Anomalous Scattering of Ultrasonic Pulse

许多超声散射的研究都是通过分析障碍物的散射波来获得障碍物的几何特征或物理性质的信息，在这种情况下，爬行波有时会以一种神秘的方式产生有害的影响，表现为它们在绕障碍物背面运行后的反射。研究的目的是通过公式化爬行波现象，发展一种与数值和力学实验相比较的理论。发展这样的理论是出乎意料的困难：它需要大量的数学知识，如索末菲-沃森变换和方法的最速下降。现有的理论仅限于完全理想的情况，如平面波和剪切波，而超声波由于其波长较长，比光波相干性更强.本文首先用一种新的方法进行了数值实验，并与实验数据进行了比较，得到了刚性圆柱和刚性球体周围爬行波的速度和衰减常数.我们对已有的验证理论进行了修正和改进，消除了其中的许多缺陷。所得的理论与实验结果吻合得很好。其次，我们发展了有限维圆形活塞振子近场中自由放置的球体周围的爬波理论。得出以下结论。蠕变完全是由超声波的衍射引起的。因此，在无线电波的情况下，它可以被看作是围绕地球的全球爬行波。

Many of the investigations on ultrasonic scattering intend to obtain information on the geometrical characters or physical properties of the obstacle by analyzing scattering waves from it. In such a case, the 'creeping waves' sometimes exert a harmful influence in the way of a mysterious goast, which behaves as if they reflected after running around the back side of the obstacle. The purpose of the study is to develop a theory which is comparable with numerical and mechanical experiments by formularzing the phenomenon of creeping waves. The development of such theory was unexpectedly difficult : it required much mathematical knowledge such as the Sommerfeld-Watson transformation and the method of steepest descent. Existing theories are restricted to fully ideal cases such as plane waves and sherical waves, while ultrasonic waves are much coherent than light waves because of their long wave-length.In the present study we first have made a numerical experiment by use of a new method to compare them with the experimental data on the velocity and attenuation constant of the creeping waves around a rigid cylinder and a rigid sphere. We have modified and improved the exisiting theories by eliminating much vagueness from them. The resulting theory has shown a good agreement with the experiment. We next have developed the theory of the creeping waves around a sphere placed freely in the nearfield of a circular piston vibrator of finite dimension. The Following conclusion is reached. Creeping is entirely caused by diffraction of ultrasonic waves. Therefore it can be seen in the case of a radio wave as a global creeping wave around the earth.