Why does DD say not to use my box building program?
We make the subwoofers to work in our highly efficient box system tailored to the transfer function of a vehicle's interior. In todays high energy audio systems, high amplifier power and the resulting high moving mass force are more dominant factors than enclosure air load effect on the woofer's suspension compliance and motor force. Therefore, we've built and tested countless woofer variations to work in specified enclosure sizes with the best possible sound quality for a given power bandwidth, we build the woofer to work within a box parameter, not the other way around.
In addition, high motor strength, overdamped, controlled suspension woofers are difficult to measure accurate T/S parameters, nature of the beast. We have found that the parts used in a woofer to make pretty T/S parameters at less than 1 Watt of power are not the right parts to use in a woofer that will be used in the real world.
With a DD woofer, you'll notice a high BL product, much higher than the Qts indicates. The Fs looks high, but the suspension is built for moving mass control under real power levels. This also raises the Qts.
After a DD sub suspension breaks in, the Fs will drop down in the mid 30s. When you use the DDbox system, the sub will play any music content you throw at it and do it with clean accuracy. The sub and the box work as a system, it is what all the parts in the sub were designed for.