Choosing a bass speaker

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Sorry if this has been covered; please direct me to relevant posts if so.
I need to find a suitable bass speaker to fit my existing cabinet. Changing cabs is NOT an option, so please don't waste your effort suggesting it!
The volume is just under 70 litres, closed box, and no room for a front port or vent. Bottom (or even top) ARE possible, although I would prefer to stay closed box, or variovent at most.
The speaker opening is exactly 300mm x 350mm. I could scoop out the 300mm a little, but the carcass is only 18mm thick at this point. I am using KEF B139s at the mo, isobaric clamshell. The rear of the cab is 400mm from the wall, which has 90mm dense rockwool spaced 110mm from the wall; 200mm total, and I can't move the cabs more than a couple of inches further out.
So, 10" speakers are probably the limit, unless there is a 12" which could have it's edge trimmed a little.
How do I use the published TS parameters of speakers to search for a suitable driver? Bearing in mind I used the KEFs because I can't afford Revelators etc.
Do I need high or low Qts, VAS, etc? Thinking sealed box. Sensitivity is not really an issue as I have an analogue active crossover, 24dB slope, and only need to go up to about 150Hz.
At the moment the mid unit is a Jordan JX125 aluminium driver, but I may change to something like an L. Cao 8" paper driver.
I have a pair of Seas 10" paper coned drivers, as used in the original Heybrook HB3s; one of them was rubbing, but I seem to have cured it. http://www.seas.no/images/stories/vintage/pdfdataheet/h108_25f-ewx.pdf
Any help welcome.
 
I don't see you mentioning any sub-enclosure, to isolate the mid cone from woofer pressure. That is a big problem, you either need to change to a sealed mid or put in a sub-enclosure.

Other than that, there are many drivers that could work in 70 liters. but it sounds like you are trying to put a round driver in a clamshell hole. Despite your detailed description, a picture is worth 1000 words.

Also, tell us WHY you want to make all these changes, and we can help more.
 
The MF and HF are in separate cabs. The bass is too slow, so I want to explore the possibility of changing the drivers. Here's a pic.
An externally hosted image should be here but it was not working when we last tested it.
The MF/HF cabs are temporary; they will match the bass eventually. Which are 18mm birch ply, cross braced, bitumen damped, with sand filled curve at the front, and 15mm sandstone side panels compliantly glued on.
 
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Its a myth that bass is slow because of the bass dirver - it is, as said before, usually because of crossover and driver integration problems. Any crossover, active or passive, can be sub-optimal. What type of crossover is it and what measurement equipment did you use to design your crossover?
 
The bass is too slow, so I want to explore the possibility of changing the drivers.

Consider these 10" drivers from Parts Express.
1) Dayton RS270-8 (8 ohm)
2) Dayton SD270A-88 (4 ohm - dual voice coil in parallel)

They will give you the dynamics that is currently lacking. All you need is one unit, do not use "Clampshell" design.

Play some music and adjust your crossover for the best mid-bass and low bass. It should be somewhere between 75Hz to 100Hz.
 

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Its a myth that bass is slow because of the bass dirver - it is, as said before, usually because of crossover and driver integration problems. Any crossover, active or passive, can be sub-optimal. What type of crossover is it and what measurement equipment did you use to design your crossover?
Lol! Crossover was built in 1982; what measuring equipment?
Design by Ben Duncan, Hi Fi News.
 
Sealed or Ported Enclosure Calculator

Speaker Box Enclosure Designer / Calculator

You have to manually enter the QTC value. I'd personally go for a qtc of around 0.7 (stay between 0.6 to 0.8).

Some info here:
Speaker Box Calculations

According to winISD (free download) a pair of Kef b139s only need 30ltr for a qtc of 0.71. 70ltr should give you a qtc of roughly 0.54 (depending on stuffing).

Qtc and tightness
Thanks for the links, that helps a lot.
 
Consider these 10" drivers from Parts Express.
1) Dayton RS270-8 (8 ohm)
2) Dayton SD270A-88 (4 ohm - dual voice coil in parallel)

They will give you the dynamics that is currently lacking. All you need is one unit, do not use "Clampshell" design.

Play some music and adjust your crossover for the best mid-bass and low bass. It should be somewhere between 75Hz to 100Hz.
Thanks, I'll have a look at those, although buying from Parts Express in the UKnis expensive!
And getting the crossover below 100Hz is hard, as there is no room for big caps.
 
Sorry about that. OK then, the crossover is NOT the most likely cause. It has been updated regularly over the last 30 years, and has sounded VERY good with other bass speakers.
And I am certainly not allergic to measuring equipment. I simply don't know how to use a computer well enough to use it. Unlike some younger members I didn't grow up with them, and find it VERY difficult.
Actually moderator, the topic is "how to use T/S parameters to find a suitable bass driver". It seems that others are making unfounded assumptions based on there own experiences, without knowing details of mine.
And I am grateful for the help given on the original topic.
Is that any better?
 
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Its a myth that bass is slow because of the bass dirver - it is, as said before, usually because of crossover and driver integration problems. Any crossover, active or passive, can be sub-optimal. What type of crossover is it and what measurement equipment did you use to design your crossover?

+1, absolutely...I have even experienced very small adjustment to tweeter XO paralel inductor having influence on bass definition...phase related, ofcourse

but in case of the KEF B139 ...it really is 'different' than most other woofers
 
re:"How do I use the published TS parameters of speakers to search for a suitable driver? " - download Unibox (free), obtain the T/S parameters of several candidate drivers, enter them in Unibox using the box size & port size of the existing cabinets, & see how the resulting LF frequency response graphs compare. The learning curve isn't very steep at all...

P.S. - there's no such thing as 'slow' bass; for optimum integration with the mids, the crossover will need tweaking as well.... (& possibly the room)
 
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