Acoustic admittance of organ pipe jets

The admittance o• a turbulent jet as the generator in organ flue pipes was investigated experimentally with respect o the relevant parameters, that is, the frequency, the jet effiux velocity, the mouth end-correction, and the jet tip deflection at the cdge. First, an experiment was performed using a jet with a pulsating velocity to drive a pipe to isolate and identify the various drive mechanisms occurring in the jet-pipe interaction. The two established rive mechanisms, momentum drive and volume drive, were found to be operating, but the relative proportion of momentum drive was greater than expected. Turning then to a typical flue pipe arrangement, as a preliminary experiment, the jet admittance in the pipe was shown to be independent of the jet tip deflection so long as the deflection was less than about 0.7 jet widths. This constant admittance regime was then used to facilitate measurement of the pipe excitation as a function of the three remaining parameters. The admittance magnitude and phase were used, in conjunction with a knowledge of the jet behavior, to quantify the relative importance of the two drive mechanisms here, revealing once again a greater proportion of momentum drive than expected. To a good approximation, the admittance magnitude multiplied by the efflux velocity, and the admittance phase were each found to lie on a universal curve when plotted against a parameter consisting of the frequency divided by the efflux velocity, provided the cutup was constant. A deviation from expected behavior, as yet to be explained, was found in the jet admittance and phase for very low frequencies and high blowing pressures.