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The effects of turbulence on organ sound quality and its minimisation in blowing systems

By Jude T.A. Ngu


This research sets out to study the effects of turbulence on organ sound and examine ways\ud of minimizing it in blowing systems.\ud The efficacy of application of statistical and spectral techniques for a quantitative analysis of\ud the effects of turbulence on the organ sound has been established.\ud The sound generated for different inlet levels of turbulence intensity were analyzed. The\ud inlet turbulence variation was achieved by mixing in different proportions, the wind\ud generated by a centrifugal fan which has high levels of turbulence and turbulence\ud attenuated wind. The latter was obtained by suppressing the levels of turbulence through\ud flow stratification in narrow channels. Turbulence attenuation by flow stratification was\ud used in this research as it could attenuate turbulence for very high Reynolds number (> 105)\ud flows such as encountered in the study.\ud The effects of turbulence attenuation were evaluated aurally, in a qualitative manner and\ud also analyzed quantitatively. The aural evaluation indicates that changes in the sound with\ud changing levels of turbulence were perceivable. In addition, the lower the turbulence levels,\ud the better the sound quality - the instrument (organ) is said to have a better "degree of\ud articulation... and expressiveness. " Spectral analysis of certain notes with and without\ud turbulence attenuation showed changes to the spectral shape. Ripples in the spectra in the\ud leading and trailing edges of the fundamental were much reduced. The spectra were\ud smoother. The fundamental frequency shifted, on average by 0.3°%, up or down, depending\ud on the nicking condition of the pipe. In one note, F#4, the fundamental had two peaks\ud without any turbulence attenuation. One of the peaks vanished when turbulence attenuated\ud wind was used.\ud A technique was developed for quantitatively evaluating the fundamental from the\ud frequency spectra. The fundamentals were sampled, normalized and moments taken about\ud the central frequency. The 3rd and 4th moments computed gave an indication of the\ud changing skewness and "peakedness" of the note with turbulence. It was noted, from\ud experiments conducted on a test rig, that the notes got more negatively skewed and peaks\ud more with increasing turbulence attenuation. It was also noted that the notes underwent a\ud non-monotonical increase in asymmetry with falling levels of turbulence. There were\ud similar increments in the "peakedness" of the pipe fundamental amplitude with decreasing\ud turbulence.\ud An electrical analogous circuit of the organ flue pipe for turbulence studies was developed.\ud A circuit model with linear components was simulated and tested. It was found that most\ud of the circuit component parameters were dependent on the geometric dimensions of the\ud pipe, especially the pipe length. Calculations and a simulation of the circuit using HSPICE\ud with parameters determined using the effective length of the pipe gave resonant frequencies\ud much less than those obtained from acoustic experiments. The concept of an active length,\ud La, which works out to be a third of the effective length, Le, was introduced. Calculations\ud and simulations using La, gave more accurate results. A hardware implementation of the\ud circuit developed was also done in order to study the effects of turbulence.\ud The statistical analysis and electrical analysis studies undertaken in the current work may\ud find application in the design of computer organs and other areas were turbulence in flow\ud plays a significant role

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