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Dual 15 + coax

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doober View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Topic: Dual 15 + coax
    Posted: 12 July 2021 at 9:23pm
I'm planning to make some 2x15" reflex boxes with a BMS 4593 coax to handle mid and high. They need to be compact and light enough to go on a pole. Here is a sketchup of the idea




The horn will be in the centre, an RCF HF950, which measures 300x300mm and claims a 400Hz cut off.





15" drivers are Faital Pro 15pr400. These boxes will be used with separate subs, so don't need to go very low, and will probably cross to the coax comp driver around 5-600Hz.

I'm not sure about the port tuning for the 15s. The chamber volume needs to be quite small to keep the box a sensible size and weight, so I have been looking at 57 litres per driver- considerably less than I would expect to use for a 15". Tuning the port so that the impedance peaks are equal (which I have read is preferable) gives a reasonable response, tuning is at 35Hz in this case. If I raise the tuning frequency to ~47Hz the SPL plot gains a little at the lower end, and the driver excursion reduces.











The question is which tuning is best? Will using the higher tuning affect the sound of the box, or make it sound boomy/one note?

Is the layout with the tilted baffles sensible? I'm sure I have seen other boxes using a similar layout. I don't expect to gain any extra output from what looks like short horns in front of the drivers, certainly not at the frequencies it will be used. The point of this layout is to keep the frontal are as small as possible, and keep the centres of the drivers close to avoid over tightening of the vertical coverage.

Any thoughts would be welcome

Kieran

Edited by doober - 12 July 2021 at 9:34pm
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dlyxover View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dlyxover Quote  Post ReplyReply Direct Link To This Post Posted: 12 July 2021 at 10:29pm
35Hz seems too low for this application, I would be looking for a tuning frequency of around 45-60Hz.

Having the 15" drives in this arrangement will limit the useable top end range to 750-800Hz so you will need a horn and CD combo that will go low enough to meet them.

All in looks like an interesting build ahead, good work.
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billso View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billso Quote  Post ReplyReply Direct Link To This Post Posted: 12 July 2021 at 11:46pm
Hi why bother with 15s rather than 10s or 12s if you are going to put them on sticks and use poles and subs ? 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2021 at 8:22am
Not to dissuade the fun of designing something yourself, but you’re essentially remaking the PM90 from Peter Morris. That uses the same HF horn and arrangement, but is a proven design that has a partly horn-loaded LF that can actually keep up with the coaxial HF unit, and it goes on a pole.

Check it out, at the very least for inspiration.
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doober View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 13 July 2021 at 8:31pm
@dlyxover- I would have thought a higher tuning too, but the bump in the SPL graph concerns me. The horn/driver combo might be okay down to 400Hz, I think more like 500 or 600Hz is more likely though.

@billso- I want these boxes to have good output down to 80Hz, this requires air to be moved. If I went with smaller drivers they would need larger Xmax to move the same amount of air, and would probably be aimed more towards dedicated bass use with heavier cones and surround, and less efficient. I modelled quite a few drivers and the Faital 15" is hard to beat for efficiency in this box.

@toastyghost- yes I've seen the PM90, it looks like a great box. Unfortunately it is too tall and slim to match my other boxes, I like them all being the same frontal size. I also don't need as much output (I seem to remember the designer wanted performance comparable to an EAW kf850) I have other boxes for that size of event, and need something simple and compact for smaller gigs around 2-300 capacity, and maybe small outdoor events. I do however want very clean well controlled mid and high, which is why I chose the coax driver and hf950 horn. If I end up with loads of headroom in the coax driver that's really not a problem :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kipman725 Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2021 at 5:47pm
Seems like a good idea.  You might need to use steeper than the usual LR4 filters on the horn if you run it down to 500Hz.  Regarding the ports I would look at the expected air velocity.  Sometimes I have chosen low tuned ports to avoid acoustic gain and cuffing and just have driver cooling.  This is another driver to consider:
(more xmax might go louder)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billso Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2021 at 6:30pm
Thanks for the clarification.I was just thinking no way could it be light enough to go on a pole but  that driver combination is a lot lighter than I imagined.
What is your prediction for final weight ? 

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doober View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2021 at 9:23pm
@kipman- 600 would be a nice actual crossover point, and as the level will already be 6dB down at this point it should be low enough at 500. I'll try all this out when I've built a box, but there should be flexibility to go a bit higher if necessary. I have already looked at the B&C 15CL76, it is definitely on the short list.

I see your point about the port velocity, it does indeed go up as the tuning rises. No free lunch hey?

@billso- target weight is 30kg, 35kg is the maximum really. This might have got a bit easier to achieve as there is a shortage of 15mm ply around here so I am now looking at using 12mm which should save 4kg in the box, although it might need some more bracing inside.


Edited by doober - 14 July 2021 at 9:27pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kipman725 Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2021 at 9:43pm
my speakers are about 32kg and pole mounted... you want to be at 30kg max it all gets a bit wobbly and hard to lift them high.  This build may be of interest for some bracing ideas:


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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 14 July 2021 at 11:15pm
You may find that using a pair of 12's which would be happier in that box volume might actually give a better low end response.
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doober View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote doober Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2021 at 9:22am
Yes I tried modelling a few but didn't find anything better than the Faital 15s. Do you have any suggestions for 12s I could look at?
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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 9:46am
Few drivers which I would start off looking at:

18Sound 12ND930
Oberton 12NMB35
B&C 12NDL88

I've considered doing something similar a few times.  Inspired by the D&B Q1 but on steroids! Tannoy VQ60 is another way of doing the same kind of thing.
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