Augmented Passive Radiator (APR) Designs - Anyone seem them? I have questions.

KEtheredge87

Hey All,

Investigating the "Inner Sanctum" theme for InDIYana 2024 now, and I saw that suggestion for Augmented passive radiator designs. There seems to be little information out there besides some AES papers and some alignment table style design guidance from Dickason's LDC 7th edition. The design style calls for two passive radiators of different sizes, but the literature specifically says there's no basket involved in the passive radiators. That presents a few questions for me.

1) Does that mean I'm just gluing a surround and cone to the cabinet? No mechanical fasteners involved?

2) In order to satisfy my OCD I would want a matching visual appearance for the external PR and Active woofer. Any PR that I could buy these days tends to come with a spider and basket. I assume the spider is for cone rocking motion control/mechanical support / anti-sag with washer weights, and the basket is then for mounting ease. It sounds like a wasteful PITA... but I guess I could carefully cut the spokes of the basket away and cut the spider to leave the cone and front visual setup intact?

3) I need to think through this more... but I fail to see why "no basket" would be a hard requirement. My first thought is that the acoustic behavior would be the same as long as the chamber volume ratios, PR cone areas, etc all match the design guidelines.

Has anyone seen DIY designs for APRs before? All I seem to find are napkin sketches of the concept, but no one building them. Google only seems to show the same 5 or 6 results before random garbage that doesn't help.

Thanks all!
Keith

Wolf

I don't see a "no basket requirement" being mandatory in such a design. I speculate that it would be a lot of empirical process for most of these alignments listed by next year's theme. I also think a couple jam nuts, some all thread, and lock washers would be an easy way to use 2 already available PRs for such an APR build. Essentially, you have an air spring as the rear cabinet to work with the rear and smaller PR, and then the larger front PR is attached to it via a linkage of some sort. In the LDC, it's noted as the base of the cones connected, but that is no different than PRs minus the weight if you ask me. If 2 of the same sized PRs are used, then there be equal pressure on both cones and no movement will result. The difference in Sd is what allows this to work.
If you use PRs, you have the added option for added mass as well, and is something that was likely not plausible when this kind of design was thought up. Another option I've thought about, is using a driver as the rear, and shorting the coil with resistance to change the damping aspect. Then the rear chamber is less important, but the linkage of cones would not be as easy. Tuning the PRs or resistance could vary the results in this quite heavily.
I welcome your thoughts...

DrewsBrews

PRs without a spider can have rocking modes AKA "flop around". The spider attached to the frame gives a second point of contact to try and force pistonic motion only.

If you are connecting two PRs together that don't have spiders; you then have two points of contact again via the two surrounds. Basically the basket and spider becomes redundant and is not necessary in that configuration. I think they are coming from that angle.

---

Now that my brain started going down the rabbit hole... Going out on a limb (probably wrong).

Is possibly the second PR/chamber bringing it's own tuned damping to the primary PR? Would you be able to make a facsimile of that with a circuit attached to an otherwise disconnected woofer and no second PR/chamber? Would probably still need to add weight to the cone somehow.

kenrhodes

https://patents.google.com/patent/US4076097A/en

6thplanet

Wooshhhh, right over my head.

kenrhodes

I hope no one assumed I understood the contents of my post. Ever...

