The last video in this series covers how to design a passive radiator enclosure. Passive radiators have some upsides and some downsides that you should be aware of should you decide to build a project with one. If you have tried playing around with many subwoofers and enclosure designs, you might have come across a few that had ridiculously long ports for a given box size, especially if you needed a large port area to keep from exceeding port velocity limits. A passive radiator will allow you to tune an enclosure lower without having to worry about port lengths that won't fit inside your box. They also don't suffer from vent velocity problems, i.e. port chuffing. In some instances, they can also prevent a compliant woofer from unloading below tuning and therefore can be used to control woofer excursion better than a port. On the downsides, actually less efficient than a properly sized port since there are mechanical losses through the suspension. They also are significantly more expensive. As with everything in audio, it becomes a series of trade-offs.
Once again, the software being used is the Jeff Bagby's Woofer Box and Circuit Designer Excel spreadsheet and the subwoofer being modeled is the same subwoofer used in my first three videos, the Dayton Audio RSS256HF-4.