Final colloquium Daan Ratering

26 October 2021 09:00 till 09:45 - Location: lecture room D (James Watt) - By: DCSC

"Towards silent open windows - An open-loop wave-domain algorithm for control of noise through apertures"

Noise pollution is a major health threat to society. Active noise control systems that attenuate noise propagating through open windows (apertures) have the potential to create quieter homes while maintaining ventilation and sight through the aperture. Such systems are commonly realized with closed-loop LMS algorithms. However, these algorithms require a large number of error microphones inside the room and provide only local attenuation. A large number of error microphones lead to slow convergence and high computational effort, indicating additional disadvantages and resulting in algorithms infeasible for real-world application. In this study we develop an open-loop wave-domain algorithm that converges instantaneously, operates with low computational effort and does not require error microphones. This approach inherently controls sound in all directions from the aperture in the control region located in the far-field of the room. We derive new acoustic transfer functions to obtain frequency responses of the aperture and loudspeakers. Those are used for soundfield calculation. The sum of the soundfields, from the aperture and the loudspeaker array, is then expressed in orthonormal basis functions. By minimizing this sum in least mean square sense, we can calculate the filter-weights that minimize the sound energy in the control region. Implementation of these filter-weights with block-wise processing using the Short-Time Fourier Transform generates the signals for the loudspeaker array. This processing induces a delay. Two methods to compensate for this algorithmic delay are compared. The first is positioning the reference microphone further in front of the aperture. The second method uses an autoregressive model for signal prediction. Both lead to a loss in attenuation performance compared to the optimal algorithm. We compare the optimal wave-domain controller with a LMS-based reference system, as well as both the algorithmic delay compensation methods. The controllers are tested with a sparse and grid loudspeaker array, and we use rumbler-siren, airplane and white noise signals as incoming noise. Furthermore, we compare performance for three incident angles. Our simulation results indicate that wave-domain processing has the potential to outperform LMS-based methods in practical active noise control for open windows. More specifically, we obtain an average -10~dB global reduction up to 2~kHz for all signal types with the optimal wave-domain approach. In comparison, the performance of the closed-loop algorithm ranges between -5.2 and -9.2~dB, depending on the signal type. Furthermore, we proof the limited impact of the incident angle on performance for the wave-domain controller. Positioning a reference microphone in front of the aperture outperforms the predictor approach in all scenarios, and its performance compares to that of the closed-loop LMS algorithms. Eventually, the absence of error microphones and inherent global control of the wave-domain algorithm present a method for a feasible product. Future work could improve the algorithm by reducing loss of performance with short window-sizes, due to time-delay wrapping in the block-wise processing. Moreover, a natural continuation of this study is to develop and test the wave-domain controller for scenarios with a moving primary noise source, to further emphasize its advantage over the closed-loop LMS algorithm.

Dr. R. Ferrari