I have a somewhat technical question about the VMPS downfiring passive radiators. Conventional passive radiator wisdom says that you don't point a passive radiator downward, because the mass causes the suspension to deform and sag down over time, making the available excursion less and less until the suspension or the PR itself must be replaced. However, it is obvious to me that what VMPS is doing with their passive radiators is not conventional. I got into a discussion of slot-loading technology on another audio forum, and we agreed that the purpose of slot-loading is to deepen the output by adding virtual mass to the driver or passive radiator. If this is what VMPS is doing, then it makes sense because it would allow the passive radiator to carry a much lower mass, allowing it to be used in a downfiring position without problems. Then the little bit of Mortite can be added and removed to fine-tune the box.
What I want to know is, how does one design a setup like this? Trial and error is always a possibility, but are there pat mathematical equations to determine the dimensions of the slot or to predict the amount of added mass that corresponds to a slot of particular dimensions?