Active noise cancellation in ducts

Abstract

This paper summarizes the state of active noise cancellation in ducts. The application of active feedback to reduce the sound energy propagation through the duct is primarily important at low frequencies because passive silencers are very large, and the attenuations achieved are relatively small. In its classical configuration, the noise field detected by a signal microphone is phase inverted and time delayed by an adaptive filter. This signal is then reradiated into the duct from a sound source at the duct wall or located remotely and connected by means of a waveguide. The adaptive filter is controlled by a microphone located in the duct to sense the noise reduction and adjust the entire system for optimum operation. This paper analyzes the effects of the signal microphone position, type, and location of the cancellation sound sources, and sensing microphone location. Optimum operation requires consideration of higher order modes and the nearfield of the cancellation source, including evanescent waves. Results obtained with practical constructions illustrate the great potential of active systems to attenuate noise with discrete spectra, random noise, as well as transients are presented.