In this paper, experimental data on the propagation of acoustic energy in the vicinity of a vertical chain of discrete air bubbles are presented. The acoustic energy was created naturally during the formation of each bubble at the bottom of the chain. Previous work has reported that the root-mean-squared pressure distribution is highly anisotropic in the vicinity of a bubble chain. A new experimental set-up has been developed to obtain 'snapshots' of the instantaneous acoustic pressure field using a triggering technique with two hydrophones. This methodology allowed coordinated measurement of the acoustic signal in the near and far field and the data were used to construct the instantaneous spatial distribution of acoustic energy around the bubble chains. The results show that the phase speed in the direction of the bubble chain has values substantially lower than the speed of sound in pure water. Bubble chains of different configurations were investigated and it was found that this speed of propagation is reduced for chains consisting of larger and more closely spaced bubbles.
Journal of Sound and Vibration,
Vol. 306, no. 3-5 (Oct 2007), pp. 507-523