Abstract
Measurements on an artificially blown and mechanically excited flute head joint provide values of the complex acoustic back pressure generated by the blowing jet. The magnitude of the acoustic back pressure is calculable from the jet momentum and is approximately twice the static blowing pressure times the ratio of the lip-aperture area to the tube cross-section area. The phase of the induced back pressure relative to the oscillation volume velocity is determined by the lip-to-edge distance and the velocity of propagation of a wave on the jet. Adjustment of this phase is demonstrated to be the major means by which the flutist selects the desired mode of oscillation of the instrument. The efficiency of conversion from jet power to acoustic oscillation power is low (2.4% at 440 Hz) and is about equal to the ratio of particle velocities in the air column and the jet. Nonlinear (turbulent) losses are measured and are substantial. Stroboscopic views of the jet motion under explicitly stated oscillation conditions show the large amplitude of the jet wave and its phase relative to the stimulating acoustic disturbance.
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