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White-light coronagraphic images of Coronal Mass Ejections (CMEs) observed bySOHO/LASCO C2 have been used to estimate the density jump along the whole frontof two CME-driven shocks. The two events are different in that the first onewas a "radio-loud" fast CME, while the second one was a "radio quiet" slow CME.From the compression ratios inferred along the shock fronts, we estimated theAlfv\'en Mach numbers for the general case of an oblique shock. It turns outthat the "radio-loud" CME shock is initially super-critical around the shockcenter, while later on the whole shock becomes sub-critical. On the contrary,the shock associated with the "radio-quiet" CME is sub-critical at all times.This suggests that CME-driven shocks could be efficient particle acceleratorsat the shock nose only at the initiation phases of the event, if and when theshock is super-critical, while at later times they lose their energy and thecapability to accelerate high energetic particles.

Super- and Sub-critical Regions in Shocks driven by Radio-Loud and Radio-Quiet CMEs