Wolf

I understood and read the whole thing. Essentially, the example uses an active driver that is 6.5" (5" effective diameter) and 13g Mms, with Fs=30Hz. The inner diaphragm is flat, and 7", and the outer is 8.5", both of which comprise 52g Mms and diameters are effective, or surround peak to surround peak. The Sd of the outer is 3x that of active driver, and the ratio of total Sd to difference in Sd of of the passives are also 3. Front chamber of all drivers is half a cubic foot, and rear chamber is 1.5 cubic foot. Materials of cones in example are lightweight polystyrene. Others are said to be suitable.
My takeaway is that this is a method of taking the conventional PR setup of the front active driver and inner flat PR, and finding a new way to amplify or increase the efficiency of the PR setup, so that it moves more air than a conventional PR setup of the same kind by mechanical transfer of the motion of a more typical PR to that of a larger PR.
To simplify, you can't move more air in a typical PR box even by increasing the PR or drone's Sd. The active driver can only move so far, and the PR moves as a direct result of the pressure wave it's given, and no further. This means there is a limit to the typical PR output, that by way of the larger PR strapped to said typical PR, this adjustment is able to generate more output than a typical PR setup, and avoid the notch in rolloff of typical PR setup. It's as close as I can come to calling it a mechanically horn loaded PR. The document claims increase of 3dB over that of conventional PR designs in the bass range.
There is another drawing associated with leaving the back open and placed close to a wall to gain more bass output with less cancellation for console style designs. My thought here, is that it could be translatable to designs wanting cardioid bass response profiles. This is because they have output to both sides and actually want more output in the front than the rear.
All of this is said related likely to the tech of the times this was invented. There were lower power amplifiers, drivers were limited in bass output and xmax, and having a more efficient speaker design was quite important. Nowadays, these are obviously ot seen much if at all because of the improvements over time. However, it is still a means to an end to be considered, and possibly improving such an example with modern drivers and PRs would create an exceptional bass design.

Drummer

Wolf, your head is like a computer. I just sit in the back, and keep my mouth shut, trying to learn something, but way over my head. 😁Glenn

Wolf

I'm reminded of the conversation between Dave Grohl and Rupert Neve in the documentary film Sound City where Rupert is describing the transformer wiring methods of his soundboard. Dave Grohl pops in a sound bubble whilst nodding with a goofy grin that says, "He knows I'm a high school dropout, right?"

While I am not saying that you guys are saying you are dropouts or less wise, the feeling Dave had and the look on his face is likely capturing the same feeling some of you are having. I've been there too, trust me! The thing that does not compute, and makes you astounded exclaim, "WHAT!?", or even the rating system icon this forum employs. it was just a memory in film of the same kind of moment.

Oh, and if you've not seen the movie, highly recommended!

Drummer

I've seen it multiple times and loved it! And I have many of those WHAT!? moments in life! 🤪Glenn

charlielaub

@KEtheredge87 said:
Hey All,

Investigating the "Inner Sanctum" theme for InDIYana 2024 now, and I saw that suggestion for Augmented passive radiator designs. There seems to be little information out there besides some AES papers and some alignment table style design guidance from Dickason's LDC 7th edition. The design style calls for two passive radiators of different sizes, but the literature specifically says there's no basket involved in the passive radiators. That presents a few questions for me.

1) Does that mean I'm just gluing a surround and cone to the cabinet? No mechanical fasteners involved?

Just a comment on your Q#1 above: The suspension that sits in the bottom of the driver basket acts in tandem with the surround connected to the edge of the cone to stabilize the cone so that center of mass travel can only occur in a line. When there is just one ring of compliant material, the mass suspended by it will tend to develop rocking modes. There have been some attempts at constructing a PR by gluing a disc of MDF to a high excursion surround and that supposedly does not have a very good outcome.

Marveloudio

LspCAD 6 has a complete ready module for APR style reflex systems!

http://www.ijdata.com/

(demo version with some restrictions)

Not sure if the APR module is in the standard version ... the Pro version with dongle is much more expensive!

website has price info ...

LspCAD is only for hardcore audio guys .. not posers like those using REW and their fancy clickidiclick VituixCAD

KEtheredge87

Thanks everyone for the good discussion so far. I am reading all of it and absorbing as much as I can while still finishing my MWAF Chimera entry. The APR idea has piqued my interest for InDIYana 2024, so I'll try to work up an idea before the tent sale.

Thinking from a commercial perspective - I don't think I've seen or heard of any recent commercial designs that use an APR arrangement. Maybe the benefits didn't justify the design complexity for larger companies to mass produce?

Wolf

I have to say I am very intrigued as well.