BMF 15s - Two-way Active DSP Build


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Nov 18, 2005
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The Fen Edge
My word, it feels like putting on a very old slipper being back on a forum and in this sub-forum in particular. However, outing an intention publicly is a great motivation for getting it done, so here we go...

By the way, even when I was reviewing on here, I liked to tell a story so for those unfamiliar, I don't do short.

I cannot be arsed with speakers AND subs any more. I've been running a pair of Klipsch Forte IIIs underpinned by a pair of 15" 96L sealed DIY subs. Performance, not to mention SPL capability has been epic but it's too many 96L boxes to look at. So I removed the subs, had a rejig of the speakers and you know what? I don't really miss them.

I've heard myself say it in the past - How subs lower the demands on the main speakers and amps, lowering distortion, etc, etc - but when your speakers have 12" bass drivers and do 96dB off one Watt and are frankly deafening by the time you're using a whole 16W (and yet the bass drivers have barely visible travel) you realise this dogma is really for stand mounted or floor-standing arrays of 6" mid/bass dog whistlers...

Clearly the Klipsch aren't delivering something on their own, or a build wouldn't be on the cards. They're not doing two things:

One. They have a 15" passive radiator. Even once EQ'd with Dirac live, the 36dB/octave PR roll-off delivers a pretty much brick wall drop at 35Hz. Don't get me wrong, 35Hz off big drivers has a natural power and easy grip that no 8" or less driver can aspire to make felt in the same way. They feel a LOT deeper than a paper 35Hz suggests, but when they're done, they're done,

Two. When the subs were present (crossed at 80Hz in 2.2 and in reality, taking the PRs out of the equation) it kicked that much harder. Visceral is a word I've heard used. As I've long since grown out of surround sound movie demo pieces (!) and am very much more a music listener, there really isn't much sub-30Hz info even in some much vaunted bass-demo music pieces and no, I can't be arsed with them any more either.

Don't get me wrong; we get up and dance in the living room to 70s funk/soul/disco at club SPL levels, so I'm not pretending we're being some sort of straight laced audiophools. Yes, we like it to sound good, but it also has to feel good!

Basically it's all about a large diaphragm moving air efficiently (No replacement for displacement, etc) going deep enough and punching hard at all frequencies. And thus, this project is born.

I've built a dozen plus subwoofers, but also speakers both passive and active/DSP. The speakers didn't make it on here for commercial reasons, but I'm unshackled by all of that now. This is not my first DIY rodeo, but because of the scale it could get expensive fast, because big physical volumes mean big materials bills, so I'm looking at some 'alternative' finishes borrowed from other arenas. As it stands the spec looks like this:

  • 175L nett volume
  • 34Hz port tune delivering a marginally sub 30Hz in-room response at 100dB(SPL) plus.
  • 18 Sound 15W750 mid/bass driver
  • 18 Sound XT1464 60x40deg dispersion horn
  • Tweeter TBC - Suffering analysis paralysis on this, but either 18S ND1480 of Faital Pro HF1440...
  • 800Hz LR4 acoustic crossover
  • Hypex FA123 DSP plate amps
  • Hypex IR RC kit
  • 18mm Medite MDF carcass with 36mm front baffle. 12mm (or even 9mm) internal bracing
  • 4mm sound deadening applied to all internal panels
  • Leather effect vinyl wrap to front, back, top & bottom.
  • 9mm MDF/hardboard side cheeks vinyl wrapped in 'exotic' wood and gloss lacquered
  • Vintage conical furniture legs to elevate and layback speaker by 2.5deg
  • Chrome!!!

Yep! You read the vinyl wrap right. Have you seen what the car boys are doing with stuff that can survie years of weather?

Watch this space!

Surely a 34Hz port tune won't give you that much more than you have already?
Surely a 34Hz port tune won't give you that much more than you have already?
That will be refined once in room, but I have some pretty healthy room induced peaks down in the low thirties and mid twenties of Hertz and lots of room gain. The 36dB/octave roll-off of the Klipsch passive radiator means you really are over and done by 34Hz on a speaker with a -3dB of 38Hz. I mean it's like a brick wall - literally stops dead. Running an EQ sweep from 20Hz starts with nothing and then all hell breaks loose as the PR chimes in

Going ported will 'only' result in a 24dB/Octave roll-off which will be worth a few extra Hz on the bottom end and should see me into the high twenties which will be plenty.

