Building 21" sealed subwoofer w/ PD.21 drivers

Hi, I want to build some really subs that go down very low without sounding muddy.

I came across this design for a 21" sealed subwoofer.

  1. Homemade Hifi_Serious Sub.pdf
  2. Homemade Hifi_CAD_Serious_Sub.pdf

I like the approach and the designers thinking. I've got the space and making this would be cheaper than anything I'd consider buying.

The original sub bass driver used in the design was the PD.2150. That has since been discontinued and they now make the PD.2155 in a 1N (Neodymium) and 1F (Ceramic) variation. Although stock is limited on both of these.

  1. (discontinued)
Is there anything about these newer version of the PD.215X which make it more or less suitable for this use? I know I'll have to adjust the designs slightly to hit the 0.5Qtc specced in the design.

If they're both fine, is one going to be better suited than the other? I note the Ceramic version has a lower state frequency range (28 vs 30) but a highly higher f0 (35 vs 33).

Any input about driver suitability for this project is greatly appreciated as well as any thoughts on this particular design concept.

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Skip the lead sheet. The cabinet is extremely well-braced, and these low frequencies will not benefit from mass damping IMO.
The difference between those drivers is going to be negligible too, maybe a decibel here and there.

I hope you are going for the DSP processing, that's by far the best way to get non-muddy bass, and you can control system Q electronically.
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That's exactly right. I am going to DSP the bass. But I am hoping to keep the DSP out of the signal path for my main Speakers (Kef 105s), I might build a simple external hi pass filter for them.

(although I am sure DSP would be better, there is something I do like about my current all analogue setup up on an intrinsic level).
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A great choice of driver and size, and enclosure type and Qtc. Instead of starting with the worst-shaped box possible (cuboid) and 'fixing it' with bracing, you could save yourself a huge amount of time, money, materials, and weight by building a multi-sided box. This pushes the resonances way higher than your passband without adding lead lining, which simply pushes them lower - the last thing you want...
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is a cube that bad? 22" inside dimensions give a first quarter wave resonance at 154hz.
increasing one dimension to keep the cabinet volume constant is only going to make that lower.
standing wave resonances and panel resonances are two different things.
this cabinet won't have any panel resonances I'm pretty sure
A great choice of driver and size, and enclosure type and Qtc. Instead of starting with the worst-shaped box possible (cuboid) and 'fixing it' with bracing, you could save yourself a huge amount of time, money, materials, and weight by building a multi-sided box. This pushes the resonances way higher than your passband without adding lead lining, which simply pushes them lower - the last thing you want...
Not sure it saves time IME due to the extra time to do angle cuts, assembly. That said, his total panel thickness has slightly more MOE than 18 mm BB, apple, ply + whatever laminating might yield, so bracing need not be as elaborate as he did, though to keep it anchored to the floor it might need a heavy removable object perched on top if the lead is left out.
Are you able to expand on that or link me to some sort of resource where I can read more about it.

Do you mean something like an extruded hexagon?
18" subs, 55l, reverse mounted drivers for cooling. First major panel resonance is above 400Hz and entire speaker can be lifted - just! - with one hand. The enclosure is invisible, acoustically, to my ear compared with the cuboid prototype.


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Thanks everyone for your contributions. I am glad it sounds like some of the inputs ingredients seem to be getting approval, namely large driver, sealed unit.

A benefit of a cuboidal design is that all the cuts on the table saw are 90 degrees, as this is the first real speaker build I've done and that's quite appealing to me to be honest. At the end of the day MDF is cheap compared to 21" drivers. I can build a more complex design down the line and reuse these drivers.

@MrKlinky those enclosures in your pictures look fantastic, but they also look like a level of skill and time went into it that I am not sure I have posses right now and it's basically a totally different design. Did you document that particular build? I'd like to read about if it you did.

Another thing is I suspect the crossover with my main speakers is going to be relatively low as my main speakers, Kef 105, have 2x12" sealed bass cabs themselves and are no slouch in the low end. I don't know what implications that'd have on resonate frequencies - but I'd like to learn.

I feel quite committed to 21" drivers, I do want to be able to call this build an ancestor of that original design document I've linked above and a part of me loves the idea of a massive over spec driver then been driven to hard break a sweat. So I'd like to stick with that.

The other thing is, although this is a monster driver and I will be playing it loud - I don't think I'll be getting it any where close to it's limits as my main speakers just won't be able to keep up and I'll get deaf even faster than I am already. So I suspect a lot of the concerns whether the design can handle it will be moot in the real world - although please convince me otherwise.

@GM the design I linked is 2x18mm MDF. Benefit of that is prices haven't gone quite so berserk as ply.

Thanks again for all your input, I am learning a lot already.
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More bracing than in design I linked? It seems to have a about as much bracing as the enclosure can take with 4 braces horizontally and 4 vertically. The walls of the design are doubled up 18mm MDF and the braces are each 18mm MDF too.

Can I ask how you'd expect the sound quality to be negatively affected with the linked design? Let's assume I also get some measuring equipment (which I'll need for the DSP part of the project anyway), what would I be looking out for?
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All the work on these Mark II's was done on a Triton workstation using only a 9" circular saw and 1/4" router and a couple of simple jigs; I suppose about a couple of weekends and a couple of weeks of evenings. Once a few tricks were learnt they are really not that difficult. I did not document the build but will dig out the few photos I took of the build. I'm very lazy in documenting stuff...

The Mark I's were kerf cut plywood cylinders which I was never happy with and the 12-siders are easier to build in most respects. I am also building 4 x 21" versions using RCF drivers which will have a face width of 150mm and a length of about 600mm built in 18mm ply. I used 1" ply on the 18's pictured which was complete overkill, and the six new Mark III 18" enclosures using B&C drivers are made in 15mm birch ply which still has its first major resonance over 400Hz. Each of the ten subs has its own dedicated amplifier.

Good to see that you are using 21" drivers, which absolutely offer the best bang for your buck when you consider displacement volume. Larger cone area also couple to the surrounding air better (acoustic impedance) and I just know you will not be disappointed. Just one question - why Precision Devices, with their particularly small Xmax and particularly high price?

Good luck with the build, Carl.
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Reason for PD drivers is a few fold. The main one being the original design doc used a PD drivers. I am really new to all of this, so was trying to limit the possibility of a dud.

But also, I do quite like the fact they're made down the road from me (slightly figuratively) and there is something to be said for that.

That said, I am just not hugely aware of what else is out there and I used the design doc above as the starting point. I'd be interested to know what else you'd recommend. I am guessing RCF.

Is there a reason you're using Ply rather than MDF? I am guessing this is a stiffness vs density balance? Just going off current prices, MDF is appealing.
Guys, a DSP can split the low frequencies just fine, but any DSP adds delays to the signal. If you pass the signal through the DSP to the main speakers (your KEFs), the sound quality will undoubtedly degrade. If you bypass the DSP, there will be a time lag in the bass.
How are you going to solve this problem?
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