I'm planning to buy some Sonotube for a sub. Since I've never purchased this stuff before I would like to know if the diameter measurement is an interior or exterior measurement.
It's a bit variable. The best way is to measure the outer circumference (use a piece of string and then measure the string). Once you have the circumference, you divide by 2*pi (around 6.2832) to get the radius to the outside of the sonotube. Multiply by 2 to get the diameter.
If you need the inner radius, subtract the measured thickness of the tube from the original outer radius value. Multiply that by 2 to get the inner diameter value.
I think you misunderstand. I'm speaking of the manufacturers specifications and what they mean when they say, for example, 14 inch tube. Is that 14" the interior diameter or the exterior diameter.
Remember that this stuff is originally designed to pour in concrete. The vendor is going to tell you the diameter of the CONCRETE when its done, which, logically would be the inside diameter (since when the concrete is hard, you basically remove the sonotube). As for "variable", since this is construction, not sculpture, they are going to be a bit "variable" though more than 1/4" off would seem to be excessive since all they are doing is wrapping reinforced paper around a mold...
The variance may be different for each manufacturer. I would think that all Sonotube brand tubes are pretty close to the same whereas other manufacturers have a different tolerance. It also may be similar to lumber. We all know that a 2x4 isn't really 2x4. I guess I'll know for sure tommorrow. I picking up some "14 inch" tube to build a 6 foot sonic cannon that will lay horizontally in the back of my room. Should be interesting. I'll let you know how it goes.