Starting transients of flue organ pipes in relation to pressure rise time
A. W. Nolle, T. L. Finch · The Journal of the Acoustical Society of America · 1992
A wind supply with adjustable pressure rise time is used to study the initial transients of adjustable stopped and open flue pipes which approximate classic voicing. The precursor, at several times the fundamental frequency, and a subsequent oscillation dominated by the next mode above the fundamental, are strongest when the pressure rise time is somewhere between 1 and 10 fundamental periods. These features may vanish for much longer or shorter rise times. Decreasing the inharmonicity of the resonances increases the next-mode content in the attack, as does reducing the cutup. External sound at the fundamental frequency, applied either at the pipe mouth or through the air supply, suppresses both the precursor and the subsequent next-mode burst. The precursor does not involve a longitudinal pipe resonance. Pipe frequencies were 198–256 Hz. Air supply pressure was 530 Pa. A computer simulation, based on an oscillator model with three resonant modes and time-delayed nonlinear feedback, incorporating ideas of N. H. Fletcher and others, accounts for major features of the experimental results. The suppression of the initial next-mode burst when the pressure rises slowly is caused by the extra time spent in a pressure range where the feedback loop gain (particularly the phase) for the next mode does not support self-oscillation. With sufficiently fast pressure rise, impulsive excitation favoring the fundamental enables it to dominate the attack, preventing the next-mode burst.