Anomalous Negative-Pressure Effect of Ultrasound

当障碍物放置在超声场中时，即使没有声流，障碍物也会受到单向的力，这种力称为声辐射压力或辐射力。辐射力的方向通常与入射波（斥力）的方向相同。另一方面，我们有时在球形波场和驻波场中观察到引力和斥力：这种引力据说是由于异常负压效应造成的。虽然已经证明在理想的平面进行波场中不存在负力，但是产生异常负压效应的原因还没有得到阐明。本研究的目的是对有关现象作一个完整的说明。本文首先对声辐射压力的典型理论进行了溯源，并建立了统一的声辐射压力理论。验证了球形声波负压效应的原因归结于伯努利定理本身，并阐明了驻波场负压效应发生的条件。此外，我们已经证明，即使在有限维声源的近场辐射中也会发生相应的效应。结果表明，由于声源本身的衍射效应引起声压的急剧梯度，圆形活塞散热器近场内的小而轻的障碍物通常会受到负力的作用。研究发现，出现异常负压效应的条件与声源的尺寸、与声源的距离、障碍物的密度和大小有关。

When an obstacle is placed in an ultrasonic field, it experiences a unidirectional force even if there is no acoustic stream, which is called the acoustic radiation pressure or radiation force. The direction of the radiation force is ordinarily in the same direction as the incident wave (repulsion force). On the other hand, we sometimes observe a force of attraction as well as repulsion in a spherical wave field and a standing wave field: such an attraction force is said to be due to the anomalous negative-pressure effect. Although it has been proved that no negative forces occur in an ideal plane progressive wave field, the causes of the anomalous negative-pressure effect have never been elucidated yet. The purpose of the present research is to give a complete account of the phenomenon concerned.We first have traced in the present work the typical theories for the acoustic radiation pressure back to their principles and have established a unified theory for it. We next have verified that the cause of the negative-pressure effect of a spherical sound wave is attributed to Bernoulli's theorem itself, and clarified the conditions in which the negative-pressure effect occurs in a standing-wave field. Furthermore, we have shown that the corresponding effects can occur even in the nearfield radiated from a finite dimension of sound source. The results shows that, in general, small and light obstacles in the nearfield of a circular piston radiator experience negative forces due to the sharp gradient of sound pressure caused by the diffraction effects of sound source itself. It has been found that the conditions for the appearance of the anomalous negative-pressure effect depend on the dimension of the sound source, the distance from the source, the density and size of the obstacle.