Anomalous Negative-Pressure Effect of Ultrasound

当障碍物被放置在超声场中时，即使没有声流，它也会受到单向的力，这称为声辐射压力或辐射力。辐射力的方向通常与入射波(排斥力)方向相同。另一方面，我们有时在球面波场和驻波场中观察到引力和排斥力：这种引力被认为是由反常负压效应引起的。虽然已经证明理想的平面行波波场中不存在负力，但反常负压效应的原因一直没有得到阐明。本研究的目的是对这一现象进行全面的描述。本文首先对声辐射压强的典型理论进行了溯源，并建立了统一的理论。其次，我们证明了球面声波负压效应的产生归因于伯努利定理本身，并阐明了驻波场中产生负压效应的条件。此外，我们还表明，即使在有限维声源辐射的近场中，也可以发生相应的效应。结果表明，一般情况下，由于声源本身的绕射效应导致声压的急剧梯度，圆形活塞散热器近场的小而轻的障碍物会受到负力的作用。发现出现反常负压效应的条件取决于声源的大小、距声源的距离、障碍物的密度和大小。

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.