Implementation of decentralized active control of power transformer noise

The active noise control of sinusoidal sound in free space has received considerable attention in the last few years. In a number of practical situations, the sound radiated in free space by extended sources needs to be globally reduced. This is the case of electrical power transformers, for example [1]. Nelson and Elliott have demonstrated that sound power attenuation can be achieved by active control with secondary sources, loudspeakers for example, located in the near field of an extended radiator (also called the primary source) [2]. An effective global attenuation of the sound power implies that the secondary sources must be in the vicinity of the radiator : the distance between each secondary source and the radiator must be less than λ/4 where λ is the acoustic wavelength [3]. The secondary sources are driven by a controller in order to cancel the sound field measured by error microphones for example. An effective global attenuation of the sound power can be achieved by independent active controllers with collocated loudspeakers and microphones located in the near field. The figure 1 presents experimental implementations of 24 independent control units on a real power transformers. A noise reduction of 10 dB of the first harmonics (120 Hz) have been obtained [9]. The main problem of this approach is to design a disposition of loudspeakers and microphones such that the active noise control system is efficient. A practical method is proposed in order to predict the stability and the attenuation of the system. Several experimental results with 7 control units are presented.