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Updated: Aug 9, 2026

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers
Published on: December 27, 2012
Causal impedance matching for broadband hybrid noise absorption
1Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong. mmjyuan@polyu.edu.hk
Abstract:
The complementary strengths and weaknesses of passive and active noise control (ANC) methods have motivated many researchers to develop hybrid noise absorbers that integrate both control strategies. The impedance matching technique (IMT) is the most effective for such a purpose. An unsolved problem with available IMT schemes is the a priori reference signal that limits IMT applications. This study proposes the use of the forward wave, available by the two-microphone method, as the reference signal. Due to inevitable errors in wave separation and inlet reflection of the control signal, the absorber becomes a feedback system. A simple and stable ANC is developed for impedance matching without the a priori reference signal. The proposed absorber has an absorption coefficient of 0.9 or above in a frequency range of 60-850 Hz. It is stable in the presence of sensor mismatch and robust with respect to significant variation of inlet boundary conditions.
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