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Dual 15 + coax

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Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FOO Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2021 at 10:06am
https://boyzpc.home.xs4all.nl/Bouwtekeningen/18Sound/K24xov60.pdf

Give this a go Smile

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toastyghost View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2021 at 10:19am
I’d also suggest considering the B&C coaxial compression driver over the BMS if you want to cross that low.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2021 at 10:27am
I would also just warn you that although a horn may be specified as having a 400Hz cut-off, it's usable range (i.e. crossover point) may be up to an octave above that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smoore Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2021 at 7:03pm
With the drivers arranged like that you have a little air mass chamber infront which will act like a band pass and, as mentioned previously, probably cut off your top end quite dramatically but also add some roughly (as an extremely loose one eye closed guess) ->600hz output from each 'horn' as that mass resonates.

I also agree with some others that I would probably choose 12"s over 15"s for that volume but if your sims suggest 15"s then go with whatever works! With regard to port tuning, free output is free output and you can always reduce it with EQ. A tuning of 40Hz is probably useless to you if you are crossing over at 80Hz? Just for reference I have tuned lots of boxes to 70Hz (measured) and they don't sound 'one-notey'. You can even still high pass them at 40Hz and push in some EQ at the expense of displacement and they still sound great.

Looks like a cool project! Excited to see the outcome. +1 on the B&C DCX coax driver suggested, they do sound good! Expensive though..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 18 July 2021 at 10:39pm
Lot's of good suggestions so far, thanks everyone, it looks like this project will happen soon.

The B+C coax looks good, specs and price are similar to the BMS. Has anyone heard both and done a comparison? I'm not sure which to choose.

Whichever driver I use I will do some tests to choose the crossover point. The horn sizes look good to reach 400Hz (just!) but I anticipate the crossover to be around 650Hz, a 24dB/oct filter which is -6dB at 650 will be way down at -24dB around 400Hz. If the 12s or 15s aren't happy that high it might have to be a lower steeper filter.

I had a bit of spare time today so I knocked up a dry run to think about the internal layout, ports, bracing, handles pole mount and such. It's very light so far, I can easily carry it with one hand. That will change though...


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Post Options Post Options   Thanks (0) Thanks(0)   Quote kipman725 Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2021 at 2:46pm
If you reduce the height of the cavitys above and below the drivers to the minimum that will allow for driver excursion the frequency where you get a resonant peak followed by a low pass effect will be pushed up.  You could also try just having the bass speakers fire through narrow slots like this:
you would possibly need some kind of volume filling plug that followed the cone contour to keep the air volume down.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2021 at 3:20pm
Originally posted by doober doober wrote:

The B+C coax looks good, specs and price are similar to the BMS. Has anyone heard both and done a comparison? I'm not sure which to choose.


The BMS sounds a little smoother from 14 kHz up, but the B&C absolutely craps all over it for midrange detail and clarity. It seems to be far more linear in that region.

Assuming you’re planning to bi-amp these and use a passive network on the coaxial HF, then the B&C is the more resilient option too. They have a passive crossover for the unit that by all accounts works well:
https://www.bcspeakers.com/en/products/crossovers/0-0/8/FB4648

They also have a bloody excellent horn for it, if you can fit it in the box!

https://www.bcspeakers.com/en/products/horn/1-4/0/ME464

Unfortunately I can’t say the same for the BMS units. I’ve seen far too many shattered VHF diaphragms from those, since the power handling disparity between that part and the main diaphragm is pretty huge.

If you get the output you need at 17 V or ~20 W peak then cool. If you need more, then you need a custom passive network with complex bulb protection circuit, or to run the driver itself on two channels with DSP.

