THE ACOUSTICS OF THE TROMBONE: HOW DO PLAYERS LIP UP AND DOWN?

摘要

Four advanced trombonists and three beginners played the note Bb2 (the lowest normal note played with the slide retracted), then 'lipped up and down'. The normal playing frequency lies above that of the bore impedance peak, so the bore is a compliant load. However, the range reached while lipping is approximately centered on this peak. To investigate how the lip oscillation is regenerated with inertive or compliant acoustic loads, acoustic pressure and flow waveforms were determined both up- and downstream from the lips, while playing. The lip opening area and the flow component due to the lips' sweeping motion were also estimated using a transparent mouthpiece and high-speed video. The lips move forward before separating, then backward before closing. Consequently, the acoustic flow into the mouthpiece becomes positive before the lips open. Further, the volume V of air swept by the lips in the direction of the flow is positive around one cycle. A model in which the lips execute out-of-phase simple harmonic motions in the forwards and vertical directions is consistent with the results. The pressure difference ΔP across the lips does nett positive ΔPdV work on the lips, providing sufficient energy for regeneration of the oscillation. As the note is lipped down to inertive load, the phase of the mouthpiece pressure moves ahead of that of the flow, and ΔP moves further ahead of the volume of air swept in the horizontal direction. The latter effect decreases ΔPdV work, so, for very inertive loads, the oscillation stops. Lipping upwards is limited partly because negative mouthpiece pressure must always overlap largely with the lips being closed, and the limited magnitude of sweeping flow limits the extent to which flow can lead the lip aperture. Consequently, further increases in lip stiffness produce jumps to the next impedance peak.