An investigation is done on the effects of evanescent modes on the performance of active noise control in waveguides. The effect of the location of the control actuators relative to the location of performance sensing microphones and the waveguide termination is investigated. Both the sound power radiated from the waveguide and potential energy in the controlled section of the waveguide are monitored. In the analysis, the effects of the waveguide termination are modeled using a modal reflection coefficient at the termination. Experimental results for several cases are included to illustrate the predicted effects. [This investigation was supported by Nelson Industries, Stoughton, WI.]