# Performance Evaluation of Active Noise Control for a Real Device Casing
## 1 Introduction  
Noise is one of the most common hazards in industrial environments. High sound pressure levels may cause hearing loss, induce stress, reduce comfort and also bring voice communication problems. Thus, noise reduction is an important topic. The best solution is to reduce the noise level at the source. However, existing possibilities have many limitations. Commonly used passive noise reduction systems have poor performance for low frequencies. The barrier must be too thick (and in many cases also too heavy) for practical usage. Such limitations influenced interest in active systems, which may be very effective for low frequencies and complement passive methods in their weak points [1; 2; 3]. A number of single panel systems were developed [4; 5; 6], using which, vibrations of a thin plate were controlled by piezoelectric or electrodynamic actuators and the noise emission from the plate was reduced. For control of noise emission in three-dimensional space, multiple flat panels are required. The idea of using such an approach for the whole casing was initially proposed by [PERSON] et al. [7]. A similar idea was later strongly developed and applied by the authors of [8; 9].  
For higher reduction levels, walls made of two panels were also investigated [10; 11; 12]. Such an approach can also be used for the whole casing. However, double-panel walls have some disadvantages: the walls must be thicker; the mass must be higher; and the mass-air-mass resonance might be a potential problem. Moreover, double-panel walls have negative influence on thermal conductivity of the casing.  
Less effective alternatives for active control are semi-active control of walls with shunt systems [13] or systems with energy recovery [14; 15], which could be potentially used for active control. However, both possibilities are out of the scope of this paper.  
Dedicated, additional active noise-reducing casings provide good global noise reduction. However, they increase the dimensions and weight of the device, and they also have an influence on thermal dissipation. Hence, whenever possible, the best way would be to use original device casing. Nonetheless, this approach is much more complex than using a dedicated casing because the device casing was not designed on purpose for active control. The mechanical structure of the original casing is usually sophisticated, often including numerous bends, embossing and internal mountings. Hence, an optimization of actuators and sensors placement is much more difficult. Also, there is a vibrational coupling between the device and its casing. The active control system should, thus, not only provide noise transmission reduction, but also control noise emission from walls excited by vibrations. Taking all these into account, the original device casings are vibroacoustic plants that are very challenging to control.  
A simplified problem, where a loudspeaker was placed inside the device's casing, was already investigated by the authors of [16]. In this paper, real noise generated directly by the enclosed device was reduced during its regular operation. It is a factor that also increases the difficulty of the subject, because the real device is a spatially-distributed noise source that excites the original casing both vibrationally and acoustically.  
The approach proposed in this paper has been tested on a regular, off-the-shelf washing machine with its original casing, during spinning, when it generates high sound pressure levels. Experimental results are presented, proving for the first time under real conditions, the concept of the proposed active casing method. Moreover, details about the active control system are also provided, including switched-error modification to the original filtered-x least mean square (FxLMS) algorithm and a distributed control scheme that facilitates implementation on multiple simpler controllers.