自由落體撞擊水面過程中所形成之聲音特性是經常被探討的研究主題之一。過去文獻中較多研究水滴入水時的聲音特性，但是較少文獻提及固體撞擊液體之聲音特性。由於固體撞擊液體的瞬間現象太過複雜，為了減少實驗變因，本研究以最基本的自由落下之球體為主軸，探討球體衝擊水面至觸底前所引發的聲音特性，以及在不同高度、不同尺寸、不同球面粗糙度及不同固體形狀條件下，所帶來之影響。本實驗欲分析球體撞擊水面過程中水聲之時域訊號及頻域訊號，並找出球體入水瞬間之訊號特性。聲場頻譜特性的分析方法為快速傅立葉轉換 ( FFT )。本實驗在成大水利系光纖感測及聲學實驗室的透明水槽中進行。實驗架構以鋼球自懸吊裝置自由落入水中，並利用自動控制程式同步驅動高速攝影機以及水下麥克風進行量測。聲音取樣頻率為100 kHz，高速攝影機取樣張數達每秒 2000 張。本實驗結果經比對聲音訊號與影像資料，發現其物理現象與訊號特性具高度一致性。本研究除了瞭解球體撞擊水面之脈衝水聲訊號特性外，未來可以應用於實海域演練與港區防護等議題。

The sound characteristics resulted from the freely falling object impacting on the liquid surface have been discussed for several decades. Most of the investigations were concentrated on that the water droplets interacted with the sea surface. Literatures hardly mentioned that the sound characteristics of a solid object impacted on the liquid surface as a result of the complicated phenomena of the impacting process of a solid body within liquid. This study is to identify the sound characteristics of the solid object impacted on a liquid surface by using freely falling spherical stainless steel balls to reduce the uncertainties in this experiment. The sound characteristics of a spherical ball impacting water surface in different weights, heights, surface roughness and appearances were examined. Experiments were carried out implemented in a transparent water tank in the Department of Hydraulic and Ocean Engineering. Stainless steel spheroids with different sizes were released by an electromagnet, and the impacting photographs and acoustic signals were measured by a high-speed camera and hydrophones respectively. Therefore, both of instruments were simultaneously triggered by a desktop computer. Fast Fourier Transform (FFT) was implemented to analyse acoustic data within the time interval of the spheroid from the water surface to the bottom of the water tank. The results of this investigation indicate that the acoustic method could be a practicable technique to understand the phenomena of a solid object impacting on the liquid surface even though the liquid is opaque.

1.Cooley, J. W. and Tukey, J. W., "An algorithm for the machine calculation of complex Fourier series," Math. Comput., Vol. 19, pp. 297-301, 1965.
2.Duez, C., Ybert, C., Clanet, C., and Bocquet, L., “Making a splash with water repellency,” Nat. Phys., Vol. 3, pp. 180-183, 2007.
3.Duclaux, V., Caillé, F., Duez, C., Ybert, C., Bocquet, L., and Clanet, C., “Dynamics of transient cavities,” J. Fluid Mech., Vol. 591, pp. 1-19, 2007.
4.Franz, G. J., “Splashes as sources of sound in liquids,” J. Acoust. Soc. Am., Vol. 31, pp. 1080-1096, 1959.
5.Gilbarg, D and Anderson, R. A., “Influence of atmospheric pressure on the phenomena accompanying the entry of spheres into water,” J. Appl. Phys., Vol. 19, pp. 127-139, 1948.
6.Guo, Y. P. and Ffowcs Williams, J. E., “A theoretical study on drop impact sound and rain noise,” J. Fluid Mech., Vol. 227, pp. 345-355, 1991.
7.Lemon, D. D., Farmer, D. M., and Watts, D. R., “Acoustic measurements of wind speed and precipitation over a continental shelf,” J. Geophys. Res., Vol. 89, pp. 3462-3472, 1984.
8.Laville, F., Abbott, G. D., and Miller, M. J., “Underwater sound generation by rainfall,” J. Acoust. Soc. Am., Vol. 89, pp. 715-721, 1991.
9.Laverty, S. M. Jr., “Experimental hydrodynamics of sphericl projectiles impacting on a free surface using high speed imaging techniques,” Master Thesis, B.S., Mechanical Engineering Brigham Young University, 2003.
