The harmonic noise produced in an enclosure is actively controlled with phase compensators. The active controller is designed in the frequency domain using optimal control theory. This controller can be implemented in a simple form of FIR filter to ensure stability. The FIR filter has a time delay of half the filter length to satisfy causality. This time delay corresponds to the phase shift of the response of the harmonic components. Phase compensators are used to correct this response, and the requirements for the phase compensators are determined from a numerical analysis. The performance of the controller is investigated with various types of phase compensation. The results show that the performance is better with time-domain compensation, regardless of the target frequency. The acoustic power reduction of the system is also determined by experiments. The results for the real control model show that the acoustic power is reduced by approximately 15 dB and 10 dB for the first and second harmonic components, respectively, while the overall acoustic power reduction is about 10 dB. To control the harmonic noise transmitted through an enclosure window, a controller for acoustic power reduction can be designed by applying a time delay for the fundamental frequency.