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.
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