a cab with two different LF drivers

[Sid the Kid]

If the OP would put the 6" and 8" into seperate cabs then he would end up with 4 cabs instead of 3 cabs (which actually follows his intention).

The OP could do that. I am equipped with 2 hands so carrying 2 cabinets instead of 4 or 3 always seems like a better option when public transportation is a consideration.

I agree there is no problem with him building one cab instead of 2. My suggestion would simply maximize versatility. You could continue this with me via PM if you would like. I feel like the OP has plenty of suggestions from other users, where you only offered criticism.

 

[ThisBass]

The OP could do that. I am equipped with 2 hands so carrying 2 cabinets instead of 4 or 3 always seems like a better option when public transportation is a consideration.

How do we know if public transportation was a consideration?
Personally I think that the OP does know well enough what might be best for him in any regards of "public transportability"?

I agree there is no problem with him building one cab instead of 2.

Correct, cause optimized tuning and performance are NOT impaired if there was only ONE 6" 8" cab built that consists of two seperate chambers.

To the contrary (and for comparison reasons) here you are with your previos suggestion:
#19
"...and having identical drivers will help the enclosure be tuned optimally which will maximize performance."

My suggestion would simply maximize versatility.

Correct, versatility was improved by building two cabs but, this additional versatility of stacking format is actually NOT WANTED by the OP!

 

[jj.833]

OP says: thank everyone for your generous input! @agedhorse you explain it very simply and leave no space for doubts, which is very helpful to me.
Following a story why I want this weird speaker combination: in 2014 i made a 1x8" cab, because I gigged a lot in Prague center where you can't park. I had Markbass CMD 121 back then, and DB with Realist Copperhead pickup. It seemed too heavy to carry, and too bass heavy to dial a nice clear sound. Back then it was not really possible to buy a really small cab, perhaps just in my area. So I made this 8" with very little knowledge, but it worked out so well that I eventually sold the Markbass. Then I found that this setup is not strong enough for BG usage, and that I miss the highs above say 3khz. So I made the 6" cab. This lasted me till now, works perfectly together, and on DB gigs, I'm usually okay with 6" alone.
Nowadays I gig regularly in two towns, so the idea is to build generally the same thing twice to avoid the eternal cab transport planning. And since in mass transit ( mostly tram and bus) it is handy to have one free hand, I'm thinking to put them both in one box. This thread is leading me to reconsider, and make two separate boxes that would be transportable in one bag (or lockable together).
To make my confession complete, I also made another 6" cab, a bit bigger, being curious on how the bigger than optiomal litrage affects the lows. It's okay but boomier, not better. But at least I checked 2x6" combination, which in my case sounds too much mid heavy and 'too loud' without loud enough bass response.
The only situation I need the lows strong enough is when I play BG on very small gig where there's no PA. With PA, I always hear the big lows from the PA, no matter whether the gig is inside or outside.
And the last thing, I mostly play a good part of my bass lines improvised, so I really need to hear the pitches. If I play right, the ground shaking lows somehow always appear.
Again, thanks everyone, you've been very helpful!

 

[Sid the Kid]

Correct, cause optimized tuning and performance are NOT impaired if there was only ONE 6" 8" cab built that consists of two seperate chambers.

To the contrary (and for comparison reasons) here you are with your previos suggestion:
#19
"...and having identical drivers will help the enclosure be tuned optimally which will maximize performance.

The ship of quote you chose here was taken out of context. In the context of the quote each enclosure would have the same drivers in the same airspace, where my advice is correct. I never once suggested building a cab with 2 drivers in separate airspace but one enclosure, as it was already established that doing so is perfectly acceptable. There was never a problem, I simply offered a slightly different suggestion that seems like it may have helped.

Have you helped this thread, and do you feel better now?

 

[ThisBass]

In the context of the quote each enclosure would have the same drivers in the same airspace, where my advice is correct.

Correct!
Exactly the same can be "easily" done with a single 6" 8" cab where the air space (and tuning) of the existing 6" cab does match the 6" chamber (and tuning) of the 6" 8" cab and the air space and tuning of the existing 8" does match the 8" chamber and tuning of the 6" 8" cab.

Technical (and performance) wise there was no benefit (nothing at all) by building two single cabs where each new cab then may match to one of the two existing cabs.

Furthermore the OP is planning to have:

Nowadays I gig regularly in two towns, so the idea is to build generally the same thing twice to avoid the eternal cab transport planning.

If I understand his words right then the OP is planning to have ONE 6" 8" cab rig placed in one town and a second ONE 6" 8" cab rig in a different town.

So technical wise and performance wise it makes really no difference if the OP is building now ONE new cab or two another new ones (based on his existing two cabs).
Versaitility does not count in this context cause both 6" 8" cab rigs shall be placed on different places/locations.

