We report on an experimental study of the impact of a water drop on a liquid surface in the regime of the so-called irregular entrainment. The hydrodynamics of the phenomenon has been correlated finely to the features of the acoustic signal, both underwater and in the air, thanks to the synchronization of images and sounds in a home-made setup. If the origin of the acoustic signal is known to be caused by the capture of a bubble during the hydrodynamic flow following the impact, for the first time, a new mechanism responsible for the formation of the air bubble is highlighted. The latter is caused by the closing, like a liquid zipper, of the cavity induced by the retraction of the Rayleigh jet, by a secondary droplet detached from this jet. The comparison of the experimental data with the Minnaert model and plane wave theories reveals: (i) the time-dependence of the instantaneous oscillation frequency, (ii) a dominant frequency about 30% higher than the Minnaert prediction, (iii) a higher damping characteristic time, and (iv) a two orders of magnitude higher water–air transmission coefficient. All these results can be explained by the proximity of the bubble to the air–water interface, and by the too small dimensions of the tank to avoid underwater echoes in the measured underwater signal.

中文翻译：

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关于液滴撞击液面的空中和水下声音机制的新见解

我们报告了在所谓的不规则夹带情况下水滴对液体表面影响的实验研究。由于自制装置中图像和声音的同步，这种现象的流体动力学与水下和空中的声学信号的特征密切相关。如果已知声学信号的起源是由撞击后流体动力流动过程中捕获气泡引起的，则首次突出了导致气泡形成的新机制。后者是由瑞利射流的回缩引起的空腔的闭合，就像液体拉链一样，由与该射流分离的二次液滴引起。实验数据与 Minnaert 模型和平面波理论的比较揭示：(i) 瞬时振荡频率的时间依赖性，(ii) 主频率比 Minnaert 预测高约 30%，(iii) 更高的阻尼特性时间，以及 (iv) 两个数量级的水-空气传输系数。所有这些结果都可以通过气泡靠近空气-水界面以及水箱尺寸太小而无法避免测量的水下信号中的水下回波来解释。