| Author: | Yang, Weiping |
| Title: | Modelling of unbaffled long enclosures for noise control |
| Advisors: | Choy, Yatsze (ME) Zhu, Jie (ME) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Noise control Environmental engineering Acoustical engineering Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxvii, 187 pages : color illustrations |
| Language: | English |
| Abstract: | Unbaffled long enclosures can be widely observed in practice, such as ventilation systems, traffic tunnels, and railway stations. These facilities bring people convenience however also cause noise pollution. Sound waves inside a long space do not dissipate but rather reverberate if they are not properly treated. Excessive exposure to such noisy acoustical environments exerts adverse effects on people's physical and psychological health. Besides, the sound radiated from the openings of long enclosures also produce noise pollution to the surroundings. To predict and reduce the radiated noise, the sound pressure fields of unbaffled long enclosures are investigated, according to which, noise attenuation strategies are proposed. A theoretical model is first formulated based on the Wiener-Hopf (W-H) technique in conjunction with the mode-matching method to predict the sound radiated from an unbaffled long enclosure. The geometrical configuration represents a practical scenario in which noise is produced inside the long enclosure and radiates to the outside through the opening. The sound field inside the long enclosure is expressed in terms of the superposition of acoustical modes, while the radiated sound field is described by a far-field directivity pattern that can calculate large acoustic domains effectively. The detailed implementation procedures of the model are introduced and the physics behind the sound radiation phenomenon is explored. The modelling procedures using the W-H technique build a theoretical foundation for problems regarding sound radiation from unbaffled long enclosures. For the purpose of predicting and attenuating the noise radiated from sound-proof tunnels, a theoretical model is proposed applying the W-H technique. Both the ground and impedance boundary conditions are taken into consideration. As a result, the sound distribution in the current configuration is totally different from that of the rigid long enclosure without the ground. Owing to the ground reflections, more directivity lobes appear outside the long enclosure. Besides, from the investigations on the impedance boundary conditions, the inner wall of the long enclosure is the most effective location to mount noise control devices. Subsequently, a partial lining is employed to abate the radiated noise. The results demonstrate that SPLs inside and outside the unbaffled long enclosure are significantly decreased. Aiming at suppressing the peaks in the SPL spectra of the sound fields, Helmholtz resonators (HRs) are proposed to reduce the modal responses inside the long enclosure so that the radiated SPL field around the targeted frequencies are suppressed. A hybrid method based on the finite element method (FEM) and the W-H technique is established for the purpose of dealing with discrete noise control devices mounting on the enclosure wall. The mechanisms of using HRs to suppress the SPL peaks are then explored using the hybrid method. In addition, the interaction between the HRs and the acoustical field inside the long enclosure is investigated. The HR locations, optimized to achieve a high sound reduction, are obtained. Numerical results demonstrate that noise reduction can be achieved inside and outside the long enclosure around the targeted frequencies with an appropriate number and locations of HRs. To attenuate the higher-order acoustical modes inside an unbaffled long enclosure and achieve a broadband sound absorption performance, A Z-shaped micro-perforated panel absorber (ZMPPA) is proposed. To calculate the sound absorption coefficient of a ZMPPA under an oblique plane-wave incidence, an FEM-based numerical method is established. The acoustical performance of a ZMPPA is compared with that of flat and corrugated MPPAs. Numerical results demonstrate that the ZMPPA outperforms the others, especially at the first dip and middle-frequency range of the sound absorption coefficient curve. Parametric studies are carried out to study the effects of corrugation depth, offset distance, and the incident angle on the sound absorption performance of the ZMPPA. Besides, a liner consisting of an array of ZMPPAs is employed to reduce the noise radiated from an unbaffled long enclosure including the ground. Satisfactory insertion loss is obtained. In addition to theoretical formulas, FEM-based simulation results are presented to validate the proposed models. Indoor and outdoor experiments are also implemented to figure out the spectrum characteristics of environmental noise. Furthermore, a scaled-down quasi-two-dimensional test rig is developed. The theoretical models are verified and the sound attenuation performance of HRs and ZMPPAs are investigated using the experimental results. |
| Rights: | All rights reserved |
| Access: | open access |
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