Have you guys heard of a Copyleft? Its the exact opposite of Copyright. It requires that all modifications or enhancements made to a free design shall also be made free. Thus it is not possible to copyright a copylefted design; it is truly free (the design, not the speaker
).
Unfortunately, the innovators who have made their designs free to everybody did not think of the property rights issues involved when somebody adds their own designs to the basic design. How much (or how little) modification would make it the original inventor's property or the new inventor's property? The copyleft resolves that issue. As long as you are using the basic copylefted design, it is still free no matter how much modification was made.
As far as I can tell, the Voigt and Nagaoka speaker designs were not copylefted probably because IPR was a non-issue to the designer. But i salute them for sharing their genius with the rest of us.