Ok, stupid question. I have noticed most of the time that the airspace of a sub is measured in cu feet. Looking at some sites like adireaudio.com, it does not measure airspace in cu feet or at least I don't see it. What is the value that I need to look for on these tech sheets that tells me the airspace? Is it the Sd measurment? Also, if that is the case, what is the conversion to cu feet? Thanks.