Bubble curtains for noise mitigation: One vs two

Bubble curtains are widely used to protect marine life from exposure to harmful noise during offshore pile driving. However, operating a bubble curtain is costly and compliance with government noise regulations remains a challenge. It is therefore important to optimise the acoustic effect of the available compressed air. An interesting approach to achieving this is to split the air flow rate into two separate bubble curtains, rather than one single bubble curtain. This concept is tested both experimentally and numerically in this paper. The experiments and the model show the expected increase in performance of the supplied compressed air when it is split between two manifolds. An increased insertion loss of up to 11 dB is measured. This increase in performance is possibly due to the fact that the reflective properties of the bubble curtains are maintained even when the air flow rate is halved. In effect, by splitting the air flow between two manifolds, a second acoustic barrier is added. Additionally, the variations in the bubble curtain performance between individual measurements are shown to be largely caused by temporal variations in the air distribution within the curtain. The applicability of equivalent fluid models for bubble curtains is discussed, and it is shown that accounting for a gap in the bubble curtain, close to the manifold where the bubble curtain is not yet fully developed, results in better agreement between the modelled and the measured values of the insertion loss.