北南海陸坡沙丘地形之三維水下音傳之研究

Study of Three-Dimensional Underwater Acoustic Propagation in the Continental Slope of South North China Sea with Sand Dunes

葉翰融(Han-Jung Ye)

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(2015年)

https://doi.org/10.6342/NTU.2015.10405

摘要

在北南海陸坡上，存在許多因海洋內波所造成的特殊地形-沙丘(Sand Dune)。這些特殊地形關係，當聲音在水下傳導時，會產生強烈的散射與耦合效應。本研究運用三種不同聲學模式(二維RAM、Nx2D及三維FOR3D)探討聲音在海床沙丘地形上之音傳特性及分析南海沙丘在同調時間內對水下聲音傳遞所造成的三維耦合效應。在數值模擬方面，預觀察水平方向音傳變化所造成的耦合效應(θ-coupling)故採用以三維拋物線方程式為基礎的三維數值(r, z, θ)來模擬。實驗聲源採用移動式聲源(EMATT)拍發訊號，並利用錨定串接收訊號。由於相位的擾動對聲壓能量會造成顯著影響，故模擬時須考慮複數聲壓之變化，並定義當複數聲壓之能量下降一分貝(dB)所需的這段時間，稱之為同調性時間(Coherence time)。
使用由美國海軍研究院(Naval Postgraduate School)合作之實驗室所提供之聲學實驗資料與本研究所得之數值模擬計算進行比對，發現其一，地形環境之改變對音傳損耗及同調性時間影響甚大; 其二，當聲源以接收器為圓心進行圓周運動時，由模擬與實驗均可發現，音傳損耗會隨沙丘地形之變化造成顯著之差異; 其三，當音傳路徑與沙丘走向垂直時，同調性時間越長，但從同調性時間裡，並不能看出三維效應; 其四，當音傳路徑與沙丘走向平行時，由FOR3D之結果可觀察出顯著之三維耦合效應。雖然模擬與實驗數據上，仍有不少差異值，但比對之下FOR3D仍與實驗資料趨勢較符合，未來若能加入更多的詳細的海洋環境條件，修改FOR3D內初始條件以及更新計算軟體上矩陣記憶體的限制，必能可更符合實際海域狀況。

關鍵字

水下音傳 ； 三維擾動 ； 沙丘 ； 移動聲源 ； FOR3D ； 同調性時間 ； 音傳損耗

並列摘要

On the continental slope of Northern South China Sea, there are many special terrain, Sand dune. Because of this special topography, when the sound conducts underwater, it will be affected by these sand dunes and cause strong scattering and coupling effects. In this study, three different acoustic models (2D: RAM, Nx2D, 3D: FOR3D) are used to investigate the acoustic characteristic on seabed-dune terrain, and analyze coupling effect caused by the conduction under South China Sea within the coherence time. In numerical simulation, we use three-dimensional value(r, z, θ) that based on three-dimensional parabolic equation to observe the coupling effect (θ-coupling), which produced by horizontal acoustic transformation. We used mobile sound source (EMATT) during the experiments, found that the phase perturbation will make significant impact on the variation of sound pressure level (SPL). To observe the phase perturbation need to notice the complex of sound pressure and the coherence time, which defined as the duration when SPL decreases 1dB.
From the comparison of simulation and experimental data. First, we found the topography of sand dunes will affect the TL and coherence time a lot. Second, the model and experiment display that there has significant difference in TL along with change of topography. Third, when the propagating way vertical with the sand dune, the coherence time is longer and can’t be find coupling effect. Forth, when the propagating way parallels with the sand dune, there has significant coupling effect from the simulation of FOR3D, In the future, plus more ocean environment, improving the initial condition in FOR3D, updating the software, correspond the actual situation of the ocean more.

並列關鍵字

3D effect ； Sand dune ； FOR3D ； Coherence time ； Transmission Loss(TL)

參考文獻

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