Application of extra reference signals for feedforward active noise control

By the increasing computational power of the controllers, multiple reference feedforward active noise control is of increasing interest. It is often utilized if the unwanted noise is generated by multiple sources or the positions of the sources are not known in advance. The performance of multiple reference feedforward systems has been investigated in the last decades regarding the attainable suppression, convergence speed, and computational burden. In the meantime, many applications have been published proving the efficacy of the approach. Several experiments show that in reverberating enclosures greater suppression can be obtained if the number of reference signals exceeds the number of noise sources. This paper provides the analysis of such control systems in the case of broadband excitation. It is proven that the application of extra reference signals shortens non-causal decay in the inverse of the reference path, enabling higher suppression, compared to the case where the number of the reference signals and noise sources are equal. The paper introduces the motivating experimental examples explained by the theoretical results and simulations.