Consideration of active noise control in space

Abstract

Active noise control in space is classified into four groups based on physical aspects, which can be further reduced into two groups, namely, the sound pressure or the sound power, and the ability or nonability of an additional sound source to act on the sound radiation of an original source. The latter is equivalent to the problem of whether the mutual impedance between the two sound sources must be considered or not. The noise reduction process in each group is analyzed using the impedance of the sources. The desirable condition of the additional source is discussed, and, when this condition is realized, the noise reduction effect is estimated. In general, when the additional source is placed near the original one, the more tightly they are coupled, the more effectively both the sound pressure and the sound power are reduced. It is difficult, however, to find the most desirable condition for reducing the sound power by means of an experimental procedure.