Wave propagation in long compound acoustic transmission line: numerical simulation and measurement

This paper describes the FEM-BEM simulation of and measurements on an acoustic transmission line consisting of three connected components; a long thin tube, a narrow solid cylinder, and a long narrow column of water. The total path length corresponded to about 8500 wavelengths at the highest frequency used, 5 MHz. Applications for this topic are in the field of high resolution ranging and is particularly important in leadscrew position measurement, where high precision over long travel lengths are required. The new aspects of this study lie firstly, in the development of special hybrid FEM-BEM simulation techniques to handle the extremely long lengths, secondly in the optimum choice of waveguide modes, and thirdly in the details of the experimental measurements, especially, when applied to industrial leadscrew position detection. Current techniques for measuring leadscrew positions are based on electromagnetic sensors and give a range resolution of only about 10 mm. The ultrasonic range detection provided a precision of 1 mm.