Abnormal scattering of ultrasound

本研究的目的是调查我们所谓的异常散射的声波散射体的固体材料发生在频率范围从1到20 MHz的自由振动的共振频率附近。在大多数情况下，自由放置在超声场中的物体在波传播方向上经历时间平均力。我们把这种单位截面上的力称为声辐射压力，它是由声音能量密度的空间差异引起的，与散射本身密切相关。针对传统的声辐射压力理论只考虑理想场（如声波的球面波场）的情况，本文提出了一种新的理论，用于计算在圆形活塞振子近场中自由放置的固体弹性球上的声辐射压力。本文介绍了从本研究中得出的基本结论。1.异常散射的模式不依赖于散射体的形状，无论是球形还是圆柱形。2.散射体的质量密度对散射图样的影响很小，可以忽略不计。3.纵波和横波速度的影响最为强烈。我们可以根据散射模式将材料分为四类：（1）铋和铅等材料，其共振现象主要表现为一系列明显的极大值;（2）镉和锡等材料，其共振现象既表现为极大值又表现为极小值;（3）铝和镍等材料，其共振现象主要表现为一系列极小值;（4）铍等材料，其共振效应最小。

The purpose of the present study is to investigate what we call the abnormal scattering of sound waves by scatterers of solid material which occurs over the frequency range from 1 to 20 MHz in the vicinity of the resonance frequencies of free vibration. An object placed freely in an ultrasonic field experiences a time-averaged force in the direction of wave propagation in most cases. We call such a force per unit cross section the acoustic radiation pressure which is caused by the spatial difference in energy density of sound and is closely related to the scattering itself. We have developed, in the present study, a new theory for the acoustic radiation pressure on a solid elastic sphere placed freely in the nearfield of a circular piston vibrator, while the conventional theories for the acoustic radiation pressure have considered only ideal field such as the spherical wave field of sound. The basic conclusions which were drawn from the present study are presented here. 1.The patterns of the abnormal scattering do not depend on the shape of scatterers whether it is spherical or cylindrical. 2.The effect of the mass density of scatterers on the scattering pattern is negligibly small. 3.The effects of the velocity of compressional and shear waves were most intensive. We can classify materials into four groups according to the scattering pattern; (1) materials such as bismuth and lead, for which the resonance phenomena are manifested mainly as a sequence of pronounced maxima; (2) materials such as cadmium and tin for which the resonance phenomena are manifested as both maxima and minima; (3) materials such as aluminum and nickel,for which the resonance phenomena are manifested mainly as a sequence of minima; (4) materials such as beryllium, for which the effects of resonance are minimized.