Active local noise control in open space

Local noise control, which creates quiet zones in desired areas, is a practical option of active noise control in open space when the control sources and the primary sources are not able to be very close. In most cases, the local control system attenuates the sound pressure in some areas at the cost of total sound power output increase. As a result, while the control system creates a quiet zone in some areas, it causes the sound pressure to increase in others. Two indicators of control efficiency, quiet zone size and total sound power output, are dependent upon the configuration of the control system. It has been found that for the multichannel control system, there exists a range of optimal configuration of control system, in which the control system can create the largest quiet zone with the lowest increase of total sound power output. Outside this range, the control system either becomes useless in creating large quiet zones, or causes great increase of sound pressure in most areas.