I also have the benefit of having had my Arendal 1723 Monitors THX in here, which have a -3dB of, you guessed it, 34Hz. They measure and sounded a lot deeper than the Klipsch, even with both ports open, so I'm drawing on previous experience of similar tunes.

It'll only cost a couple of lengths of 3" tube to play with port tunes, but I have to start somewhere!
This just got real. Drivers and horn are ordered, but why these ones?

18 Sound 15W750. This started with the perennial Faital Pro 15PR400, that has appeared in countless DIY projects, not least of which the mighty Humble Home Made Hifi Calpamos. The link opens a PDF as the design is now archived. Indeed, the original idea was just to buy that parts list and stick a plate amp in the back and DSP the crossover.

However, I got thinking that that design and indeed the 15PR400 are now a decade plus old and the passing of time may have delivered better drivers. You'd still want a low Mms, Fs and low Le, but the Xmax of 5.75mm seemed a bit parlous.

I modelled a lot of drivers for the enclosure volume I'd settled on and the 15W750 consistently seemed to be strong in all areas. Yes the Le is 1.1mH against the 0.72 of the Faital, but with an intended crossover of circa 800Hz the difference in SPL at the crossover was fractions of a dB. Fs of 39 versus the 15PR400s 35Hz in a box of this volume was irrelevant. Mms at 88.5g against 85.2g was again higher, but all but a few drivers were north of 105g, so it is again a difference small enough not to matter. However an Xmax of 8mm (Xlim 18mm) gave the opportunity to chase quite a lot more bottom end oomph.

15W750 Response.PNG

15W750 Excursion.png

For those used to worrying about Watts, the SPL graph is showing 106dB off a 20W (10.1V) input. Even a typical AVR (stripped of lies) with and actual 80W all channels driven with crest 112dB. The Hypex will have 250W bridged. With real music program rather than bass test tracks, my ears will give up long before the driver is breaking a sweat and that's before we factor in room gain, etc bringing up the 96dB Xmax limited 20Hz. With Xlim of 18mm, you're miles off pushing the driver into trouble.

18 Sound XT1464 Horn. If you're going to insist on a 15" two-way (and I am) then you need a driver & horn that will reach down low enough to beam-width match a large driver significantly below its first cone break up mode AND play up to 20kHz, or thereabouts.

The low end is limited by the largest dimension of the horn and the upper end requires a small thoat. That narrows the field down to two easily available off-the-shelf options. The 18 Sound XT1464 and the Faital Pro LTH142 both of which are designed to match their respective 1.4" mouth drivers. As small differences in the exit angle match between the driver and entrance to the horn are undesirable, nailing your colours to one horn or the other makes sticking with the same manufacturer for the tweeter a common sense decision.

For those not conversant with compression drivers, the nominal size is not the size of the diaphragm. Indeed the output of a much larger diaphragm is 'compressed' into a smaller opening, hence the type of driver and their extraordinary (by 'hi-fi' standards) sensitivity - 110dB/W is not unusual or even exceptional.

Again, the diaphragm size has bearing on the FR available. 4" won't play high enough and dome breakup modes constrain this size to midrange duties and 2" horns. 2" diaphragms lack the area to support a low crossover and are really just tweeters mating with 1" horns. 1.4 or 1.5" exit drivers with a 3" diaphragm are the sweet spot, but there is a trade off in controlling the dome (the diaphragm is an inverted dome) breakups which can broadly be summarised as 'cost'.

£200 is pretty much the baseline of acceptable performance. £200 for a tweeter. The real gooduns start at £300+. I decided in a fit of hubris that 'in for a penny, in for a pound and bought...

18 Sound ND3SN. £329. I must need my head seeing to. Read around but the only slightly less costly ND3ST is one of the best measuring 3" CDs you'll ever see. At the point that test was published, the TPM nitrate coated version of the titanium dome wasn't released. This coating pushes the visible wiggles above 15kHz to 20kHz where it has been measured in other drivers without the ND3s bottom end capability or smoothness from the 4-slot phase plug. I just couldn't help myself.

Hopefully, the box of goodies will arrive later this week, but now I've skint myself the MDF will have to wait until after pay day!
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The looks are a big issue. The speaker will stand about 107cm tall and be 60cm wide. It's got a big face to present to the world. It is 'only' 450mm deep and it is tempting to turn that around and trade the width for depth, but I want a retro look to the speaker.

