This paper presents a study of the effectiveness of using actively controlled panels to block the transmission of sound. Ideally, the acoustical pressure behind the panel is driven to zero by actuators mounted on the panel surface. Microphones are used as the sensors to detect the acoustical pressure, with the adaptive FXLMS algorithm being used for feedforward control. It is found experimentally that major limitations of this approach are non-uniform cancellation of noise across the panel at high frequencies, harmonic nonlinear behavior at very low frequencies and actuator saturation at high sound levels. These limitations restrict the controllable frequency range and noise level, and in some cases can lead to instability. Approaches are suggested to overcome these limitations. Non-uniformity can be lowered by using multiple or distributed actuators, harmonic nonlinearities can be reduced by using back-to-back panels and instability from saturation can be avoided by using a proposed half-power FXLMS algorithm.