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

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