本文主要目的為利用蘭姆波性質，藉由實驗和模擬觀察經過疊代後不同材質大小的探測物收斂頻率趨勢，判別水下探測物的性質。研究中使用空心圓管設計模擬和實驗來做驗證，產生的蘭姆波會在相對應頻寬下有最大的能量，並經由疊代時間反轉法去增強蘭姆波的能量以便於辨別物體特性，此外我們可以藉由波的傳遞時間定位探測物的位置。實驗方面發射訊號利用LabVIEW經由水下超音波探頭發射，反射的蘭姆波訊號則由另一個水下超音波探頭接收並在數位示波器上擷取資料和波型。利用疊代時間反轉法聚焦特性，增強接收訊號收斂頻率的振幅。藉由比較不同大小和材質的收斂頻率，結果證明這方法能夠辨別不同的探測物。模擬部份則利用Goodman and Stern定義的方程式去建立殼的模型找出收斂頻率。

The purpose of this paper is developing a procedure to distinguish target characteristics such as material and size by observing convergence frequency through iterative time reversal process. The proposed procedure is verified from designed simulations and experiments by using slim shell circular tubes as targets. These slim shell structures produce lamb waves that contain energy within corresponding frequency bandwidth. The iterative time reversal process is then performed to enhance energy of responding lamb waves and makes it possible to distinguish target characteristics. In addition, target distance can be determined from wave propagation time. In experiments, interrogating signals are controlled by LabVIEW and emitted from an unfocused broadband underwater transducer. Specular echo and responding lamb wave signals are then received by another same type transducer and recorded by a digital oscilloscope. Due to the enhancing effect of iterative time reversal, energy of iterative receiving signals converges to certain frequencies. After comparing these frequencies from varying target material and target size, the result proves that the proposed procedure is capable to distinguish properties of different targets. The simulation of a thin and fluid-loaded elastic shell is computed using the classical theoretical formulation of Goodman and Stern to create shell model and find convergence frequency

1. 劉金源, “水中聲學—水聲系統之基本操作原理,” 國立編譯館, 2001.
2. A.P. Brysev et al., “Phase Conjugation of Acoustic Beams,” Sov. Phys. Acoustics, Vo1.32, 408-410, 1987.
3. M. Fink, C. Prada, F. Wu, and D. Cassereau, “Self Focusing in Inhomogeneous Media with Time Reversal Acoustic Mirrors,” IEEE Ultrasonic Symposium Proceedings, 681-686, 1989.
4. D. Cassereau, F. Wu, and M. Fink, “Limits of Self Focusing using Closed Time-Reversal Cavities and Mirrors-Theory and Experiment,” IEEE Ultrasonic Symposium Proceedings, 1613-1618, 1990.
5. M. Fink, “Time-Reversal of Ultrasonic Field—PartI: Basic Principles,” IEEE Trans. UFFC, Vo1.39, No.5, 555-566, 1992.
6. B.-H. Wu, G.-P. Too and S. Lee, Audio signal separation via A combination procedure of timereversal and deconvolution process, Mech. Syst. Signal Process. 24 (2010) 1431–1443.
7. D. Caesarea and M. Fink, “Focusing with plane time-reversal mirrors: an efficient alternative to closed cavities,” J.A.S.A., Vol.94, No.4, 2373-2386, Oct 1993.
8. D. Caesarea and M. Fink, “Time-Reversal of Ultrasonic Field—PartIII: Theory of the Closed Time-Reversal Cavity,” IEEE Trans. UFFC., 579-592, Sep 1992.
9. C. Prada, F. Wu, and M. Fink, “The iterative time reversal mirror: A solution to self-focusing in the pulse echo mode,” J.A.S.A., Vol.90, No.2, 1119-1129, Aug 1991.
10. R. Hickling, “Analysis of Echoes from a hollow metallic sphere in water,” J. A. S. A., Vol.36, No.6, 1124-1137, Jun 1964.
11. 賴耿陽, “超音波工學理論實務,”復漢出版社, 2001.
12. 陳俊吉, “超音波在UIC60鋼軌旅熱季和道的品質檢測及破壞試驗,”交通部台灣鐵路管理局工務養護總隊
13. P. L. Marston “GTD for backscattering from elastic spheres and cylinders in water and the coupling of surface elastic waves with the acoustic field,” J. Acoust. Soc. Am. 83, 25-37 (1988)
14. S.G. Kargl, and P.L. Marston, “ Observations and modeling of the backscattering of short tone bursts from a spherical shell: Lamb wave echoes, glory, and axial reverberations,” J. Acoust. Soc. Am. 85, 1014-1028 (1989)
15. L. R. Dragonette, D. M. Drumheller, C. F. Gaumond, D. H. Hughes, B. T. O'Connor, Y. Nai-Chyuan, and T. J. Yoder, “The application of two-dimensional signal transformations to the analysis and synthesis of structural excitations observed in acoustical scattering,” Proc. IEEE 84(9), 1249–1263 (1996).
16. D. Pierson, “Buried-object detection using time-reversed acoustics,” Ph.D. dissertation, North Carolina State University, Dec 2003.
17. A. Sutin, J. TenCate, and P. Johnson, “Single-channel time reversal in elastic solids,” J. A. S. A., Vol.116, No.5, 2779-2784, Nov 2004.
18. Z. J. Waters, B. R. Dzikowicz, R. G. Holt, and R. A. Roy, “Sensing a buried resonant object by single-channel time reversal,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56(7), 1429–1441 (2009).
19. R. Goodman and R. Stern, “Reflection and transmission of sound by elastic spherical shells,” J. Acoust. Soc. Am. 34, 338–344 (1962).
20. S.D. Anderson, K. G. Sabra, M. E. Zakharia, and J.P. Sessarego, “Time-frequency analysis of the bistatic acoustic scattering from a spherical elastic shell,” J. Acoust. Soc. Am. 131 (1), (2012)