Modal power distribution in a duct at high frequency

Baxter, Stephen Michael (1983) Modal power distribution in a duct at high frequency. University of Southampton, Doctoral Thesis.

Abstract

The project described in this thesis was motivated by experimental results obtained in the Absorber Facility of the National Gas Turbine Establishment, Farnborough. Peak values of duct attenuation were found to occur at frequencies different from theoretical predictions. A possible cause of this discrepancy was a test duct's modal power distribution, previously assumed uniform. Since at frequencies of interest in a typical test duct there are many propagating modes, a new modal analysis technique was developed to replace previous mode-by-mode resolution techniques. The new technique was based on the reverberant field theory of architectural acoustics. The new analysis technique was applied to data from a test duct microphone traverse experiment. The analysis showed that the duct's modal power distribution was not uniform, and the physical origins of the distribution were studied. It was found that the use of the new distribution results produced significant differences in lined duct attenuation predictions, but did not resolve the peak frequency discrepancy.

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