SB Acoustics SB20FRPC30-8 (8" Fullrange Cheap Monster II)

Greetings ladies and gents, glad I found this post very very informative. Loaded with a ton of outstanding information not only on tht SB20FRPC30 but in loud speaker design in general.

I am in the process of planning and building a hybrid Tower, like the Alta Audio Aphrodite:

https://altaspeakers.com/product/aphrodite-speakers

But a the same not send $3k in transducers to build it.

I was going to use single 12" Full-range the Lii Song Fast 12( great unit in it own right) as the mid/high unit and than after some general tests and extensive amount of research an 8" fullrange is nearly the perfect size for an Open Baffle mid/high driver. It has the best mixture of fullness, speed and staging/imaging potential.

So started looking for some great 8" Fullrange drivers that would brake the bank and honestly have already ordered a pair of the in this discussion mentioned 8" Betsy Full-range units, which very nice also. But out of no where the SB20FRPC30 popped up. Not sure how I have over looked them but anyways, I am a huge fan of SB Acoustics/Satori drivers. I have used many of there products of other projects.

So my design is going to be as stated, much like the above mention Aphrodite, but difference in that my towers will an all active crossover system.

So in each tower:

A pair of the SB20FRPC30 (Open Baffle) and a Radian Dipole Planar Tweeter in a MTM configuration.

SB Audience BIANCO-12OB150 12" as midbass (Open Baffle)

GRS 12SW-HE in a Bass-flex enclosure tuned to 24hz (Should give in room to 19hz).

Why a pair of SB20FRPC30 in each tower? Simply to get closer to matching the sensitivity of the mid-bass and the tweeter.

Going at always me the able to adjust and fine tune until it to my likings. Also defeat the tweeter to make it a 3 way if feel the desire to. And the use of 4th slopes also. Which affords the to get to about 85% of knowing what need to have nice passive crossover designed built also, I would be able to active or passive an hear the advantages and disadvantages of both designs.

Any input encouraging or criticism is welcome. This is how we learn. This is how we get better. Thanks for all your input and advance.
 
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don’t forget that your mid bass will have much lower sensitivity after baffle losses. Especially open baffle due to cancellation.

Also, crossing a dipole planar low enough to limit lobbing with 8” drivers in an MTM configuration is going to probably be tough.

All that is to say that I didn’t look up all the drivers you mentioned, but I think you might do better with a single SB20FRPC30 unless you have a specific reason for going MTM, like vertical dispersion control.
 
Great point on the Mid-bass. Other side of it is the amount of power on the Mid-bass also it 5 times the amount the full-range will see. 120 watts on the Mid-bass Class AB vs. 25 watts pure Class A ÷ 2 on the full-ranges. But level matching is possible via the active crossover modules to balance things out.

Next to great point the Planar playing low enough they will play done to 1.8khz with 2nd order slopes. However they will not be playing anywhere near that low. Honestly my first experiment will be playing the full-range all the way up and just play with different crossover points with the Planar to bring the top end sparkle out.

On to your other point iof the MTM configuration, vertical dispersion was honestly the second thought, sensitivity was my first thought that is why I thought about going with 2 SB20FRPC30 and the Alta design came to mind. Putting the 2 full-range above each may work better with keeping the 2 centers of the full-range drivers as close as possible to each other and planar on top.

But One SB20FRPC30 would be just fine with me if it works better. I will be playing around test baffles the SB20FRPC30 and the Planar once the enclosure and midbass baffle is done. Do see what works best. To be honest I hope 1 works better.
 
How small is small? You might be able to get down to 50-60L with a PR, but that’s just a guess. I found them really good sealed though in as small as 25L. It all depends on how low you want them to go.

I built sealed 1.5 ways in 40L total giving 25L to the SB20FRPC30 and around 15L to the SB20PFCR30-8. This gives both of them nearly identical roll offs and a Q of just under 1 if I remember correctly. F3 is only around 70Hz, but with the combined SD and mid bass bump they sound pretty impactful and like they play lower than that (and in room, they do). The upside is that I get almost 94db in a reasonable box.

Here’s what they look like disassembled and waiting for me to get around to veneering them. Not stand mounts, but reasonably small imo.

View attachment 1414151View attachment 1414152
Wery nice.
I've been thinking about building something similar.
Do you have have construction design on the speaker and what kind of crossover did you use?

Regards
 
I’d be happy to share it all with you, but I didn’t renew my SketchUp Pro subscription this year, so I can’t open the files at this time. Unless you have SketchUp pro, the best I could do is give you some dimensions. I do have the XO schematic though.
 
I just found this thread.. and wanted to share my experience with this Wonderful driver!
I have made 11 different Mass Loaded Folded Voigt Pipes, using this driver, hoping to get the version best sound..
And with my room, gear and Ears believe that I have arrived!

I have tried to make the front, top and back something like the top plate of an acoustic guitar 🎸 and there is now doubt that it brings out the detail and speed of this driver! Following ideas by Janos on Real World Audio YouTube.

I’ve never considered a “ baffle step or similar… you can tune the little bit of harshness that you sometimes get around 3=4 khtz with a little bit of stuffing inside the top of the box and a little bit on the back directly behind the driver, otherwise I run them Stuffing free!

“Hard Feet” under the box help to tighten the bass, of which I have plenty…
This driver does need some “ run in time”, I’ve had 5 sets of drivers, ( I’ve sold 3+ pairs of speakers), and each time I’ve noticed them settle down after some hour’s work..
A photo of number 11. 24mm Birch 🌳 ply sides and cut LVL front, top and back.
 

