The use of axial splitters inside ducts for active noise control of higher order acoustic modes

Studies of noise reduction in ducts have become, over the years, a field of intense application mainly because of the industrial potential and feasibility of practical systems. The traditional approach to noise control in ducts uses passive techniques such as dissipative and reactive silencers. However, with the advance of the technology and limitations of conventional noise control treatments, active noise control (ANC) has become a practical and economic method due to powerful hardware for the digital signal processing in real time. However, in industrial applications, the design and employment of active noise control not always is simple and practicable. In exhaust systems, the ducts have geometry which allows high order acoustic modes to propagate at the frequencies to be controlled. These characteristics make it difficult to implement an ANC system. Higher order modes in duct sometimes result in lower ANC performance. In this paper, a methodology based on splitting the duct into parallel axial sections is presented in order to ensure that only plane waves are present in the region of the ANC system. The aim of this work is to show that this artifice can be a viable and cost effective solution. Numerical acoustic simulations were carried out in order to demonstrate the viability of the suggested technique. A finite elements method is used. An optimization using evolutionary algorithms is also used to select optimal actuators positions. Attenuation of 26 dB was obtained for tonal noise in a duct with splitters, while simulations with the conventional duct had 3 dB of mitigation.