【摘要】 本文所采用的激光宽带声谱仪包括Ｎｄ：ＹＡＧ激光，光声转换介质（水银或蓝玻璃），宽带超声换能器，瞬态波形数字化仪和计算机系统．超声脉冲源有单极性型和双极性型二种，谱仪的有用超声谱范围为２．５～５０ＭＨＺ．本文研究了几个有机玻璃样品，有机玻璃的声吸收随频率变化为线性关系，这结果与哈脱曼所得结果一致．他采用通常的超声谱方法，最高频率为３０ＭＨｚ．更多还原

【Abstract】 In this paper a new technique for studying polymer has been presented. Thet6chnique is called laser excited wide-band acoustic spectroscopy with beatrues of havingvery wide bandwidthh. It is diffiCult to obtain anything wider than 50MHz by usingconventional ultrasonicspectrum technique. However, it is easy to obtain something widerthan 50MHz band width for laser excited wide-band acoustic spectroscopy.The new technique is based on the newly developed theory of thermoelastic sound andexperiment results obtained. According to the theory it is anticipated that when the blueglass or mercury is used 4s the optoacoustic transform material, ultrasonic signals will beexcited at the free surface of the blue glass or th surface of mercury which is located at theback side of a polished SiO2 plate owing to pulsed laser irradiation .The profile of theultrasonic signal (OA-Pulses) at the surface of mercury or blue glass has a single-polarityor bi-polarity shape, respectively. For the YAG pulsed laser irradiation (τ≈ 8ns). thewaveform of the OA-pulses excited in mercury coincides with that of laser. Thesephenomena have been observed in our experiments.In our experiment the OA-pulses are detected with the aid of wide-band ultrasonictransducer produced by the international Laser Center of Moscow state University. Thedetected signal was digitized with a time resolution of half nenosecond. It is then transferred to a computer to be displayed, stored and further processed. Two plyxiglass sampleswith the thickness of 3.8 ±\ 0.1m and 7.5 ± 0.1mm have been investigated. The experimmentis performed under two different conditions: (1) Optoacoustic transform material is mercury and the couplig medium is mercury too; (2) Optoacoustic transform material is blue glassand the coupling medium is silicone oil or water.The setup of the experiment has been described in detail in the paper. In our experiment, a YAG pulsed laser is used as excited source of OA signal.The ultrasonic transduceroutputs (with and without the sample) are recorded and digitized by a wave-transient digitizer (type I9-2), in the time domain. The outputs of digitizer are processed by a computer. First the FFT is carried out. From the spectrum given by FFT, the absoptioncoefficient of the sample in frequency domain can be calculated with knowing ultrasonictransducer sensitivity, sample length, coupling medium length and absorption conefficientacoustic transmission at the boundary between the coupling medium and sample.The experiments show a linear frequency dependence of sound absorption α forplyxiglass. It is consistent with the result of Hartman et al. who did the measurement at thefrequency up to 30MHz. In our experiments, the frequency range has been extended up to50Mz under the experiment condition (1), while the frequency range is 2.5 to 20 MHz underthe experiment condition (2).Finally,the authors believe that the laser excited wide-band acoustic spectroscopy isvery usefull for studying polymer, as well as other materials. And it is a new effective inspection tool for materials NDE.更多还原