Narrow/deep speakers are an '80s invention that uncoincidentally coincided with the invention of the term WAF. The marketing boys got to work on selling the idea a bunch of small drivers can equal one large driver because the radiating area is the same, blah, blah blah.

Technically it is true the area is the same, but anybody who has had a chance to compare a line of 5" dog-whistlers to one 12" mid-bass will immediately notice that it does not feel the same. There is an efficiency of coupling to the light, fluid air mass over a wide area that a narrow area cannot replicate. A wide baffle gives the air less room to escape out of the way and it has to move and it does so more easily. Yes, you can engineer your way around this (go listen to a KEF Blade) but that engineering costs. Big cabinet and big drivers is cost effective route one.

So, this big face demands some tricks to soften it's impact, or indeed make features of otherwise unavoidable voids, like the 15" hole in the front that is the horn. To whit, I scoured the internet to see what how other far better resourced manufacturers approach the issue. Many don't, but there are some gorgeous examples of how to out there.

A personal fave is the Klipsch Fifteens that never made it to market for no apparently good reason given the rest of the stable. The stripe of exotic veneer down the centre of the front baffle breaks up the expanse of wood and it you Google further, compress the visual impact further by wrapping the top, bottom and sides in grille fabric. Hell, they even deliberately left the grille magnets on display as a feature in some of the prototypes. Gorgeous.

Not so gorgeous but possibly more apposite is a speaker I discovered once I started looking at the 18 Sound drivers; the equally Italian Unison Research Max 2. Corner round overs, leather (effect?) wrap and smaller gloss wood side panels conspire to suppress the size of the speaker, whilst doing little to soften the industrial looking drivers, not to mention pin and cup grill fixings on a £6k speaker...

There are others, but after multiple nights of Sketchup bashing, I've settled on this:

18 Sound Narrow.jpg

18 Sound Narrow Rear.jpg

The front, rear top and bottom of the will be wrapped in the most textured dark brown leather effect vinyl I can find. The box beneath will be the best 25mm MDF I can lay my hands on; Medite unless somebody can suggest better. The vinyl immediately sidesteps painting and veneering and will hide neo magnets. The side cheeks will be a 6mm high quality hardwood faced plywood.

The silver bits you can see, plus the horn you can't (because life is too short to model it!) will be chrome. Chrome? Have you seen this paint? Chrome really works well on curved surfaces and as there's no hiding the horn, I plan to make a feature of it. The chromed trim ring on the mid-bass will cover the industrial driver frame and fixings, the strip on the side will be just that with each side panel split into two and the sofa legs chrome also.

Grilles will be 9mm of something I have in the garage and be covered in an antique coloured 12 count or less tapestry fabric. This stuff is really acoustically transparent, pulls taught and stays taught and is cheaper than anything attached to words like 'hi-fi' or 'speaker'. I used some before when I found out buying the official Klipsch lambswool grilles to replace my black ones was £450!


It'll be either too much, or look great. It's certainly more than I've taken on before, but broken down into it's constituent parts, there's nothing exactly ground breaking here. We shall see...
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The box beneath will be the best 25mm MDF I can lay my hands on; Medite unless somebody can suggest better.
I prefer to use valchromat instead of MDF, I find it nicer to work with and get a much cleaner edge than when cutting/routing MDF. Reportedly it is stronger than MDF though whether that is marketing blather or not I couldn't say.

It does come up nicely to my eye when oiled though that probably doesn't matter given the finish you are after.
I prefer to use valchromat instead of MDF, I find it nicer to work with and get a much cleaner edge than when cutting/routing MDF. Reportedly it is stronger than MDF though whether that is marketing blather or not I couldn't say.

It does come up nicely to my eye when oiled though that probably doesn't matter given the finish you are after.
Is that the stuff Troels uses?

Once you get over the scale of these things, the finish is extremely impressive. Very tight, very even and tough:











Time to get the timber in.
And so the endless cutting, routing and milling begin. The horn in particular and because of the way the cabinet will be covered will be something I've never attempted before.

Still, got to start somewhere and tonight we managed the [double thickness) baffles, rear panel, tops and bottoms.


Saturday. We had to go to Scotland for a funeral so I completed the initial cutting of wood, double checked dimensions (a table saw doesn't half make this easier) and marked up all of the routing out that needs to be done.



