The generated noise caused by the fluid inside a duct is difficult to estimate accurately due to the difference of power between fluid and noise caused by the fluid. However, it would be required to estimate the distribution of sound pressure inside the duct to introduce the active noise control techniques effectively. The objective of this paper is to estimate the radiated acoustical energy and to visualize the distribution of the sound pressure inside an air-conditioning duct using the acoustical power balance method. This method is suitable for analyzing complicated random noise at a fairly high frequency. Also introduced was a new method of the boundary integral equation to analyze the dynamic behavior of sound fields. The analysis of sound fields controlled by an active noise control system will be shown with the dynamic time-domain visualization of the sound-pressure field.