Analytical and experimental studies undertaken for controlling noise in the interior of a three-dimensional enclosure with a flexible wall are presented. The rigid walls are constructed from acrylic material, and the flexible wall, which is clamped along all four edges, is constructed from aluminium material. Noise generated by an external speaker is transmitted into the enclosure through the flexible boundary and active control is realized by using lead zirconate titanate (PZT) piezoelectric actuators bonded to the flexible boundary. Condenser microphones are used for noise measurements. For harmonic external disturbances, optimization analyses are carried out in the frequency domain to determine the optimal voltage inputs to the piezoelectric actuators for global and local noise control. In the associated experiments, analog feedforward control is implemented by using acoustic error signals for different panel and cavity controlled modes.