Active noise control of time‐varying signals in ducts

Abstract

A sound cancellation system has been developed to reduce the flow‐induced noise in piping systems under both steady‐state and transient conditions. A hybrid controller that combines both open‐loop and closed‐loop responses with adaptive digital signal processing and filtering techniques is utilized to speed up the response of the system controller for the transient condition. Autoregressive, moving average with exogenous input (ARMAX) alogrithm has been utilized in the on‐line and off‐line modeling of signal characteristics. Off‐line model data are introduced into an on‐line adaptive system model via a parameter leakage mechanism. The convergence of the resulting algorithm is improved at the expense of bias in the final system parameters. The bias depends on a weighting or leakage factor, which controls the relative amount of off‐line data to be used. A preliminary test has been conducted to demonstrate the active cancellation concept for synthesized steady‐state signals. Results showed that up to a 30‐dB reduction has been obtained for pure sinusoidal waves ranging from 100 to 500 Hz. For confined jet noise, the noise reduction is about 10 dB, and similar results have been observed for the random broad‐band noise.