Picking a woofer for 2-way Synergy build?

So, now that my first fullrange build is well under way, I've been browsing the forum as a mean to keep furthering my newly found speaker building abilities. I ended up reading all I could find about synergy horns, taking pages upon pages of notes, and decided and I wanted to have a go at it once my first speakers will be up and running. Because what's better than immediately thinking about another build while the current one is still in the ropes?

After seeing @xrk971 and @bushmeister's amazing build, I figured some pretty insane things could be achieve with a regular 2-way horn with two woofers per side coupled with a FR driver, XO'd at 500/600Hz. But it's when I saw the insanely low-distorsion low-end extension bushmeister was able to achieve that I figured I absolutely needed to build one myself.

My goal for this project is therefore to end up with a highly efficient, very low-distorsion point-source speaker that can achieve great, clean low-end extension. They'll be used at nearfield volume (never quite exceeding 60/65dB) and listened to at no more than 1/1.5 meters. I figured this would allow me to really make use of the very high efficiency, betting on very low need for displacement, and therefore vanishingly low distorsion levels.

I set out to look for a good woofer driver to use, basing myself on the guidelines I had gleaned after reading what felt like thousand of forum pages. I wanted something that would allow me to reach close to 30Hz and possibly below, at reasonable levels. Here were my pre-requisites.

  • Preferably 8"
  • Large excursion capability for low HD
  • Low-ish Qts to optimize horn loading
  • As small Vas as possible
  • Strong motor to further reduce excursion
  • Highest possible volume displacement (Vd)
  • High BL preferred
  • 8Ohm so they can be wired in parallel to achieve a 4Ohm load

I made a quick Excel sheet to input different drivers' parameters, trying to stay below 150€, because I'll be buying 4 of these. I calculated an idealized version of electromechanical damping to get an idea of the motors' strength by doing (BL*BL)/Re. I know this doesn't account for losses, but it still gives a workable number. I also calculated an idealized version of Vd by doing Sd*Xmax.


Here are the results. The grey line is the driver bushmeister used. I used it as a reference since he stated the TS parameters were seemingly perfect, and also because I saw the measurements of his finished speaker, and that sweet, distorsion-less extension into sub-30Hz...

The clear winner based on my research is the SBAcoustics SB20PFC30. It is not only the cheapest of them all, but it also has:

  • High Xmax of 7.32mm (measured by third-party, not by manufacturer)
  • Highest efficiency of the bunch at 90.5, giving me about 93.5db when wired in parallel. Would allow for 'easy-ish' matching with a very efficient horn loaded FR.
  • Highest volume displacement of them all
  • Good BL / Motor strength
  • Reasonable Vas. I can live with giving each horn 60 to 100L to breath. I intend on running them sealed to get the cleanest bass transient possible.


The only thing keeping me from immediately pulling the trigger is this: am I missing something? What's the catch? The driver will have to deal with everything below 500/600Hz.

It models really well. I even tried a tapped horn sub, and I get a very respectacle extension down to 27Hz at about 90/91dB before it rolls off. Usable all the way to 100Hz.


So I'm calling upon the Synergy boys to help me figure out if I'm making a sane decision. I've been itching to pull the trigger.


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Hey @Mitch311,

I did end up buying four SBAcoustics SB20PFC30, but due to time constraints, I didn't quite get a chance to have a go at implementing them quite yet.

I've played around on paper with several topologies. Before diving into a complex Synergy build, I wanted to try using them in a Ripole configuration with a horn loaded full range up top.
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Hey Pardon,
I’m thinking about the same thing. Staged progression towards a MEH. Trying to find the right horn to buy or build is a hell of a chore though.

I don’t think I have it in me to learn akubak or any of that stuff at the moment. I just want a nice set of wooden horn plans to copy 😂🥲.

Have you looked at any horn options yet?
I think you might have found the cheapest woofer that could work and its a reasonable choice. Faital makes a series of 8" woofers with more extension. Th e 8PR200, e.g. is rated for 8mm Xmax. and has a triple roll suspension that needs less clearance when placed against a horn wall to avoid hitting the wall at full extension. But the Faitals cost much more$