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Thanks!!
So, it’s a single fold… but the narrow part is at the back of the cabinet ( following suggestions by Janos on Real World Audio YouTube.
The mouth/port is 70mm high and 70mm deep from memory (sorry I’m on holiday so I can’t measure it or find a pen 🖊️ to draw you a diagram..)
Internal width is 225mm.
Outside, 1100 high, 480mm deep.
The sloping Center piece is 2 pieces of 25mm hard eucalyptus plywood with mass loaded vinyl in between. ( I’d like to try green glue).
I have access to 40mm x90mm LVL structural beams. So I cut them to 30mm and re glue them back together to form a 30mm thick panel, with the pieces in a brickwork pattern.
These panels become the front, top and back of the speaker.
The idea is that it behaves like an acoustic guitar 🎸 top plate( the bit with the hole in it).. All the timber grains are in the same direction.. ( Janos says build your Voigt pipe like a musical instrument… Mostly I believe what he says!!!
the solid timber boards required to achieve the same goal are going to be Very Expensive in today’s world, so this is My way around that..
Also, my ideas with “Stuffing “….. I use Carpet… yer, I know some are saying “ where is the Science in that???”.
Use the Minimum stuffing that works.
With the folded design and this driver, it’s only around 3000 htz that benefits from some control. So, that corresponds with the top of the box.. I usually just use one piece of carpet held in with a couple of wood screws.
When the driver is brand new, you will feel that you want more carpet. Then behind the driver and maybe the sides at the top of the box… but you may take them out once the driver “ runs in”….
I never bother the light stuffing up the narrow end… Why slow down your bass speed?

Probably some people are thinking/ saying “ that doesn’t make sense, what a load of rubbish!!”
That’s ok. That’s my journey, after 11 boxes, one is an experimental box, that I screw together and has been used for testing different plywood species, and different internal shapes.
If you need more info or diagrams, I’m more than happy 😃 to share!!
Ps. It sounds Amazing to my ears!!
 
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Very cool! I’m going to have to check out this Janos guy…
I’d love to see a plan, even a simple sketch of the design you’ve developed. I’d ask more questions but a basic drawing would probably answer them all. I’m heading away for a holiday as well so no rush. Again, very nice result!

Edit: Oh, like the middle design shown here? I was trying to picture what you meant by “narrow part at the back”. Very interesting!
 

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Ah, it looks like you found Janos!!
I have also learned a bit from Harley Lovegrove ( YouTube) about developing his Sibelius speaker. Also folded voigt pipes.
So I managed to draw up my design on the iPad!! I have tried nearly every depth from 370mm to 480mm. My room is large, so 480mm provides more bass, not necessarily lower htz.
Instead of LVL beams, on older speaker boxes I’ve cut 25mm plywood Into 30mm Strips rolled them 90 degrees and glued them into panels.
This will give you 1/2 of grain running front to back, but the LVL has better dynamics and detail.
I hope the drawing helps a bit.
Ps. Enjoy your holiday!!
 

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Nice job on the drawing! The color coding makes it very simple to interpret.
Yes, Janos is fantastic! I’ve watched about an hour of his videos and I’m looking forward to more. His method of explanation is perfect for me. I feel I understand more now after a couple of videos than in many months of reading. Certainly his description of tuning the Voigt pipe size and opening based not just on driver specs but also room size was very informative.
Thanks for the recommendation! I’ll look into Lovegrove also.
Now I really want to experiment with Voigt Pipes!
 
Thanks Soarsimon!

I love your enthusiasm!
Try not to make 11 boxes as I did! Hopefully you will get Amazing results

The good part about this type of design is that it’s all rectangular/ square so the wood work is pretty simple..
The original design from Nandappe uses 15mm plywood… personally I can’t recommend thin plywood as I like a fair bit of volume to my music and even my first 18mm Malaysian hardwood plywood box resonated pretty badly!!
It loved Michael Buble though!!! 🤣😂😆.

I found that to my ears, if the box is too stiff, it reduces the detail..
Janos recommends Strong sides and softer front, top and back, and probably that’s true.

On a straight Voigt pipe ( I’d love ❤️ to build one, but the Lady Wife will send me to the Doghouse as they are a bit ugly… so, it isn’t worth it!! 🤣) , the back panel resonates into the room.

With a folded pipe, I believe that the “ middle sloping panel, corresponds to the back panel, so you don’t want the sound vibrating from the front to the back of the pipe, as I suspect it muddies things, hence the 2 x 25mm panels and MLV or green glue in between..

With the port tuning, I used to have 100mm high port and 100 deep in the past, but I added a 30mm piece of wood in the slot and the bass went noticeably lower in htz, and felt happier with the balance of sound, so now I stick to 70mm high.
Probably I should do more tuning..
As Janos suggests your room might prefer different tuning… or it might suit your musical choice better, so maybe you can plan something you can adjust, but these are some starting numbers!

If your listening room is “ normal sized”, like 4=5 metre x 4-5 metre you may be happy with just 370mm deep boxes.. especially if the boxes are close to the corners..

Speaker stands, as simple as blocks of wood 30=40mm high will tighten your bass response.

Plan to remove the drivers to adjust the “stuffing”… I made a box with a removable lid, but my gut feeling is that the removable lid messes with box rigidity…

Enjoy 😊 the journey! With SB Acoustic 8 inch drivers, it’s cheap fun 🤩!
I believe you will know when you want to spend more on drivers!

Always happy to help! 😀
 
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