Is it true to say that round ventilation ducts with a single gasket generally achieve airtightness Class C?

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Although this seems to be true, this is a dangerous generalization. There are many factors that govern airtighness of round ductwork, including roundness (any slight ovality can cause leakage at the joints) and the flatness of the seams at the joints between two ducts (protuding or convex seams can cause point leakages at the joint irrespective of the gasket type).