On the way back today, because it was only a small detour on the way home, we popped into who are a bricks and mortar operation with an actual showroom you can visit. This proved to be a good idea for a couple of reasons.

For starters the colour accuracy (or consistency between websites) of webshop photos leaves something to be desired. How brown? How dark? How matte? etc, etc.

I hadn't appreciated this until I had the actual vinyl in hand, but there's a large variation in the stretch of the fabrics and you can't tell that from a website. Or at least wityh my experience of vinyl, I can't!

I need the stretch to get over and round the corner roundovers and the stretch will be needed to accomplish this without bunching the vinyl up. For once, fortune smiled on me and both the look and requisite stretch came together in one vinyl that was the bottom end of the price range on offer. Also fortunate is that they had four metres left on a roll and I need slightly over three metres to get over and round each speaker. That leaves some for practicing.

Here's three more inaccurate photos for the web. It's a matte brown distressed leatherette:



Been a bit busy with funerals, cycle sportives and Covid rescheduled concerts over the last few weeks, but managed to sneak in some workshop time here and there. The multiple window braces and routing meant a lot of time with a respirator on in increasingly warm conditions, but the end result is a tight fit with no faffing about with shaving off bits here and there.


You can see the amp box nestling in there which need to be glued to the rear baffle which then needs to be routed out in situ to match the box. I'm pleased to say this went well.



The eagle eyed will spot the extra millimetre or so around the recess for the plate amp. This is required on the amp, driver, horn and ports to allow the 1mm plus glue thickness of the vinyl leatherette to lap under all of them. That's not to hard with plain old round or straight things, but the horn is both.

I could have just bolted the ting down straight onto the vinyl, but there's a 2mm vertical 'face' around the edge that I felt would look better recessed. This posed a problem as better woodworkers than I could make a template from the horn and use that to guide a router, but trying to get my head around this AND add a millimetre or so extra was too much for my increasingly small brain. I elected to measure the horn accurately route out four circles, four straight lines and do the rest freehand. Lots of potential for slip-ups and burning a baffle. Got away with it in end!






10 hours over two days and everything including the four port 1" flared openings which I won't bore you with but can be seen in the background of the picture with the horn shown. Gluing starts in the next couple of days when I've figured out where to start with the eleven separate bits of brace inside each box...

A little more progress today. Front baffle glued up and grills started. I'll glue the top and bottoms on tomorrow with any luck.

Before I put the sides on, I'll place all of the 4mm Silent Coat Extra bitumen damping sheet. It would be a nightmare job working around all the the window braces, even with holes the sizes these drivers require. Question:

Has anybody used this stuff (or stuff like it) before? If so, did you apply a PVA wash to the surface to aid adhesion? My head says it would be a sensible belt and braces approach.




Grill frames have been screwed to each other with a view to routing them out as one. It doesn't save on cutting time, but it does save time in setting up and it does mean the grills will be identical. Equally, if I've messed any measurement up, they're both scrap...

Buddy Simon for scale. It's his workshop I'm building them in. Top fella and a lot more handy at woodwork than I am, although he's never built loudspeakers. It's very useful to have someone who looks at a problem from a different angle on hand and he makes a mean cuppa.
Has anybody used this stuff (or stuff like it) before? If so, did you apply a PVA wash to the surface to aid adhesion? My head says it would be a sensible belt and braces approach.
I have used that specific product a few times, in my experience it has stuck firmly without doing anything else extra to it (and has stuck in places for years without a problem)
I have used that specific product a few times, in my experience it has stuck firmly without doing anything else extra to it (and has stuck in places for years without a problem)
Nice one. Thank you. 👍
I have used that specific product a few times, in my experience it has stuck firmly without doing anything else extra to it (and has stuck in places for years without a problem)

Same here but I used some additional pan-head nails on the edges just to make sure. My OCD was working overtime. :)
A little more progress over the last couple of evenings.

4mm Silent Coat Extra applied to the top, bottom and rear of the cabinet. It really does deaden the panels, scientifically tested using the same knuckle and ears on a treated, versus untreated top panel.



I'll finish off the few odds and sods after the top and bottom panels are glued which they're doing now:
Following here and DIY Loudspeaker project pad! Loving it
It's slow going as I only get a couple of hours here and there and inevitably, when you do get the chance of a good hit, the next job is to glue something which means a lot of sitting and staring at them!
Had decent run at it on Sunday, finishing all the Silent Coat on the side panels before starting to glue them on one of the cabinets.

