中文摘要
本文主要是以實驗方法探討雨滴降落水面產生之噪音及氣泡形成過程。實驗分為兩部分，一部份是以數位式高速照相機拍攝雨滴撞擊水面所產生自由水面之變化，以及氣泡形成之過程。另一部份則是以水中聽音器量測雨滴撞擊水面產生之噪音，並用FFT分析噪音頻譜之特性。不同雨滴大小及高度，對氣泡之生成及噪音特性的影響，本文做了有系統的討論。實驗結果顯示雨滴撞擊水面產生之氣泡有兩種模式。第一種模式為只有單一氣泡發生，此種氣泡文獻上常用來解釋降雨噪音的來源。第二種模式為當雨滴降落水面會形成一向下之射流，此射流會演變成一渦流環(vortex ring)，渦流環上產生一圈成環狀排列的小氣泡。第一種模式氣泡直徑較大，頻率範圍在3~5kHz之間；而第二種模式氣泡之噪音，頻率在8~20kHz之間，且在14kHz左右聲壓最大。在雨滴距離水面較近處降落時，只有第一種模式的氣泡發生；而當雨滴離水面較遠處降落時，有時會出現第一種模式氣泡，有時會出現第二種模式的氣泡。此兩種不同的氣泡模式及其噪音特性，可說明為什麼多雨滴降雨噪音的音頻會有兩個主峰，一在3~5kHz，而另一個則在14kHz附近。最後，不同降雨率產生之降雨噪音特性，本文也做了分析研究。

ABSTRACT
The underwater sound and the bubble formation caused by rain drops were studied experimentally in the present paper. A CCD camera was set up to photograph the drop impact and the process of bubble formation. Underwater noise was received with a hydrophone connected to a charge amplifier and a FFT analyzer for capturing the main characteristics. The experimental results showed that two types of bubbles
are generated as a single rain drop strikes the free surface. The first one is the formation of a single bubble, radiating sound of frequencies ranging from 3 to 5 kHz. This type of bubble has been accepted by many researchers as the main source of rain drop noise. The second one is that as the rain drop impacts the free surface, a downward jet is formed which develops into a vortex ring. Several bubbles are generated along this ring. The latter bubbles are smaller than the former one, hence radiating sounds of higher frequencies, ranging from 8 to 20 kHz, with a peak sound pressure level at about 14 kHz. The first mode of bubble generation happens to rain drops of lower impact velocities. However, as the impact velocities increase, both modes of bubble generation are possible. These two different types of bubbles and the radiated sounds can be used to explain the double peak pressure of the real rain drop noise, one locates at 3-5kHz, the other one at about 14 kHz. Finally, the sound spectra of different rain rates were also studied in this paper.

中文摘要 I
ABSTRACT II
謝誌 III
目錄 IV
表目錄 VI
圖目錄 VII
符號說明 IX
第一章 緒論 1
1-1前言 1
1-2降雨噪音之特性 1
1-3研究目的 3
1-4本文組織 4
第二章 理論分析 5
2-1衝擊脈衝與流體動壓力 5
2-1-1初期研究 5
2-1-2水滴撞擊水面的聲學物理機制 6
2-1-3起始衝擊脈衝（水錘波） 7
2-2氣泡聲音 8
2-2-1氣泡振盪產生的聲音 8
第三章 實驗方法與佈置 15
3-1實驗佈置 15
3-1-1水下聲音接收系統實驗佈置 15
3-1-2 雨滴撞擊過程觀測及記錄系統 16
3-2實驗設備 16
3-3實驗方法 18
第四章 實驗結果與討論 20
4-1背景噪音分析 20
4-2單雨滴降落水面之噪音 21
4-2-1模式I氣泡形成之過程 22
4-2-2 模式I之噪音特性 24
4-2-3模式II氣泡形成之過程 24
4-2-4模式II氣泡噪音特性 26
4-2-5雨滴大小對噪音的影響 27
4-3多雨滴降雨噪音特性 27
第五章 結論與建議 29
5-1 結論 29
5-2 建議 30
參考文獻 31
附錄一 實際聲壓轉換公式 74
附錄二 水中收音器特性圖 75

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