minimum distance from vent to floor/wall

Imagine a down-firing circular vent, 4 inches diameter. The box is ~ 80 liters, tuned to 22 Hz.

The box will be elevated on short legs. How much clearance is needed between the floor and the box so that the box resonance frequency is not affected?

My thought is 1 diameter (4 inch), based on the area of a 4 inch diameter cylinder 4 inches high... but thoughts?
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