A "new" approach to active noise control in ducts

In this paper we consider the fixed-filter design approach for active noise control (ANC) in ducts. Based on a large body of literature during the past decade that deals strictly with adaptive filter solutions (see [1] and references contained therein) the fixed-filter approach appears to be all but discarded for this application. The ANC system under consideration has a similar configuration to those used in most adaptive implementations; i.e., a duct equipped with both feedforward and feedback sensors and a control speaker. Our design of a linear, time-invariant controller involves three steps: (1) system identification from the experimental duct frequency response, (2) H_∞ synthesis using the identified plant and model errors, and (3) use of Quantitative Feedback Theory (QFT) to further improve performance by sequential tuning of the feedback and feedforward filters using the experimental data. Preliminary experiments indicate that attenuation levels of 10-30 dB are achievable over a 400 Hz band without amplification in the immediate range. An important consequence of LTI designs is that performance is guaranteed for wide- band noise as well as pure tone disturbances - in adaptive schemes stability and performance may be very difficult (if impossible) to guarantee beforehand. This paper focuses on the third step of our three-step approach - the QFT design aspect - (the other two steps are described in [2]).