Abstract: Active noise control (ANC) system formed of a corner microphone-amplifier-loudspeaker system has been shown to be effective to reduce the reverberant noise in a room, based on the Normal-mode-Canceling prinetpie, and the theory and practice of snch has also been clarified. But sieady state noise was considered only in the past, and few cases are stable noise sources, and there is time that even stable noise source is started or stopped. Any change of the noise source will excite transient in the ANC systsm, which might have unfavorable effect on the application of the ANC system. It is desirable to have a good knowledge of the transient behavior of the ANC system, in order to have a better utllzation of it. It is the purpose of this investigation to clarify the transient behavior of the ANC system in a room and the influence on its acoustics.