Active control of low-frequency noise in bubbly water-filled pipes

This paper describes an active silencer system for low-frequency noise in water-filled pipes, with implementation in marine vessels and structures in mind. Active noise control in underwater environment has its unique challenges arising from bubbles. In this light the effect of bubbles on active noise control in water-filled pipes is explored, and a design guideline for a robust active silencer is proposed. The silencer consists of an underwater sound source, an error hydrophone, and an electronic controller programmed with an adaptive filter. The adaptive filter is updated using the filtered-x least mean square algorithm. Estimation of the incoming noise is performed using an adaptive notch filter. The performance of the active silencer is tested for pure tone noises below 500 Hz in a water-filled pipe connected to a reservoir. Notable reduction of noise is achieved with varying degrees of success at different bubble fractions.