Placement of microphones for an active noise-reducing casing

Active Noise Control systems have been in scientific interest for a long time. Such systems allow for high reduction of low-frequency noise, especially for predictable noises such as tones or multitones generated by rotational machines. The obtained reduction level may be much higher compared to passive methods, which for low-frequencies require unacceptable thick and heavy barriers. Classical Active Noise Control systems, which use sound interference in air, except in some limited cases, can create only local zones of quiet, where the destructive interference occurs. In most cases this is unacceptable. Such problem can be solved by using actively controlled barriers, which can form a full active noise-reducing casing. Then, by appropriate control of wall vibrations, a significant global noise reduction can be achieved. However, both global noise level and noise emission cannot be measured. Some estimations or limited measurements need to be used. One of possible solutions is to measure the noise emission by using multiple microphones around the casing. In this paper different configurations of microphone placement are investigated. Microphones are used by a feed-forward FXLMS-based control system for adaptation of control filter weights. The control systems are tested on an off-the-shelf washing machine casing with electrodynamic actuators installed on four walls. The performance of the resulting control systems is experimentally verified and obtained results are reported.