[Rory B.]

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?

[Brian Cheney]

The PR is front loaded by having a constant distance from driver to the floor, which smoothes bass response considerably. The slot acts as an acoustic lowpass filter for the PR, filtering HF distortion products.  Because of this and other design considerations, the PR's moving mass is very low, around 20g, about half that of a driven driver with a voicecoil attached.  Then small adjustments of the PR moving mass allow the user to tune the system to its environment and associated equipment even though the changes are extremely small, less than 1/2g typically.

Most modern PR systems favor a massive cone stiffly suspended.  We use a very light cone compliantly suspended.  Big difference, and better results.

[Rory B.]

I assumed that because of the lower moving mass of the radiator, there would be a lot less boom or overhang in the radiator. Also, the foam surround (I assume from the pictures that it is foam) is slightly stiffer than rubber, making it so that the PR's cone won't sag quite so badly. I don't suppose I could get you to divulge how you decide where the PR's overall tuning point is? I'm interested in this for DIY purposes. I suppose I'll understand if this is a VMPS secret technology, though.

[Brian Cheney]

The tuning point is selected according to driver parameters and enclosure volume, i.e. it varies.  We start with a F3 of 10Hz.

[Rory B.]

I was just wondering about how you decide how much mass the passive radiator actually needs once you factor in the slot loading, as opposed to a passive radiator that is not slot loaded. I am familiar with how driver parameters and box size affect the length and of the port needed for a given diameter (or the amount of mass needed for an equivalent passive radiator). My whole deal is that I don't know how the slot loading affects things mathematically, such as airflow resistance, resonances, etc. all adding up to a different tuning than if you only had the passive radiator alone. If it's something you can't tell me, that's all right.

[Brian Cheney]

PR behaviour is quite complex, particularly when front-loaded; even the Thiele-Small people gave up on it.  There is no good mathematical model for PR behavior beyond the most basic.