An acoustic transmission line model applied to liquid-filled tubes: Measurements and model predictions

Aaron M Kyle, Purdue University


Acoustic propagation and distortion in liquid-filled tubes were modeled using an acoustic transmission line model and predictions were compared to measurements. The transmission line model utilized an acoustic circuit to predict dispersion and attenuation that occur with axial propagation. The model had previously been applied to air-filled tubes to predict acoustic distortion. This thesis research represents the first comprehensive investigation of the applicability of this model to liquid-filled tubes, where the interaction between the tube wall and the constrained fluid causes a significant level of distortion of axially-traveling acoustic waves. Acoustic propagation was experimentally examined in various liquid-filled tubes, ranging from nearly-rigid copper to highly compliant vinyl. Acoustic distortion was assessed using time and frequency domain techniques, and the resultant measurements of group velocity, attenuation, and transverse resonance frequencies were compared to model predictions. Additionally, simple propagation and reflection experiments were performed in interconnected copper tubes to examine the ability of the model to predict the delay and polarity of reflections that would arise at well-defined acoustic impedance discontinuities. The transmission line model was able to account for the group velocity trends and their differences from the free field speed of sound: predicted group velocities were within 20% of experimental values for all tubes and liquids. These results indicated that the model could be used to forecast dispersion that would occur with axial propagation. Attenuation was also fairly well-predicted in the polymer tubes, and it was concluded that the transmission line model, which was developed for viscoelastic tube materials, was also appropriate for predicting dispersion and losses in linearly elastic tubes with non-rigid walls. Transverse wall resonance was predicted to occur over frequency bands where the interaction between the tube and liquid precluded axial propagation through the liquid. Wall resonance bands were experimentally demonstrated via acoustic noise excitations and evaluations of the resultant power spectra. In all of the tubes, low power regions that were characteristic of transverse wall resonances were experimentally evident. The onset and cessation of these high loss bands were well-predicted using the transmission line model. The upper resonance limit predictions were particularly accurate: model predictions were with 10% of the experimental frequencies for all tubes. Also, the shifts in resonance limits that were expected to accompany varying material properties and dimensions were experimentally observed and well-predicted. Finally, the model was successfully utilized to forecast the time delay and polarities of acoustic reflections that occurred at simple changes in cross-sectional areas and variations in wall material properties. The modeling and experimental results of this thesis research form a basis for the potential development of an acoustic guidance system for fluid-filled catheters.




Wodicka, Purdue University.

Subject Area

Biomedical engineering

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