Does anybody have experience and data concerning Isobaric loading two woofers in a tapped horn, the Cubo Sub in particular?
I recently completed building a Cubo-Sub utilizing the Eminence Pro-18" woofer and after break-in we used the sub in a tri-amped PA for an outdoor gig. Results were fantastic. Great design. We have a second Eminence Pro-18 and also two Eminence Kappa-15LF drivers on hand and I started thinking about the benefits of Isobaric loading. Considering Isobaric loading benefits (textbook) are a 10-20Hz lower F3, distortion figures are halved, Vas is halved but we get a 3db decrease in efficiency, I did find discussion on this,...mostly negative because of the 3db hit. But, is it worth it to gain cleaner and deeper response???
The main feature of isobaric loading is halving Vas of the driver but keeping most other parameters equal. If you have reflex cabinet you can now halve the chamber volume to get a similar performance at the cost of 3 dB output. To keep the tuning of the reflex the same with halve the chamber volume means making the port larger to compensate. So the enclosure benefiting most from this arrangement has a large chamber and a very small port.
A Cubo Sub is 25% rear chamber, 75% is horn/ port, so the main benefit is kinda lost already. To maximize the impact of isobaric loading is to maximize the effect of the chamber size, for the output of the cabinet. Besides, the Vas of a Kappa 15LF isn't exactly large, so making it smaller isn't a real benefit. You're probably creating a dip in the part where the output relies solely on the driver (around 50 - 55 Hz).
I do have a Cubo Sub designed to fit two 15" drivers, that way you'll actually create a peak between 50 - 80 Hz rather then a dip at 50 - 55 Hz. That being said, the Kappa 15LF aren't exactly sub drivers, so they might be better of in a Cubo 15 or Cubo Kick 15.