Abstract Passive acoustic signals from the free fall of water droplets onto a liquid free surface are reported experimentally. The signals are co-registered with video images, permitting the association of the signals with various physical-hydrodynamic events such as the initial impact, first bubble entrainment, fragmentation of small bubbles and the entrainment of a second bubble. An interesting acoustical signal is observed when a satellite drop hits a previously formed cavity, creating an oscillating air body which shows great symmetry about the time axis; its envelope is similar to analytical polar-cardioid or ‘heart’ function. The heart signals reach a maximum value when four cycles are completed and then are exponentially damped. The other features of the heart signal are a total duration of ≈20–24 ms and frequencies between 1.7 and 1.8 kHz. Finally, when the cavity-eccentricity is calculated, two regions are observed, corresponding to the presence or absence of the secondary bubble entrainment.

中文翻译：

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心脏信号：在第二次气泡夹带期间观察到的声学特征

摘要 实验报告了来自水滴自由落体到液体自由表面的被动声信号。这些信号与视频图像共同记录，允许将信号与各种物理流体动力学事件相关联，例如初始撞击、第一次气泡夹带、小气泡破碎和第二个气泡夹带。当卫星落点撞击先前形成的空腔时，会观察到一个有趣的声学信号，产生一个振荡的空气体，它在时间轴上表现出极大的对称性；它的包络类似于分析型极心型或“心脏”功能。当完成四个周期时，心脏信号达到最大值，然后呈指数衰减。心脏信号的其他特征是总持续时间约为 20-24 毫秒，频率介于 1.7 和 1.8 kHz 之间。