Trapped modes in cylindrical waveguides

We prove the existence of trapped modes in the presence of two classes of obstacles in cylindrical acoustic waveguides. First we prove that trapped modes exist whenever the obstacle is thin and has a normal which is everywhere perpendicular to the generators of the cylinder. Secondly we prove that for the case of a circular cylindrical guide containing an axisymmetric obstacle, an infinite sequence of trapped modes exists, the frequency of the modes tending to infinity. In each case we consider an example where the trapped mode frequencies can be calculated numerically using the residue calculus method.