Bassleg 15"
This post describes my bass extension stand for the Aktiivinassakka speaker build. As mentioned in that build post, the midbass driver is closer to a midrange driver and I think it will benefit greatly from added woofer, that will make it a 3-way as a whole. The idea was to build a bassleg to give more capacity and power to low frequencies, but not to be a real subwoofer. That would give target frequency low frequency of 30-40 and upper frequency limit of 200-250. The midbass of the main extends easily only to about 200 Hz, so that's one good reason to not design for a subwoofer here. I got subwoofers that I can still use.
The size of the bassleg should not be much bigger than the main speaker so that they could be nicely stacked. The depth of the speaker is 35 cm, so that's also the target for the bassleg. I had commercial subwoofers in use, but their capacity (maximum SPL to lowest frequencies in music) was not even close to what the main speakers could do. I had a stack of main speaker -> small stand -> subwoofer. That height was good, so that set my target height to 70 cm. That leaves width as the only variable, and it needs to be wide enough to accommodate the chosen woofer.
Description:
- 15" woofer
- ~70 liter sealed cabinet. Width x Height x Depth: 45 x 70 x 35 cm. ~31 kg.
- Intended frequency range: 30-200 Hz
- External amplification
- Not a subwoofer
Finding Suitable Drivers
Capacity wise I thought that 15" woofer would supplement a 10" midbass well. So I made a list of all (reasonable) manufacturers that I can think of. The list is 23 names long, mostly PA manufacturers and a few hifi makers. I spent literally dozens of hours looking at specsheets of 15" drivers and writing them down on a Google Sheets sheet so that I can filter the candidates by desired parameters. I ended up with a list of more than 200 drivers.
I read somewhere that the Efficiency Bandwidth Product (EBP for short) value (Fs / Qes) of less than 50 is best suited for sealed enclosure, greater than 100 goes well with ported boxes and values from 50 to 100 are good for both. Okey, so just a bassleg, so a sealed would do nicely. That decided, I also started to prioritize fairly high Qms, because the suspension can be looser in a sealed enclosure than in a ported one, and it can be slightly more efficient that way.
Simulation and Modeling
When I started simulating drivers in VituixCAD, I noticed a couple of things. First, most often the simulated performance of woofers of the same size are (almost) identical when equalized for equal frequency response, and this includes group delay, and also in comparison to ported boxes. I have a DSP processor to be used with this bassleg, so should I really pay any attention to EBP or many other T/S parameters? I don't know the exact answer to that (even a year after), because I have not tried "all" kinds of woofers, just the one that I picked. But it sure looks and sounds like that the parameters are not as meaningful when you can just DSP it flat. Pay attention to power handling and Xmax when designing a (sub)woofer thou.
Second point that I noticed now is that woofers with low EBP value are most often the most basic models in a manufacturers catalog. That also got me wondering that if I want neodymium magnets, shorting rings and all the bells and whistles, I will need to drop this filter. There are exceptions, but they are rare. I decided to trust the simulation more than any rule of thumb, unless backed by simulation.
So at this point my preferences changed. I was now looking for a driver with good power handling and Xmax that also models well into a sealed box. There are dozens of choices, so I started to look at drivers. Yes, look at drivers, and only model those that have nice appearance that goes well with the mains. Now I finally have a short list.
Choosing the Driver
In the end I went for the Eighteen sound 15ND930, that ticks all the boxes of a high tech driver and looks fantastic, also the looks matches well with my existing speakers. The driver has very flat frequency response up to 1000 Hz, so you could easily use it as a mid-bass with a 1.4-2" compression driver. The manufacturer also advertises that it does well in subwoofer duty, so this a very versatile driver. I picked the 16 Ohm version. It's output will be more than enough for home use and my gain structure can handle it.
I have now eyed hundreds of drivers, so I got blind to certain aesthetics, but looking at the selected driver now it's visible parts kind of gives an expression of very high quality and endless power. The dust cap is just so manly. I wonder if it's the same with all large drivers...
The Enclosure
So a 15" diver is about 38 cm wide. Leaving some margin on both sides, I first though of a 42 cm wide enclosure, but I widened it to 45 cm to make sure that there is enough room for braces. With 25 mm wall thickness that results in a slightly over 72 liter cabinet, excluding braces and driver volume. That is very good for this woofer, so that it will be.
