Effects of a Passive Enclosure on an Active Noise Control System in Free Field

A kind of noise is dominated by broadband noise with low frequency tonal components. To achieve noise control on whole frequency band, it is necessary to implement hybrid control technique combining passive control and active control. There are three methods to combine passive control and active control, including active control in passive enclosures, active control outside passive enclosure and active noise control in ducts. To reduce the weight of passive enclosure, a method is to adopt active control in passive enclosure. This paper studies the effects of the passive enclosure on an active noise control system in free field. By using a rectangular panel-cavity system, the paper gives the analysis model for inner sound transmission through a panel to the external sound field. The performance of ANC system is illustrated in free field and in passive enclosure, respectively. The noise reduction of ANC system in passive enclosure may be large even for high resonance frequencies, the optimum error point may be different with that in free field and more error points in near field may be required to reach the same performance achieved with multi evaluation points in far field to optimize the strength of the secondary source.