Acoustic study of a water drop impact on a water surface
Résumé
The impact of a liquid drop on a liquid surface produces a recognizable sound. Although the generation of this sound is related to the underwater entrapment of vibrating air bubbles, most previous studies focused on the hydrodynamic aspect but very little on the acoustic one. Depending on the drop diameter and its impact velocity, e.g., several regimes for bubble production are reported. The so-called "irregular entrainment" occurs for large drops at high impact velocities. The latter received less attention because of its non-repeatability and its randomness but represents the main topic of this study. We study the acoustic signal and the hydrodynamic behavior of the liquid during an irregular entrainment. Hydrophone and microphone recordings are synchronized with a high-speed video camera. Time and frequency analysis are performed on the acoustic signals whereas the interface features are highlighted thanks to an image tracking algorithm. We present a detailed analysis of the acoustic signal in water as well as in air considering both the apparent frequency, the intensity and the damping factor. The confrontation of the results with existing models led us to propose a new mechanism that can explain all the observed phenomena.