Vitro and Numerical Investigations of the Influence of a Vocal-Tract Resonance on Lip Auto-Oscillations in Trombone Performance

Controlling the acoustic impedance of the upstream airways in brass instrument performance may be an important factor influencing the efficiency of the sound production process. Because of the complex characteristics of the lip-valve oscillator, the conditions under which a vocal-tract resonance may be favorable to the sustain of lip auto-oscillations are not easy to determine. In order to investigate this aspect of brass performance, an experimental method based on an active control approach is applied to an artificial trombone player system in order to simulate the influence of a vocal-tract resonance at the playing frequency. By varying the amplitude and phase characteristics of this upstream impedance load, we investigate the acoustical influence of this resonance on lip vibrations and on the acoustic pressure generated in the instrument. The observations reveal that variations of the phase difference between the downstream and upstream impedance induce significant variations of the playing frequency. An optimal phase tuning point characterized by a maximum of downstream acoustic pressure at the input of the instrument, and uncorrelated to a maximum of downstream input impedance, is identified. These experimental results are compared with numerical simulations, both of which produce similar findings. The optimal tuning point appears to be partly related to the displacement of the playing frequency close to a mechanical resonance of the lips. This induces a greater “efficiency” of the lip-valve system, hence maximizing the acoustic flow generated into the instrument while other control parameters (quasi-static mouth pressure, lip tension) are maintained constant. In addition to an exploration of acoustical influence of the vocal tract, this experimental method hence offers promising perspectives for the study of artificial lips under playing conditions.