Exactly. THX implies that some Lucas-blessed engineers went out to the theater and gave them hell to get their sound system set up properly, going as far as having them relocate A/C units on the roof to get the noise floor lower in the THX auditoriums.
Here in Houston, I think there's only one theater left that's THX certified (or was, at some point certified -- it's no longer listed at
THX.com). It was a state-of-the-art AMC 8-room theater back before stadium seating and 30-screen theaters warped the movie business. It's now owned by some no-name company, but it still shows first-run films and tends to be blissfully empty, since most moviegoers seem to insist on stadium seating and 30 screens. I just hope they manage to stay in business, otherwise I won't have a a THX theater to patronize any more.
The official (consumer-comprehensible) definition is at the
THX website. They even have a
snitch page, where consumers can gripe about cinema quality, possibly generating inspections and loss of certification. In a nutshell, the THX people seem to treat theater certification the way that city health inspectors must (hopefully) treat the quality of food concessions.