There were still a couple of jobs made easier by having the sides off but he problem is I forgot one, namely fixing the IR receivers/power/clip light boards to the inside face of the cabinets. Idiot. Plan B will be to route a recess into the front baffle and sink them in and cover flush with a bit of smoked Perspex they come with. This will actually be a lot easier than trying to secure a 23mm square board over a 3.2 and 4mm hole drilled perfectly square through 36mm of baffle at precisely .7mm centres. Now I type that, it was always madness. If I trim it nicely, it look like it was meant to happen!

The other easier job was to get the legs mounted using 8mm T-nuts. Given the weight I did wonder if the 18mm MDF base would be man enough, but then remembered the sofa I'm sitting on has four such legs screwed into 12mm crappy plywood fillets and that will hold the weight of the sofa and four people no probs. Seriously, don't look inside your furniture if you worry about where your money went...



The rear leg will be shortened by 20-25mm to give the requisite 2.5deg of layback. The measurement will not be super critical as I will recess M6 spikes into the bottom of feet for levelling and stability purposes. At least I can now get an impression of the size, bearing in mind there's another 30mm of width to be glued to each side yet. See photo of 5' 10" apparent Proclaimers tribute below.


The rest of the afternoon was spent shaping up the grilles which introduced me to two tools I'd never used before; a table router and a bobbin sander. The sander is essentially a spinning, bobbing bobbin wrapped in sandpaper. It takes care, but you can seriously make jigsawed straight lines absolutely vertically true and straight. Dunno how I'd never met one before. End result:


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Managed to make some progress over the last three weeks, although it's getting to that time of year when there are lots of reasons not to hide in a workshop.

First off, I finished the grilles, because I could do this at home. The Fabric is Zweigart Aida 14 count Antique. It's a needlepoint/embroidery fabric that has very little give and stays tensioned. It is acoustically very open.

Pardon the colour accuracy as I did this outside in shade. The colour is a slightly mottled pale cream colour.



Lay on a flat surface, fold over and staple at 3, 9, 12 & 6 O'clock in that order. Because it doesn't stretch, you don't need to pull too hard, but you work your way round 1:30, 7:30, 10:30 & 4:30, progressively stapling the increasing number of decreasingly small gaps.


Once done trim up the fabric behind the staples and I then rub PVA into and through the fabric and into the frame surface. Belt and braces. When you're done:

The legs were next. Bought as a set of 150mm tapered offset sofa legs with M8 threads, I needed to shorten two by 20mm to give the cabinet 2.5deg of lay-back and recess the Insert Threads used for the M6 spikes. The spikes serve two purposes. One is so the speaker can be levelled and the other is to prevent the legs leaving indents in the carpet.

the slight issue was how to drill consistently vertical holes in non-square things and for that matter, cut the ends off two perfectly horizontal.So I screwed them into a set of 7.5mm holes drilled into a flat piece of wood.


The recess for the spike locking nut is drilled first with a forstner bit (14mm to suit the 13mm OD of an M8 nut socket and then the recess for the spike shaft. A little glob of PVA is belt and braces, again.


A couple of days later, I completed the fixed (as opposed to stuffed) damping layers. I chose a 30oz recycled wool/cotton mix carpet underlay because it's cheap as and fixed it with No Nonsense Spay Impact adhesive. After trying to spray onto the Silent Coat in the cabinet and the back of the fabric, I ended up with more fluff attached to me than the cabinet. After a quick clean up, I simply sprayed ample into the cabinet and stuck the dry fabric in. Still messy, but it got the job done.



Not that it's a scientific measure of anything, but sticking my head in the cabinet and speaking (compared to the undamped cabinet) was surprisingly dead and claustrophobic sounding. I know that's the idea, but I was surprised how dead it sounded for what is only 10-15mm of material. A small mountain of hollow fibre will be used (to taste) top, bottom and behind the mid/bass driver in the cabinet, but there's now more room to play with than using the usual open cell foam and it's a lot lighter too.

Don't underestimate that because these now weigh circa 60kg each and that's before drivers, amps, decorative covering and the final solid wood end cheeks. They're already very awkward to lift on to the work bench and that's with a selection of open hand holds which will disappear once the drivers are in. I bought some cheap castors to screw into the leg threads to make them easier to move around the workshop!

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