Reflection/Diffusion vs Diffraction around a pole

In my basement studio control room I have a round support pole about 3 feet to the left of my main listening position. the pole is roughly 4.15" in diameter. It is not in the way of either of my speakers, however I want to wrap it with rigid fiberglass pipe wrap to prevent sound reflections off this pole from reaching my ears. Does anyone know what frequencies i should be concerned about here? i.e. At what frequency will the sound waves stop diffracting around the pole and begin to reflect/diffuse off of it? I've found no way to calculate this yet (not even in the Master Handbook) besides obviously looking at the physical dimesions of the sound waves, but I know better than to assume what a soundwave might do without some hard science backing me up. Any help or resources appreciated.

2 kHz and higher. Andre

Andre, just for future reference, how did you calculate or approximate that? thank you Hbomb

Wavelength of 4" ~3 kHz plus a half octave or so as the pole does not stop reflecting right at the wavelength. Andre