Improvements to the acoustic pulse reflectometry technique for measuring duct dimensions
In the study of tubular structures such as pipeline sections, musical wind instruments and human airways, acoustic pulse reflectometry has become established as a useful tool for non-invasively measuring the input impulse response, from which the internal duct dimensions can be calculated. In this thesis, the theory describing wave propagation in a duct of varying crosssection is outlined, culminating in a discussion of the layer peeling algorithm used to reconstruct a duct’s bore profile from its input impulse response. Experimental measurements of the input impulse responses of various test objects, together with the subsequent bore reconstructions, are then presented. The problem of offset in input impulse response measurements is discussed and the effect on the bore reconstruction is shown. The offset is found to consist of both a DC component and a sinusoidal component. Methods for eliminating the two offset components are explored and the resultant improvement in the stability and reproducibility of the bore reconstructions is demonstrated. Two adaptations to the reflectometry technique, designed to extend the bandwidth of input impulse response measurements, are described. The improved high frequency content brought about by these adaptations is shown to lead to bore reconstructions of high axial resolution, allowing rapid changes in cross-sectional area to be more accurately reproduced. Finally, limitations of the acoustic pulse reflectometry technique (particularly those brought about by the bandwidth improvements) are discussed and potential future ways of overcoming the limitations are proposed.