Comparison of very compact passive radiators

[Arindal]

When thinking about getting a very compact speaker to produce meaningful bass, I stumbled across these:

4" passive radiators. I wonder if anyone has experience with these very compact ones as offered by SB, Purify, Dayton and Peerless for example. Or has realized a speaker project with such succesfully?

Somehow I would like to know how they compare and behave when being pushed to the excursion limits. Just showing increasing THD?

 

[motomech]

I have the Passive Aggressive kit from Parts Express that they use a pr. of 3 1/2 " passive drivers per speaker and the tiny ND16 tweeter.
In my desktop system they are quite satisfying w/ a sweet tweeter and a bass output that plays surprising low, but they, lack a little "fullness"
Given a choice, I would take a gd. pr. ported 2-ways w/ 5 inch woofers any day but perhaps a 4 inch woofer and 2) 4 inch radiators in a larger box would do better.
I have quite a bit of experience w/ the ND series woofers (active and passive) and while it's a little disconcerting to see their cones with their high x-max jump around on their soft surrounds, but they never bottom out. Curt Campbell stress tested the ND140 when he designed the Aviatrix and they played very loud w/ no prob.s.

 

[Torin Krell]

Common theory and practice usually recommend using a passive radiator with about twice the surface area of the active bass driver. This is why when the passive radiator and active woofer are the same size most commercial systems use two passive radiators.

 

[a4eaudio]

...twice the surface area of the active bass driver....

You need twice the displaced volume of air, which is surface area (SD) times Xmax. If the passive radiator has the same Xmax, you need either twice the SD or two passives. Note, some companies make a matching passive with the same Sd but twice the Xmax, which allows a single passive to be used.

 

[DVDdoug]

I've never built a PR speaker or even researched it, but make sure to optimize your design with speaker design software. That will allow you to compare different passive radiators (and of course different woofers) or compare the passive radiator with sealed & ported designs, and you can experiment with different box size, etc.

I can't recommend software because I've only used WinISD which doesn't support PR designs

 

[Ktacos]

I can't recommend software because I've only used WinISD which doesn't support PR designs

Vcad models PR's. Pretty easy to use the enclosure tool for it.

 

[JohnnyNG]

I can't recommend software because I've only used WinISD which doesn't support PR designs

(0.7.950, 29.10.2016) does.

 

[Arindal]

Thanks @everyone, that is useful. I am aware of the basic principles like the rule of thumb about air displacement. Will go for a PR one size bigger than the woofer to have roughly double the diaphragm area plus a reserve of additional excursion.

I have the Passive Aggressive kit from Parts Express that they use a pr. of 3 1/2 " passive drivers per speaker and the tiny ND16 tweeter.
In my desktop system they are quite satisfying w/ a sweet tweeter and a bass output that plays surprising low, but they, lack a little "fullness"

That is what I would expect given the fact that their enclosure is way smaller than what TSP would suggest as ideal for a PR design. To me is sounds a bit like the capability of the two radiators is not fully used. Would rather opt for additional volume, keeping the idea of a slim baffle which is really attractive IMHO.

What's the actual resonance frequency you are using, and did you measure it? Have experienced some PR designs being actually noticeably lower in resonance freq compared to simulation, so removing weight and pushing the tuning freq to a higher range, would help ´fullness´ a lot IMHO.

Did a quick check of the Dayton ND series PR (105 and 140 size), and subjectively they make a very good impression, rigid diaphragm, spider seems to be pretty stiff (identical with the active derivates?), as if they would never wobble.

Anyone tried the SB Acoustics SB12PAC-00? Looks very promising as well, but they offer only a 4" variant in this basket design, the 5" or 6" are too big in diameter for me.

 

[Torin Krell]

You need twice the displaced volume of air, which is surface area (SD) times Xmax. If the passive radiator has the same Xmax, you need either twice the SD or two passives. Note, some companies make a matching passive with the same Sd but twice the Xmax, which allows a single passive to be used.

Agreed - this is why I put "usually" and "most" in boldface - fully realizing the more technical details associated with matching passive with active drivers, yet trying to not over complicate my post. Usually however surface area alone is a useful rule of thumb since most conventional active and passive drivers will not have a 2x Xmas disparity. What is at least one conventional well designed quality speaker system with a single passive radiator that has only the same surface area as its active driver?

 

[Arindal]

What is at least one conventional well designed quality speaker system with a single passive radiator that has only the same surface area as its active driver?

The first iteration Mackie HR 824? Admittingly not exactly the same, and completely different designs, but active/passive in the same ballpark.

 

[Heinrich]

Thanks @everyone, that is useful. I am aware of the basic principles like the rule of thumb ...

Based on the title, I would have expected a study that had already been conducted. The question as just thrown out, however, is somewhat disappointing. Unfortunately, it triggers the usual platitudes, which today can easily be replaced by the results of simulation programmes. If the parameters are known. WinISD can do PR designs too – except that it lacks easily traceable suggestions...

Note: Passive membranes are heavy, so it is always worth using two, whose inertial forces cancel each other out on opposite panels. Furthermore, the mechanical loss factor QM must be taken into account, which then also results in relaxed planning of the membrane area deemed necessary. (Never mind, the reduction in the driver's excursion is always sufficient, as the careful analysis shows.)

Hope this helps

What's ... and did you measure it? Have experienced some PR designs being ...

What did you measure? Sharing is caring ...

 

[Torin Krell]

The first iteration Mackie HR 824? Admittingly not exactly the same, and completely different designs, but active/passive in the same ballpark.

Yes - that is a notable exception and a rather popular speaker in its time! The HR 824's actual woofer cone's diameter is about 8" and the elliptical passive radiator's dimensions are about 6" x 12".

 

[Arindal]

What did you measure?

Not yet, sorry, just starting to tinker with these. My past experience with passive radiators was mainly based on subwoofer designs which I did not measure for myself therefore have nothing to publish.

Passive membranes are heavy, so it is always worth using two, whose inertial forces cancel each other out on opposite panels.

Sounds like a good idea for heavy units, but was more considering compact designs with a minimum of moving mass. The aluminium one I have shown in the initial post has a moving mass of 13.9g plus a minimum of weight that has to be added for proper tuning (in the ballpark of +5g). Nothing to worry in terms of inertia, I think.

 

[Heinrich]

Not yet, sorry, just starting to tinker with these. My past experience with passive radiators was mainly based on subwoofer designs which I did not measure for myself therefore have nothing to publish.

Thanks for clarification.