Slot-loaded woofers?

[BAM]

What are the considerations for a slot-loaded woofer? I imagine there is some sort of bass loading that is going on (some tuning of the slot that must take place), and I want to find out how it affects the sound of the driver, and how to tune a slot properly.

[Ron E]

IIRC the main effect is just a slight filtering of HF due to the indirect sound. It could also act as some additional mass on the cone.

There are no simple formulas. There could be an effect, or not, depending on what the exact details of your design is. Worst case, it would degenerate into a 6th order bandpass.

If you measured your parameters with the driver mounted like you posted (but with the top and 4 sides above the driver removed) you would probably account for most of any effects at low frequencies. Then measure in free air and see what the differences are.

[sreten]

basically a crude 6th order bandpass, the forward slot loading has no
advantage as far as I can see, compared to normal reflex front loading.

(P.S. slot loading of drivers in their immediate front cutout was
used to increased dispersion at the expense of resonances
higher up the range, now deeply unfashionable.)

sreten.

[eRiCdWoNg]

Reminds me of the EV S-181 sub I used to have. I never understood that design. Except they had the woofer in reverse of what you show, and the ports were on the same side as the 'slot'.

[BAM]

I was just on the VMPS-europe site and saw that they have slot-loaded passive radiators. Personally, I would never put a passive radiator downfiring but they do.

[kelticwizard]

I have never built one, but I have heard increasingly good things about downward firing ports. The downward firing Drone Cones would be a variation on that theme.

The action of the port or Drone Cone, (Passive Radiator), only really lasts for about an octave above the tuning frequency, so any resonances above that are largely irrelevant-the Drone Cone is not producing them anyway.

I am not so sure that you really need a perfectly centered Drone Cone the way you need a perfectly centered driver, so maybe a downward firing Passive Radiator is a decent idea. On the other hand, perhaps the added pressure of the downward firing position might change the tuning of the Passive Radiator over time. I don't know, especially since the cones of the Drone Cones frequently have weight added to them that a driver would not have.

At any rate, since the Drone Cones are really only putting out over the course of one octave, I don't thind the upper frequency resonances that Sreten mentioned are important in the downward firing Passive Radiator, (driver facing forward), configuration.

[BAM]

It was my understanding that the Drone Cones had enough mass on them that they would sag out and be damaged by a downfiring position. Perhaps downfiring them into a slot is a way of mass-loading them without loading mass onto them. The VMPS New Original subwoofer, the VMPS Larger Subwoofer, the VMPS RM/X Elixir, the VMPS Super Tower Ribbon Edition, and some other ones use downfiring slot-loaded PRs. Some of what they do doesn't really make sense to me, though. They use passive radiators that look like they have either equal or less displacement than the actual active woofers seem like they would have.

[kelticwizard]

Originally posted by BAM
Perhaps downfiring them into a slot is a way of mass-loading them without loading mass onto them.... They use passive radiators that look like they have either equal or less displacement than the actual active woofers seem like they would have.

A) Interesting possiblity. Sreten, I believe, once mentioned that downward firing ports will actually be tuned lower than ports firing forward for the same diameter and length. This would seem to be the same effect applied to Drone Cones.

B) Passive Radiators too small? Considering the expense of the VMPS, I wonder is they are not actually using a Drone Cone to tune/terminate a Transmission Line. I wonder if there is a cutaway picture on the web to check out that possiblity.

Thatch Ear of this forum did such an enclosure.

[BAM]

Consider the Super Tower Ribbon Edition. This has four woofers in a vertical arrangement. Then they have a passive radiator of the same diameter in the bottom of the tower, slot loaded. The woofers aren't spaced such that they can be loaded into a transmission line. Doesn't that seem weird?

At the same time, the down-firing ports seem longer because there is a longer path from the port to any of the edges of the bottom, and the floor makes it such that the air mass stays along the floor until it reaches the cabinet edge. I think that slot-loading passive radiators will cause there to be more resistance to their motion and thus deepen the tuning without needing to add large weights to the radiators. Perhaps a passive radiator with less mass has less boom or overhang? VMPS subs are praised for their quick transient response.

[kelticwizard]

Originally posted by BAM
Consider the Super Tower Ribbon Edition. This has four woofers in a vertical arrangement. Then they have a passive radiator of the same diameter in the bottom of the tower, slot loaded. The woofers aren't spaced such that they can be loaded into a transmission line. Doesn't that seem weird?

Yes. And I don't know enough about Transmission Lines to say otherwise. The only thing I can state is that in other speaker configurations, there is a tendency for a long source to be considered a point source in the middle. So maybe the VMPS is a Transmission Line with a woofer being considered positioned as in the middle of the array.

Originally posted by BAM
At the same time, the down-firing ports seem longer because there is a longer path from the port to any of the edges of the bottom, and the floor makes it such that the air mass stays along the floor until it reaches the cabinet edge. I think that slot-loading passive radiators will cause there to be more resistance to their motion and thus deepen the tuning without needing to add large weights to the radiators. Perhaps a passive radiator with less mass has less boom or overhang? VMPS subs are praised for their quick transient response.

I would say your statements are a reasonable guess.