氣球幕之水下低頻噪音減噪效益研究

Study of Underwater Noise Attenuation through Balloon Arrays

林天祥(Tien-Siang Ling)

指導教授 ： 陳琪芳

國立臺灣大學/工學院/工程科學及海洋工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.10339

摘要

隨著石油能源逐年減少以及綠色能源不斷增加的需求下，促使了離岸風力能源的發展。但是離岸風力遴選場址也多位於離岸較淺水域處，同時此處也是多種生物的棲息之地，水下噪音對海洋環境的影響與否也引起了關注。
在海事工程中，所產生之低頻高位準之聲壓強度對水中生物，如哺乳類、魚類等皆造成聽力傷害甚至死亡[1]。為了減少水下噪音對生態的衝擊以及保護水下哺乳類的生存環境，本研究透過氣泡之共振現象(Bubble Acoustic Resonance Phenomena)以及聲學阻抗不匹配(Acoustic Impedance Mismatching)噪音減量防治工法來達成噪音減量目的。在離岸風力海事工程噪音減量工法上，氣泡幕減噪工法在國際間為常用的工法且施工成本較其他工法低廉。但是，氣泡在實際海洋中的穩定性，會隨著海流、潮汐等因素而導致破裂，另外施工多為低頻噪音，使應用氣泡幕減噪工法對於低頻噪音之減量不盡理想[2]，因而，改以相對於氣泡更為穩定之氣球幕取代，通透過Commander and Prosperetti模式了解氣球尺寸以及體積分率對聲波的衰減，著手設計氣球參數並且嘗試從眾多控制因子找出最佳參數組合，在台大工科拖俥水槽實驗(130m * 4m * 8m)，探討聲波通過氣球幕之效益以提供未來實際海域減噪工法的應用。
在本實驗研究中，氣球半徑為5公分(cm)在體積分率為(VF=0.022)可在低頻(100 Hz~1000 Hz)達到5~25dB左右之減噪效果。體積分率(VF=0.022) 之減噪效果也較體積分率(VF=0.012)好。同時，氣球幕放置1公尺(m)處之減噪效果較氣球幕放置3公尺(m)處佳。

關鍵字

低頻噪音 ； 共振現象 ； 聲學阻抗不匹配 ； 減噪工法 ； 氣球幕

並列摘要

The excessive consumption of the petroleum and the demand for renewable energy are growing year by year, it has led to construction of offshore wind farms with high-power turbines, at the same time wind farms are being planned for the shallow water. A high productive marine life often characterizes such near shore habitats, so it has raised concerns about their impact on the marine environment.
A high sound pressure level from underwater low-frequency anthropogenic noise generated by offshore wind farms is harmful to marine life, with examples including possible changes to marine mammal migratory patterns and interference with inter-animal communication, predation. Thus, it is important to minimize the impact of noise for the marine life environment. This study is to make use of the acoustic properties such as bubble acoustic resonance phenomena and acoustic impedance mismatching to reduce the noise. Bubble curtain commonly used to reduce the noise due to low operation cost. However, bubble curtain is easily disrupted and carried away by the tides, waves, and the attenuation in specific low frequency is not effective due to the large size of the bubbles are unstable, so we propose balloon array method to be used. In this paper, the Commander and Prosperetti’s effective medium model used to understand the characteristic of bubbles, and to optimize the parameters and investigate the effect of balloons array at specific frequency range. In this work, a series of the experiments were performed in tow water tank (130m*4m*8m) of the large balloon arrays and figure out their efficiency in reducing low-frequency underwater noise through the balloon arrays.
In this study, we figure out there has 5~25dB noise reduction at the 130~1000 frequency range for void fraction 0.022 and balloon radius 5cm. The noise reduction much better if increase the void fraction from 0.012 to 0.022. Moreover, the noise reduction if placed a balloon array at 1m more efficiency compared with 3m.

並列關鍵字

low-frequency noise ； resonance phenomena ； impedance mismatching ； balloon arrays

參考文獻

[1] P. T. Madsen, M. Wahlberg, J. Tougaard, K. Lucke, and P. Tyack, "Wind turbine underwater noise and marine mammals: implications of current knowledge and data needs," vol. 309, pp. 279-295, 2006.

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[3] 周蓮香 and 李政諦, "中華白海豚棲地熱點評估及整體保育方案規劃," 行政院農業委員會林務局民國100年.

[4] K. W. Commander and A. Prosperetti, "Linear pressure waves in bubbly liquids: Comparison between theory and experiments," J. Acoust. Soc. Am., vol. 85, pp. 732-746, 1988.

[5] A. Mallock, "The Damping of Sound by Frothy Liquids," pp. 391-395, 1910.