I’ve never heard ATC. Maybe they’re onto something? But I’m skeptical. As Tom Thiel wrote, the steep filter lets you operate a driver in the range of pistonic behavior. This is *highly* desirable (and also requires diaphragm material up to the task). But this throws off phase coherence. There is no free lunch. If there were, all designers would hone in on the same design.
Thiel Audio placed phase and time coherence as a top priority. The downside is that the slow rolloff may not sufficiently suppress the inevitable break up modes. As I’ve written in this thread, I think Jim Thiel made some of the best drivers around. The diaphragms are light and rigid and the break up modes are out of the "main" region covered by each driver. Even at that, some may consider the break up modes insufficiently suppressed by a 6dB filter. Richard Vandersteen seems to have taken this even farther with his carbon/balsa drivers but you need some serious coin to move up to those. The carbon midrange is available only in the 5 Carbon ($30+K) and 7 models ($60+K).
IMO, most of the newer Thiels get it right in terms of balancing phase alignment and pistonic behavior (I’ve heard CS2.4, 3.7 and 7.2 but not earlier models). My ears tell me so, and Soundstage’s measurements of the CS2.4 confirm "very low" distortion despite the 1st order filters. Nothing is perfect but I think Thiel gets you most of the way there and at an affordable price.
That said, if ATC or others have figured out how to maintain phase alignment while also optimizing pistonic behavior I’m happy to learn! I'm not an audio engineer, either. More of a dork with a soldering gun :)
