In order to mitigate underwater noise caused by pile driving, bubble curtains are widely used during offshore constructions. These form an impedance barrier to the acoustic pressure wave, and the resulting attenuation on the interface of water to bubbly water is the main driver behind the insertion loss that can be achieved. A secondary effect is energy absorption by bubble resonance; however, normal bubble sizes are such that the resonance frequency is considerably higher than the peak in the piling noise spectrum, rendering the resonance contribution to the overall insertion loss relatively small. The broadband insertion loss and sound level reduction spectra of bubble curtains are mainly determined on an empirical basis, comparing actual project data and noise monitoring results across sites. Although several efforts have been made to capture the noise mitigation by bubble curtains in numerical models, there is no straight-forward integrated modelling method available to quantify the influence of individual design and operational parameters. Using a number of assumptions and simplifications, this paper presents an analytical model for the frequency-dependent and broadband insertion loss achieved by bubble curtains, that combines both impedance and resonance effects.