Edited by toastyghost - 21 July 2021 at 3:24pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Boom Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2021 at 6:14pm
Have tried similar and have some observations:
 - What array size you planning to use? In singles, the horn you showed will be very narrow dispersion, but the cabinet low front area (which you say is a requirement) will mean low mid will have a wide dispersion. Also check out "baffle step".
 - The diffraction slot in the horn won't do justice to the sound quality coming from your expensive comps.
 - BMS 4590 is spec'd down to 400, but should probably be rolled off 700 or higher. Not sure about 4593
 - Putting the drivers in ducts like this will effectively make a resonant low-pass filter that's hard to damp. You might get a big peak at maybe 300 or so, and then a sharp roll-off above that.

I built a design like this, using a 4590 and a pair of tens, and ended up scrapping it. The next attempt addressed the issues I listed above. It combined the same 4590 with a pair of high-EBP 8-inch drivers in a much more sophisticated acoustic arrangement and got cabs I was happy with.

One difference, though, is I was OK with a bottom end of around 130 to 160Hz, whereas you need 80Hz. That's getting you close to Hoffman's Iron Law... In layman's terms, have you ever seen anyone build "enough speaker" to keep up with 4590-type SPLs, but reaching down to 80Hz in that cubic footage? If not, there may be a reason.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2021 at 9:28pm
Kipman- the cavities are tall enough at the front to allow the drivers to be fitted from the front, avoiding the slight complication of making access hatches in the back. Of course if I used 12" drivers they could go inside through the mid/high horn cutout, its about 20mm too narrow to fit a 15" through. I've not found a 12" which out performs the 15s I'm looking at.

I will have to put some drivers in and measure where the resonance and low-pass occur, then try some experimental stuff if it is too low.


Toastyghost- I will be bi-amping the comp drivers, so no worries about over powering the HF. I don't mind sacrificing a bit of smoothness above 14k for substantially better mids, that's quite an easy decision. I have seen that horn, it looks great but is a little too large for this project. I did think of using it in it's own box and putting 2 15s in a separate box with a pole between, but that just causes other problems...


John Boom- There won't be an array, these are to be used one box per side (plus subs.) The RCF horn is 90 degrees H by 50 degrees V, shouldn't be too far off the 15s. I have heard reports that this horn does sound good, despite the slot which is much smoother in shape than many others. The 'wide' version of the Peter Morris box uses this horn, and people are raving about it.

Thanks for sharing your experiences, that gives me some good points to consider, I know there will be a fair bit of experimenting to do with the layout. I'm aware of Hoffman's law, this box more a case of as much output as possible from a given box size which goes low enough to meet subs. I don't expect the 15s to keep up with a coax comp driver at full tilt.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 21 July 2021 at 10:51pm
Bear in mind that the PM90 uses a pair of 12” that were explicitly tuned to meet a xover of 630 Hz for the same horn. I too think 400 Hz for that horn is ambitious, even just from a cursory glance of geometry.

This link might be of interest:
https://audioxpress.com/article/Test-Bench-RCF-ND950-Neodymium-Compression-Driver-and-HF950-Horn

Edited by toastyghost - 21 July 2021 at 10:52pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 22 July 2021 at 8:50pm
I mentioned the PM90 as an example of the horn sound quality regarding the diffraction slot in the RCF horn. I don't expect the horn to go all the way down to 400Hz, 630Hz crossover will hopefully be fine if the 15s still sound decent up there and beyond.

That article is interesting. I was planning to do an impedance plot myself, but from looking at the one in your link the horn unloads around 450Hz. The SPL plot is informative, it shows a roll off below ~900Hz, might need a fairly gentle low pass on the 15s, again if they go high enough. Off axis plots look pretty good.

I've ordered some components, hopefully I'll be able to test something soon.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 25 July 2021 at 9:34pm
I test fitted the drivers. Had to do some adjustments to the box due to the B&C drivers being slightly larger than the BMS and Faital I original thought of using, the clearance between 15 magnet and comp diver was a bit non-existent.

It needs some ports now. I'm favouring rectangular ports each side of the horn. The side walls of the port can also act as bracing on the panels.





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