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Dual 15 + coax

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Forum Name: New Projects Forum
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URL: https://forum.speakerplans.com/forum_posts.asp?TID=106393
Printed Date: 26 September 2022 at 6:47am
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Topic: Dual 15 + coax
Posted By: doober
Subject: Dual 15 + coax
Date 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

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Replies:
Posted By: dlyxover
Date 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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In the Truth there is no news, and in the News there is no truth


Posted By: billso
Date 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 ? 



Posted By: toastyghost
Date 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.


Posted By: doober
Date 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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Blahblahblah


Posted By: kipman725
Date 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:
https://www.bcspeakers.com/en/products/lf-driver/15-0/8/15cl76" rel="nofollow - https://www.bcspeakers.com/en/products/lf-driver/15-0/8/15cl76
(more xmax might go louder)


Posted By: billso
Date 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 ? 



Posted By: doober
Date 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.


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Blahblahblah


Posted By: kipman725
Date 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:
https://www.diyaudio.com/forums/pa-systems/366187-backpack-challenge-7.html#post6706589" rel="nofollow - https://www.diyaudio.com/forums/pa-systems/366187-backpack-challenge-7.html#post6706589




Posted By: ceharden
Date 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.


Posted By: doober
Date 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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Blahblahblah


Posted By: ceharden
Date 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.


Posted By: FOO
Date Posted: 16 July 2021 at 10:06am
https://boyzpc.home.xs4all.nl/Bouwtekeningen/18Sound/K24xov60.pdf

Give this a go Smile



Posted By: toastyghost
Date 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.


Posted By: ceharden
Date 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.


Posted By: smoore
Date 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..


Posted By: doober
Date 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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Blahblahblah


Posted By: kipman725
Date 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:
https://www.diyaudio.com/forums/multi-way/292379-syns.html" rel="nofollow - https://www.diyaudio.com/forums/multi-way/292379-syns.html
you would possibly need some kind of volume filling plug that followed the cone contour to keep the air volume down.


Posted By: toastyghost
Date 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" rel="nofollow - 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.


Posted By: John Boom
Date 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.


Posted By: doober
Date 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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Blahblahblah


Posted By: toastyghost
Date 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" rel="nofollow - https://audioxpress.com/article/Test-Bench-RCF-ND950-Neodymium-Compression-Driver-and-HF950-Horn


Posted By: doober
Date 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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Blahblahblah


Posted By: doober
Date 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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Blahblahblah


Posted By: dlyxover
Date Posted: 25 July 2021 at 9:44pm
looking very tidy, keep up the good work.



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In the Truth there is no news, and in the News there is no truth


Posted By: kipman725
Date Posted: 26 July 2021 at 3:53pm
There looks to be some scope for reducing the depth of the box if you have enough internal volume for the bass drivers?


Posted By: doober
Date Posted: 27 September 2021 at 8:49pm
This was supposed to be a quick project, but then I had a really busy summer and no spare time.... back on it now though.

I put the drivers in for a test





A quick outdoor measurement





A quite worrying dip around 500Hz. I tried moving the mic around, disconnecting one driver, adding and taking bits of box, and then realised the obvious point that a half wavelength of 500Hz is pretty much the diameter of a 15" driver, which I was listening to from the edge.


I modified the baffles and moved some bits in the box





Another measurement





That's reduced the problem. I need to do more measurements when the weather is good enough, but I think it looks promising.


I did a measurement of the mid section





I'm thinking a 24dB/octave high pass with the -6dB around 600Hz should be okay.

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Blahblahblah


Posted By: jazomir
Date Posted: 28 September 2021 at 9:25pm
Slightly off topic, but I came across this the other day - Bennett Prescott sales manager of B&C with a new 2x15 + horn design. Bit (?!) big for pole mounting but sounds very good via utoob - amazing that that it doesn't have any noticeable horn sound on vocals possibly like the Celestion aperiodic driver that removes any crossover artifacts in most of the vocal range.




