Active noise control for open air live events at low frequencies

Research output: Book/ReportPh.D. thesis

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Open air live events can be a powerful source of noise, in particular at low frequencies. Furthermore, the sound waves at these frequencies propagate over large distances with minimal attenuation from atmospheric absorption. Sound field control can reduce these low frequency noise emissions. This approach employs a secondary array behind the audience for this purpose. State of the art methods can provide up to 15 dB in noise reduction at approximately 130 m. It is of interest to increase the working range and robustness of these systems. This thesis makes strides in this direction by focusing on two subproblems. The numerical method employed to synthesize the filters for the secondary array is the first subproblem and is directly connected to the spatial properties of the solution, ease of use and insertion loss. The limited range of these systems is mainly related to the degree of accuracy in the characterization of the propagation paths which constitutes the second subproblem investigated in this thesis. A new iterative method is proposed to deal with the first subproblem. This method uses parameters that are directly related to physical quantities. This direct connection makes it easier to use and allows us to efficiently control amount of radiation outside of the quiet zone. This method has been experimentally validated using both measured and simulated transfer functions. The results show that this method using simulated transfer functions provides noise reduction performances on par with other methods with 10 dB broadband insertion loss and peaks of up to 20 dB. Solutions obtained from simulated transfer functions generalize better outside of the quiet zone and can be easily updated when propagation conditions change. Moreover, regularization not only balances the trade-off between noise reduction and amplitude of the solution but also controls the radiation pattern of the control array and the robustness of the solution against uncertainties and modelling errors. The second subproblem can be dealt with by using measurements, but this strategy becomes increasingly impractical over large distances. Propagation models and simulations provide an alternative. However, there are many concurrent factors affecting sound propagation outdoors such as reflections from obstacles, trees, ground, and the influence of a moving and inhomogeneous medium. Characterizing and including all these effects is an enormous task. This thesis restricts itself to the effects produced by a moving inhomogeneous medium to extend the range. An accurate description of the medium is crucial to achieve reliable predictions of the two propagation paths. For this reason, this work reviews fundamental aspects of micrometeorology that allow to better understand and model the wind and temperature profiles. This project makes use of numerical simulations to analyze the importance of using the right model for the wind profile. The results show that the use of an unsuitable model introduces a phase error and misrepresents the interference pattern close to the ground leading to a deterioration of the performance of an active noise control system. This thesis studies how different parts of the profiles affect the error and how sensitive it is to parameters that characterize the lower atmosphere. Finally, these results are used to derive a range in which simpler profiles can still be used in simulations without affecting the accuracy of the results.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages160
Publication statusPublished - 2023


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