Abnormal scattering of ultrasound

本研究的目的是研究在自由振动的共振频率附近，在1-20 MHz频率范围内发生的固体材料散射体对声波的异常散射。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会受到时间平均的力。我们称这种单位截面力为声辐射压力，它是由声的能量密度的空间差异引起的，与散射本身密切相关。本文发展了圆形活塞振子近场自由放置的固体弹性球体上声辐射压力的新理论，而传统的声辐射压力理论只考虑了球面波场等理想场。给出了本研究得出的基本结论。1.反常散射的模式与散射体的形状无关，无论散射体是球体还是柱体。2.散射体质量密度对散射方向图的影响可以忽略不计。3.纵波速度和横波速度的影响最大。根据散射模式，我们可以将材料分为四组：(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，其共振影响最小。

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.