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For sidefills, can we have two enormous things of a type that might be venerated as Gods by the inhabitants of Easter Island, capable of reaching volumes that would make Beelzebub soil his pants.


Posted By: toastyghost
Date Posted: 28 September 2021 at 9:36pm
It’s probably more to do with using a bloody massive horn that was explicitly designed around the (very good) driver attached to the end of it, by proper engineers using top-tier tools such as FEA-based optimisation


Posted By: jazomir
Date Posted: 28 September 2021 at 11:17pm
Still, nicely done and not too dissimilar to the JTR 215RT speakers with the co-axial HF driver.

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For sidefills, can we have two enormous things of a type that might be venerated as Gods by the inhabitants of Easter Island, capable of reaching volumes that would make Beelzebub soil his pants.


Posted By: Keen
Date Posted: 29 September 2021 at 12:40am
great to see they made that horn, thanks for sharing


Posted By: doober
Date Posted: 01 November 2021 at 7:46pm
More progress. I now have 2 boxes. Both have had 2 coats of paint sprayed on and are awaiting a gentle sand and 2 thick coats of paint. I did a test with pole mounting an empty box





The pole is a K&M adjustable with an M20 screw on the base and the expanding ring lock on the top. There is a bit of sway at full extension, but that is to be expected. A single punisher as a base gets the top box just high enough, I will always be using at least 2 though.

I didn't like the idea of a surface mounted pole socket, so I knocked this up out of 3mm steel





It has an M20 nut welded inside, and fixes with 6 bolts into the horn/driver chamber and 2 through to the exterior with a plate on the outside to spread the load.

I had hoped to have the boxes finished ready to use for a couple of gigs next weekend. Unfortunately I can't get another DCX464 driver in time, there doesn't appear to be any stock in the UK.

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Blahblahblah


Posted By: Teunos
Date Posted: 20 July 2022 at 1:21pm
Hello Doober,
Did you finish these boxes by now? I'm very curious to the results of this design Big smile


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Best regards,
Teun.


Posted By: doober
Date Posted: 31 July 2022 at 8:59pm


They are finished. I've been using them since December for live music, 2-400 capacity events indoors and outdoors.

I like them. The midrange is very clear, the boxes are light enough to get up above head height on a pole.

The coax drivers are very efficient, and could easily keep up with way more than the 15s I am using. I do like headroom in the mid and high though.

The 2 boxes work well with 4-8 punishers, crossing over around 90Hz. If I used some upper bass boxes between them and took the crossover up to around 180Hz I could get a lot more out of the 15s.



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Blahblahblah


Posted By: cobrasneverdie
Date Posted: 05 August 2022 at 1:32am
Originally posted by toastyghost toastyghost wrote:

I’d also suggest considering the B&C coaxial compression driver over the BMS if you want to cross that low.

Is there a reason for choosing b&c over bms for crossing that low?  My bms 4592nd loads incredibly well all the way down to the horn cutoff on a massive 225hz multicell.


Posted By: MarjanM
Date Posted: 05 August 2022 at 10:54am
Why did you chose that specific 15 inch driver? To me its a shame using top of the line compression driver with a budget 15.



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Marjan Milosevic
MM-Acoustics
www.mm-acoustics.com
https://www.facebook.com/pages/MM-Acoustics/608901282527713


Posted By: bob4
Date Posted: 07 August 2022 at 9:33pm
Originally posted by MarjanM MarjanM wrote:

Why did you chose that specific 15 inch driver? To me its a shame using top of the line compression driver with a budget 15.


15cl76 is a neodymium driver that retails around 175 - 200€ each, so 350 -400€ per box...... not exactly thomann the box budget level? Just wondering........

Nice and interesting project Doober! Thumbs Up


Posted By: kipman725
Date Posted: 08 August 2022 at 12:04pm
The 15cl76 looks to be a decent midbass it has 8.5mm xmax, a 3" coil and 0.34 Qes.  It also only weighs 3.2kg which is probably important if you're going to put it on a pole.



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