I drew the cabinet with FreeCAD and made sure all parts and braces are the correct size, fit together and it's easy to install even when glued together. Well, there is a 15" hole in the middle, you can fit almost anything through it! After getting it from the carpenter shop I drilled the cable terminal hole myself, this is completely passive unit.
FreeCAD is quite nice, although highly unstable (at time of designing the bassleg). I was supposed to learn Fusion360, but it was not intuitive enough, so I ended up still using the FreeCAD. I have since then used FreeCAD to design a 3D printed small speaker enclosure, I will write about it soon.
I again ordered the cabinet from the same professional carpenter shop that I used in the past. I got nice and painted enclosures that matched the drawings perfectly. Then I glued butyl damping/insulation mat (from car accessories shop, similar than I used with the main speakers horns, but without the aluminum) onto the walls and stuffed it with polyester pillows. A local furniture and decorations shop sells plain polyester filled pillows or cushions that you can upholster, but for speaker damping they are perfect as is. They come in sizes of 200 g and 450 g filling, which is the cheapest and/or easiest polyester-in-bag that I could find. I used a total of 1300 g of polyester filling per cabinet.
Amplification and DSP
I got a miniDSP SHD for DSP and preamp and NC400 mono blocks on the shelf, so I use them to test the speaker. They perform well, so I started to measure and craft a DSP program starting with a Linkwitz transformation and then moving to fine tuning with peak filters and room correction. My original plan was to get amplification from the main speakers plate amp, which has BTL for the woofer, so splitting that would give me another amp way. I had to forfeit that plan and make use of my NC400s because I knew that splitting the BTL would strangle the output of my midwoofer and would not be enough for the woofer to keep up with the compression driver. So I kept the main speakers as they were and hoped that the NC400 will have enough voltage swing to cope.
Because the main speakers midwoofer loses frequency response rapidly below 200 Hz, I decided to cross there to the bassleg. The bassleg gets quite high boost below 100 Hz, which is expected from PA drivers, but it has the power handling to deal with it. Again, I applied room correction. Even with that kind of high boosting, the driver doesn't visible move. Only when you seriously turn up the volume, you can visible vibration. They go nicely to 20 Hz, but the excursion gets high after that, so I have not tried to get them go any lower than that. I strongly suggest a high-pass "infrasonic" filter around 10-15 Hz to protect a driver from over-excursion in a DSP powered system, because it's too easy to drive a driver past its physical limits. I added one to be safe.
I had once loaned a Brennenstuhl brand power consumption meter and I measured how much one NC400 consumes when driving this speaker. The base consumption without signal is about 22 Watts with this one unit. As one would guess, a 97 dB/Watt driver needs to seriously loud before it exceeds 22 Watt consumption. And it was, I needed to turn up my preamp to single digit attenuation before the power meter flicked. This requires hearing protection even for short term tests. After that each dB raised to consumption heavily and I quickly reached 200+ Watts. The NC400 is a very capable amplifier and the driver is frighteningly good performer, there was no sign of distortion or non-linear performance (through hearing protection), just sound pressure and a wildly pumping diagraph.
Pictures
Conclusion
They rock! They exceeded my expectations and completely obsoleted my commercial subwoofers. Sure my usability went slightly down because now I don't have signal sensing power on for the basslegs and I need to select the preset on the preamp. But in the end that's not a big deal, because the mains are good enough for everyday use for my whole family and I can toggle the basslegs on when needed. Not a big thing. It also gives me a cool ritual, so that guests know that something new is coming.
I would kind of demote the EBP value from a criteria to a simple indicator. EBP will not tell you how well a driver does in sealed or vented enclosure. Simulators, especially VituixCAD, are much better at it. If a driver simulates well in a decent simulator, you have got better changes with that than with EBP values. Also a DSP processor is a lot better tool than a rule-of-thumb based driver selection. It unlocks so many drivers to choose from, you could say it's a must-have feature on a modern sound system.
If I knew from the beginning how it would go, I would have made the main speaker slightly smaller and position bassleg woofer slightly higher so that drivers would be closer together. I would need to consider lower Ohm versions of drivers so that I could better match the efficiency of the HF drivers or at least use a higher power FusionAmp. I would build them as separate boxes again, because they are much easier to handle and move around than if they were in one big coffin.
For the next thing I might want to drive these basslegs with FusionAmps, so small separate enclosures for them might be in order. Speaker wise, if I want real rumble base that goes lower and with more force than these 15-inchers, I would need to look for a quartet. Or I could repurpose these as actual subwoofers and build smaller basslegs, because these exceeded the capacity requirements so well.
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