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2x15" front loaded horn F2B

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citizensc View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote citizensc Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:09pm
Originally posted by djeddie djeddie wrote:

One thing to bear in mind with the F2B is that it's a bifuricated horn... meaning it's two throats sharing one mouth, not two throats having "half the mouth" each (for want of a better phrase). Much like a HF horn that has two CD's on it... if you see what I mean.


Does it really make much difference? Would there be a meaningful change in performance if you extended panel Ty the full depth of the cab? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote levyte357- Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:19pm
Any one interested in making this cab as 2x singles?


"Gentlemen, you can't fight in here, this is the war room!!".
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Post Options Post Options   Thanks (0) Thanks(0)   Quote citizensc Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 12:32pm
Originally posted by levyte357- levyte357- wrote:

Any one interested in making this cab as 2x singles?




Honestly, I was already thinking about doing that, was just a little worried people would get their nickers in a twist over it. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Robbo Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 1:30pm
No point in building singles when the original is very compact in the first place.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote levyte357- Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2020 at 1:40pm
Originally posted by Robbo Robbo wrote:

No point in building singles when the original is very compact in the first place.


Actually this is one of the reasons I never utilized this cab.

A double for me needs to be at least 1200H, so it can lay across the top of 2x subs.

Which is why THL828 is great size, even though it can't do what this cab will.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote citizensc Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 5:15am
Originally posted by Robbo Robbo wrote:

No point in building singles when the original is very compact in the first place.


An F2B weighs 107kg according to the spec sheet, according to the spec sheet of the PA100-375 Bass it has a shipping weight of 17 kg, so the driver is ~15kg. 

If a modern 15 inch neo driver weighs <5kg, you can get the weight below 45kg in theory. If given the option of a 107kg original or two 45kg singles loaded with neos, I know which id rather move. 



Another thing of note is the impedance curve of my sim isn't too different to the martin spec, suggesting I may not be far off the mark. 



Edited by citizensc - 16 May 2020 at 5:27am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote citizensc Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 11:39am
lil update 

I did some playing and I think I got some better hornresp inputs (See pics for method and new inputs).

I also simmed a bunch of different drivers, a lot of the 3 inch vc neos work well like the 15nd930 and 15NDL76 but they are very deep, they are all close to 180mm, the original driver is 145mm for reference. The RCF 4 inch vc neos are close and have a depth of 160mm. Both the MB15N405 and MB15N401 sim well but the 401 has a very high BL and low QTS, I dont know for sure but I suspect it will sound a bit too hard and a bit too dry. The 405 sims better than the original driver (see pics) and has huge power handling so it should be hard to kill in theory. I added a 10mm rebate to the drawing, to make space for the driver.

I did a lot of playing with different rear chamber volumes in hornresp, regardless of driver it seems happiest when left at the original volume of ~46L after subtracting volume of the driver. 

tldr: my drawing is exactly the same as the original in horn dimensions and very close in rear chamber volume, some slight changes have been made to make it easier to build and remove the need for 15mm ply, the MB15n405 may be the best driver for it.

One question, the current drawing leaves about 3mm of clearance behind the driver, is there going to affect performance? 

Current sketchup drawing showing 10mm clearance for driver and new method of finding hornresp inputs.
New hornresp inputs

1 x F2B at 4 ohm Black=PA100-375 Bass, Grey = MB15N405

2 x F2B at 8 ohm Black=PA100-375 Bass, Grey = MB15N405

Link to sketchup drawing https://www.dropbox.com/s/wmyzdnscotz79p0/hornrespv2.skp?dl=0




Edited by citizensc - 16 May 2020 at 11:39am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APC321 Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 1:33pm
3mm clearance behind the driver does not seem enough to me, bearing in mind the venting arrangements used to cool most voice coils.

I have previously taken a bass driver (with a circular central cooling vent) and fed it with a low frequency sine wave tone to get significant cone movement. This was done to check for a rubbing voice coil.

When the driver was in free space you could tell that the cooling system was working properly and "pumping" a lot of air.

But when the driver was moved closer to the top of a work bench there was a change. With the cooling vent less than maybe 15mm above a bench you could hear a change in the sound similar (but not as great as)to blocking the vent with your hand.

So I would think that the cooling system of the driver was being affected, and the driver was not operating under ideal conditions.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 1:41pm
Originally posted by citizensc citizensc wrote:


One question, the current drawing leaves about 3mm of clearance behind the driver, is there going to affect performance? 

JBL always recommended 76mm (three inches for the metrically challenged) minimum gap behind their 2226 for adequate cooling, FWIW.
I know modern drivers often have extra cooling between the chassis and suspension, so that will help, but my gut instinct is that only 3mm is still way too small a gap.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2020 at 4:40pm
The F2B was constructed with 18mm ply, not 15mm. But the baffle used two pieces of thinner ply glued together to make up an overall thickness of 25mm or so.

You will need to provide clearance for the rear vent on the drive unit. Even the original was barely adequate, and could have done with more.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote citizensc Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2020 at 11:59am
Originally posted by madboffin madboffin wrote:

The F2B was constructed with 18mm ply, not 15mm. But the baffle used two pieces of thinner ply glued together to make up an overall thickness of 25mm or so.


Interesting that the drawing from page 1 has parts that are made of 15mm. Look at the PDF for model 5. 

I have constructed the baffle in the same way on my drawing, an 18mm piece sandwiched with a 6mm piece. 6mm birch isn't that expensive and you don't need much, I don't think its too much of a stretch for anyone building this to buy some. 

Thanks for all the advice on driver clearance, I've bitten the bullet and stretched out the rear chamber by 12mm, adding 24mm of total width to the cab making it 1090mm(its the end of the world I know). This leaves 15mm of clearance including the 10mm routed out behind the driver. Considering the original only left 8mm clearance I think it should be fine. 

I have sealed off the chamber connected to the mouth of the horn in order to keep the rear chamber volume almost the same (less than 1L difference). 

Anyway I think the current drawing is close to usable, I don't foresee any significant issues with building it and if the sim is anything to go by, it should sound good. 




Sketchup drawing 
https://www.dropbox.com/s/pjupfqyumvzac77/expanded%20rear%20chamber.skp?dl=0


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Post Options Post Options   Thanks (0) Thanks(0)   Quote APC321 Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2020 at 6:28pm
When I said:

"With the cooling vent less than maybe 15mm above a bench you could hear a change in the sound similar (but not as great as)to blocking the vent with your hand."

I probably would not use this dimension of 15mm as a basis to work out dimensions when building a box. Far too un-scientific! My disclaimer!

Does seem too close to me if you want to avoid all unwanted effects.

Especially as the back of the f2b is perpendicular (at 90 degrees) to the vent. ie. parallel to the face of the magnet.

I would have thought that you would get some sort of "back pressure" effect that could affect the response of the driver in the box, as well as it's cooling.
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