Design of undirectional sources for active control of noise in ducts

Unidirectional sound sources can be used in the implementation of forward active noise control (ANC) systems for ducts, where the anti-sound needs to propagate downstream but it is also desirable that no upstream radiation occurs. This paper discusses a new approach to the design of a unidirectional source. A digital signal processing system involving adaptive filtering is proposed for automatic design of a unidirectional source The automatic calibiation of the system can be executed in 3 phases: 1) calibration of the impulse responses from the actuators to the microphones, 2) calibration of unidirectionality, and 3) calibration of the error path from the unidirectional source to the error detetector. After these procedures, the ANC operation can start. In this paper, a fixed target FIR filter for the design stage of unidirectional two-element sources is proposed. The new method is compared with an earlier one proposed by the authors. The directivity of the source can be improved at almost all frequencies, and the obtainable attenuation is improved at the frequency range which overlaps with the passband of the target filter.