I'm thinking to put them both in one box. This thread is leading me to reconsider, and make two separate boxes that would be transportable in one bag (or lockable together).

I can't suggest anything in this regard cause I have no experience with very smallish cab formats. I can't tell anything if it was more "practical" due to transportability reasons to have either two very smallish cabs or just one cab that also was rather small.
Technical wise and sound wise and performance wise there will be no difference if you'll have two cabs or just one.
If any, and if I was in your shoes, depending on needed/wanted air space for each driver, I'd try to consider if it was more "practical" respectively meaningful to built two cabs or just one.

 

[jj.833]

If I understand his words right then the OP is planning to have ONE 6" 8" cab rig placed in one town and a second ONE 6" 8" cab rig in a different town.

Sorry, I wasn't correct, english being not my language. I'll just keep what I have - 1x8" and 1x6" at one place, and will build 8"+6" for the other place (or perhaps separated 1x8" and 1x6" ... stil considering)

I can't suggest anything in this regard cause I have no experience with very smallish cab formats. I can't tell anything if it was more "practical" due to transportability reasons to have either two very smallish cabs or just one cab that also was rather small.

My experience tells me that the best format for a small cab is such: so it fits in some common zipper bag, where also amp and cables and preamps and sheets will fit. The idea of 6+8" is to test whether a slightly bigger box, say of the size of cajon, if it were really lightweight, could be used even easier. I haven't decided yet. Perhaps it really is smarter to just buy something this size, which I think can be found on the market quite easily nowadays.

 

[abarson]

If size and portability are of utmost importance, and you have decent woodworking skills, I can recommend building yourself an Arjank Piccolo 8. Be sure and use the correct Beyma speaker. You won't need a 6".
Piccolo8, cab build for piccolo bass

Plans can be found here:
The Cyclops, a tweeterless 115

Here's the link to my build:
Say "hello" to my leetle friend

 

[ThisBass]

Sorry, I wasn't correct, english being not my language. I'll just keep what I have - 1x8" and 1x6" at one place, and will build 8"+6" for the other place (or perhaps separated 1x8" and 1x6" ... stil considering)

Yes, this is how I already understood your words in your previous post.
May be the issue of "language barrier" and thus unprecise written words is actually on my side cause English isn't my first language.

My experience tells me that the best format for a small cab is such: so it fits in some common zipper bag, where also amp and cables and preamps and sheets will fit. The idea of 6+8" is to test whether a slightly bigger box, say of the size of cajon, if it were really lightweight, could be used even easier. I haven't decided yet. Perhaps it really is smarter to just buy something this size, which I think can be found on the market quite easily nowadays.

Of course, almost always there are different ways to skin a cat.

For the B&C 8BG51 I'd choose a net volume in the range of 15..25 liters
15 liters - Fb 55Hz - predicted F3 equals 55.1 Hz
25 liters - Fb 45Hz - predicted F3 equals 44.6 Hz

For the PDN.8BM30 I'd choose roughly ~10 liters of net volume (at most)
10 liters - Fb 65 Hz- predicted F3 equals 78 Hz
Personally I wouldn't enlarge the volume to more than ~10 liters for the PDN.8BM30 cause with more of volume the roll off shape will tend to develop a "edgy" roll off knee

For the very smallish 10 liter 8" cab/chamber loaded with PDN.8BM30 the "design" may provide still sufficient "lowish" content performance for a 4-string bass in standard tuning.

For the B&C 6NDL38 I'd choose about 10 liters of net volume and tuning 55..60Hz - predicted F3 roughly 60..70Hz.

If I was asked a question which cab variant and combination was best. I'd answer the question with the following words: it depends.
Personally I might give a chance to the following "combination":
B&C 8BG51 in a 20 liter chamber tuned to 50 Hz
combined with
B&C 6NDL38 in 10 liter chamber tuned to 55..60Hz

Just to keep things as small "as possible", ~15 liter for the B&C 8BG51 was also fine but, in this case some smaller cab size volume was trated off for some low end response.

Btw how big is roughly the dimensions/volume of a generic cachon?

If you have got some questions in any regards of design principles, feel free to send me a PM

 

[jj.833]

B&C 8BG51 in a 20 liter chamber tuned to 50 Hz
combined with
B&C 6NDL38 in 10 liter chamber tuned to 55..60Hz

Just to keep things as small "as possible", ~15 liter for the B&C 8BG51 was also fine but, in this case some smaller cab size volume was trated off for some low end response.

Thanks a lot! This generally confirms my Boxsim simulation results, so I'm happy I'm not way off. I'd probably make the litrage a bit smaller than optimal, as I usually turn the lows on my amps a bit down anyway (and milimeters matter in a tram). I'd add update here when I have some.