10.Mallock, A., “Sounds produced by drops falling on water,” Proc. R. Soc. London
Set. A, Vol. 95, pp. 138-143, 1918.
11.May, A., “Effect of surface condition of a sphere on its water-entry cavity,” J. Appl. Phys., Vol. 22, pp. 1219-1222, 1951.
12.Minh, D. Q. and Gustav A., “The splash of a solid sphere impacting on a liquid surface: Numerical simulation of the influence of wetting,” Phys. Fluids, Vol. 21, pp. 0221021-02210212, 2009.
13.Medwin, H., Kurgan, A., and Nystuen, J. A., “Impact and bubble sound from raindrops at normal and oblique incidence,” J. Acoust. Soc. Am., Vol. 88, pp. 413-418, 1990.
14.Medwin, H., Nystuen, J. A., Jacobus , P. W., Ostwald, L. H., and Snyder, D. E., “The anatomy of underwater rain noise,” J. Acoust. Soc. Am., Vol. 92, pp. 1613-1623, 1992.
15.Minnarert, M., “On musical air bubbles and the sounds of running water,” Phil. Mag, Vol. 16, pp. 235-248, 1933.
16.Nystuen, J. A., “Rainfall measurements using underwater ambient noise,” J. Acoust. Soc. Am., Vol. 69, pp. 972-982, 1986.
17.Nystuen, J. A., McGlothin, C. C., and Cook, M. S., “Underwater sound generated by heavy rainfall,” J. Acoust. Soc. Am., Vol. 93, pp. 3169-3177, 1993.
18.Nystuen, J. A. and Medwin, H., “Underwater sound produced by rainfall：Secondary splashes of aerosols,” J. Acoust. Soc. Am., Vol. 97, pp. 1606-1613, 1995.
19.Prosperetti, A., and O uz, H. N., “The impact of drops on liquid surfaces and the underwater noise of rain,” Annu. Rev. Fluid Mech., Vol. 25, pp. 577-602, 1993.
20.Pumphrey, H. C., Crum, L. A., and Bjørnø, L., “Underwater sound produced by individual drop impacts and rainfall,” J. Acoust. Soc. Am., Vol. 58, pp. 1518-1526, 1989.
21.Plesset, M. S., “The dynamics of cavitation bubbles,” ASME J. Appl. Mech., Vol. 16, pp. 228-231, 1949.
22.Richardson, E. G., “The impact of a solid on a liquid surface,” Proc. Phys. Soc. Vol. 61, pp. 352-267, 1948.
23.Rayleigh, L., “On the pressure developed in a liquid during the collapse of a spherical cavity,” Phil. Mag., Vol. 34, pp. 94-98, 1917.
24.Shonting, D. H. and Middleton, F. H., “Near-surface observations of wind and rain-generated sound using the SCANR：an autonomous acoustic recorder,” J. Atmos. Ocean. Technol,, Vol. 5, pp. 228-237, 1988.
25.Shi, H.-H. and Kume, M., “An experimental research on the flow field of water entry by pressure measurements,” Phys. fluids, Vol. 13, pp. 347-349, 2001.
26.Scrimger, J. A., Evans, D. J., McBean, G. A., Farmer, D. M., and Kerman, B. R., “Underwater noise due to rain, hail, and snow,” J. Acoust. Soc. Am., Vol. 80, pp. 79-86, 1987.
27.Scrimger, J. A., Evans, D. J., and Yee, W., “Underwater noise due to rain-open ocean measurements,” J. Acoust. Soc. Am., Vol. 85, pp. 726-731, 1989.
28.Worthington, A. M., “A study of splashes,” Macmillan, New York, 1908.
29.Wenz, G. M., “Acoustic Ambient Noise in the Ocean：Spectra and Sources,” J. Acoust. Soc. Am., Vol. 34, pp. 1936-1956, 1962.
30.沈建成，「氣泡振動阻尼機制之研究」，國立成功大學水利及海洋工程研究所碩士論文，2002。
31.黃清哲、田宗謨、朱崇銳、黃郁芳、李尹維、林澄貴、羅志正，「自由落體衝擊水面之聲場訊號特性分析研究」，第十二屆水下技術研討會，第 29-34 頁，2009。
32.劉金源，「水中聲學－水聲系統之基本操作原理」，國立編譯館，台北，第 207-211 頁，2001。