Anomalous Scattering of Ultrasonic Pulse

许多关于超声散射的研究都是通过分析障碍物的散射波来获得有关障碍物的几何特征或物理性质的信息。在这种情况下，爬行的海浪有时会以一种神秘的方式产生有害的影响，它的行为就像它们绕过障碍物的背面后反射一样。这项研究的目的是通过将爬行波现象公式化，发展出一种可与数值和力学实验相媲美的理论。这种理论的发展出人意料地困难：它需要许多数学知识，如索默菲尔德-沃森变换和最陡下降法。现有的理论仅限于完全理想的情况，如平面波和斜波，而超声波由于其波长较长而比光波具有更大的相干性。本文首先用一种新的方法进行了数值实验，将它们与刚性圆柱和刚性球体周围蠕动波的速度和衰减常数的实验数据进行了比较。我们对现有的理论进行了修改和完善，消除了其中的许多含糊不清之处。由此得到的理论与实验结果很好地吻合。接下来，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行波理论。得出了以下结论。蠕变完全是由超声波的绕射引起的。因此，在无线电波的情况下，它可以被视为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.