We evaluated a method of estimating the relative bubble time period of air-gun clusters with an arbitrary number of guns. This was done by assuming incompressible flow and representing the bubbles as isosurfaces of the potential field to account for coalescence. The kinetic energy at the equilibrium radius was then compared to the equivalent energy of the single gun to estimate the relative change. The results agreed well with two-gun cluster measurements, but the lack of data does not allow us to compare with clusters containing more guns than that. We found that more compact configurations, such as a triangle instead of three guns in a line, gave a more rapid increase in the bubble time period as the gun separation decreased. This indicated that compact configurations were attractive for enhancing the low-frequency output from an air-gun cluster.