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Interesting B&C Single 21 with Dual 18 IB sub
Ales Dravinec:
--- Quote from: Chris Grimshaw on August 07, 2021, 09:24:24 AM ---Interesting cabinet design - not a typical approach, that's for sure.
My gut reaction is that the 21" cone in a small enclosed space will have trouble getting rid of heat from the motor. There's also the mis-match in cone areas between the 2x18" vs 1x21". I suspect that it could lower distortion to a point (the 2x18"s pushing the 21" cone a bit further when the 21" cone itself is starting to get non-linear - perhaps), but the overall limiting factor will still be how much air the 21" cone can move.
Since B&C (who make excellent drivers) have published this design, I'd certainly be interested to try it out.
Chris
--- End quote ---
Chris, your gut reaction is exactly spot on ! Heat from 21" motor can be an issue with prolonged use. That is why two smaller holes are there on internal walls.
For 'lighter' use those holes may be omitted, dropping THD even more.
Your explanation of cone areas miss-match is EXACTLY right. THD at higher SPLs is dramatically reduced, which puts this design very high in 'clean sub' competition.
.
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Please don't ask me how I know ...
With utmost respect
Tim McCulloch:
Greetings, Ales! Good to see you posting here. :)
Art Welter:
--- Quote from: Ales Dravinec on August 09, 2021, 05:02:09 AM ---
Please don't ask me how I know ...
With utmost respect
--- End quote ---
Ales,
As the designer, I'd guess you would know ;^)
Good work!
"Cool" impedance curve, above 6 ohms at Fb with three parallel 8 ohm woofers, that's something you don't see every day!
What voltage was used for the CEA2010B tone burst test?
How much voltage before the 21" hits Xmech in the 45-50 Hz range?
Art
Ales Dravinec:
--- Quote from: Art Welter on August 13, 2021, 03:04:17 PM ---Ales,
As the designer, I'd guess you would know ;^)
Good work!
"Cool" impedance curve, above 6 ohms at Fb with three parallel 8 ohm woofers, that's something you don't see every day!
What voltage was used for the CEA2010B tone burst test?
How much voltage before the 21" hits Xmech in the 45-50 Hz range?
Art
--- End quote ---
Art,
Recalling it right, I did not pass 100V rms. I used Powersoft X8 for its tremendous capabilities.
I'd say 80V rms was limiting voltage for reaching mechanical 'end'. Above that the 'low THD' statement doesn't stand, of course.
To be honest, I was rather surprised about impedance curve when originally designing this unit. Fb is under 30Hz (looking at original graph now being 25.4 Hz), which was designed for.
But the second peak at 50Hz I simply can not explain. Please forgive me.
Lowest impedance is at approximately 110Hz (2.5 ohms), while the highest sits at Fb 25.4 Hz being around 12 ohms. Ratio between the two is rather shocking being so small.
.
.
One more secret about this design :
I originally wanted to design an isobaric subwoofer WITHOUT removable wooden panels.
The only way to do this is to use smaller back drivers that would fit through front driver hole ....
All that happened later was just a bunch of new things : responses, impedances, ... But mostly ... somebody turned off THD ... go figure.
I think it would be better that somebody knowledgable tries to model and explain why that is. Phil or Dave or Tom come to my mind.
Respectfully !
Art Welter:
--- Quote from: Ales Dravinec on August 16, 2021, 03:15:01 AM ---Art,
Recalling it right, I did not pass 100V rms. I used Powersoft X8 for its tremendous capabilities.
I'd say 80V rms was limiting voltage for reaching mechanical 'end'. Above that the 'low THD' statement doesn't stand, of course.
To be honest, I was rather surprised about impedance curve when originally designing this unit. Fb is under 30Hz (looking at original graph now being 25.4 Hz), which was designed for.
--- End quote ---
Ales,
All "normal" bass reflex cabinets have both the impedance and cone excursion minima at Fb, like Josh Ricci's 25Hz Fb B&C 21SW152 similar sized design.
I assumed an isobaric would be the same, but your design using three drivers of two different types appears to be quite different in impedance response.
Makes me wonder if the excursion minima of the 21" and 18" match, or the chamber between results in different excursion for either.
Did you measure cone excursion and find the minima at 25.4 Hz rather than the lower passband impedance minima around 35Hz?
Art
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