Design study: a budget-friendly small coaxial transmission line

[voodooless]

This is a design study about a small, affordable, but relatively high-performance coaxial speaker. The idea came to me after looking for some rear speakers for my living room. They would need to be approved by the misses, so they would need to look halfway decent. After looking at some B&O Beolab 6000 series speakers and finding them too much of a gamble, I thought: well, maybe we can create something like that.

So, it turns out we sort of can The basis will be an SB coax: SB12PFCR25-4-COAX. They are about € 50 each, so very much affordable, and the spec sheet looks quite decent. There is some decent excursion, and the response is mostly clean. Crossover-wise, I'm going to be lazy here and will just use the default crossover that SB provides:

I'm sure this is not the perfect crossover, but I can always make it better. Meanwhile, room correction will need to fix the worst of the issues.

Next, the enclosure architecture. Turns out the TS parameters are actually quite well suited for a transmission line. And what's the simplest way to build one? Well, PVC pipe, of course. It's simple and sturdy enough for such a project. The only catch is that anything but a simple cut is not easy. So, mounting the driver to the side would need some extra attention. Luckily, we live in the age of 3D printing, and thus an adapter ring was easily made. To get enough length, the PVC pipe will be split in 2, and the exit will be at the top. The top will also get a 3D-printed part. The bottom end will hold the crossover and will be mounted to a 12mm round piece of plywood or MDF.

Putting it all together, it looks something like this:

So far so good. One might even replace the 3D-printed bit with a T-junction, but I found this looked way better. Tweeter level is at 75cm, and total height is about 1m.

But what about performance? I did some basic simulations in HornResp, and the basics are pretty good:

Obviously, we have the typical suckout around 200 Hz, but in reality, with some stuffing, this should be significantly reduced.

MaxSPL? Sure, and loads of it, evidently:

In reality, I don't expect this tiny thing to hit 100 dB at 45 Hz, but the sim pretends it will be... Oh well, we'll see... At least this promises more than enough for rear speaker duty.

Will I build this? Possibly! The total cost would be around € 200 for the pair, which is a really good price for such performance.

 

[kemmler3D]

Looks like a fun project! My only thought would be to do more designing of the 3D printed t-junction (as it were) to make the whole thing less reminiscent of PVC pipe. I could imagine the whole top part looking kind of space age streamlined...

 

[voodooless]

Looks like a fun project! My only thought would be to do more designing of the 3D printed t-junction (as it were) to make the whole thing less reminiscent of PVC pipe. I could imagine the whole top part looking kind of space age streamlined...

Got any ideas? My initial idea was to have to driver protrude much less, but that isn’t really possible due to the magnet structure. I may have another 10mm wiggle room. I could also make the transition even more smooth: that would lessen the T-junction-look some more.

The top cap would also be nicer if it was less visible, bud sadly, I’ll need to cap the PVC cuts to make it look clean.

 

[whazzup]

Nice! Make the tube longer, add some weight to the base, add some hooks and now your missus will approve because there's some place to hang her clothes off of now. Which doubles up as audio diffusors! Win-win-win situation.

 

[voodooless]

Make the tube longer,

I actually thought of making the tube like 230cm high, so I could then mount the height speakers in there as well But the better alternative is to just add some caps to the junction section and put them on a high wardrobe we have. I could even turn it 90 degrees and mount two PR's in the holes.

 

[MJKing]

Looking at yout hornresp simulation results, a couple of comments.

1. If you enter a "Path" length, the linear distance between the driver center and the open end center, many of the severe peaks and dips will disappear.

2. If you offset the driver to about 1/3 of the length the 3/4 , 9/4, 15/4, ... standing waves will not be excited so fewer peaks and dips.

3. You will still need to address the baffle step, your low frequency response will be lower than shown in the simulation. Try increasing the cross-sectional areas leaving the length and taper the same to see if it improves/increases the bass, your TL might be too small so your actual bass response may be disappointing.

4. While adding damping will help control the higher frequency standing waves, the harmonics, it will also attenuate the fundamental which produces bass output. Do as much as you can with the geometry before trying to use damping materials to fix remaining problems.

http://www.quarter-wave.com/TLs/Part_5_Inside_the_Box_and_Outside_the_Box.pdf

Hope that helps.

 

[antcollinet]

Interesting. I know nothing about speaker design, but if you go ahead with this I might be interested in building a prototype. I have 3d printer, and am reasonably handy with modelling.

Aesthetically would it mess performance up to make the "speaker bulge" spherical? Might it also be worth considering lowering the centre of gravity by putting some cement (or similar) in the bottom 15 to 30cm of the tube?

 

[kemmler3D]

My very general idea was to add some volume to the shape, very smoothly transitioned, so it looks sort of like part of a KEF Blade on top of a pole. Not sure if that makes sense. Basically just kind of an aerodynamic shape mostly swept towards the back.

 

[voodooless]

Aesthetically would it mess performance up to make the "speaker bulge" spherical?

It should be fine; it doesn't have to be that shape on the inside Off axis might be better. The standard crossover should be ballpark anyway. EQ will have to fix the details.

Might it also be worth considering lowering the centre of gravity by putting some cement (or similar) in the bottom 15 to 30cm of the tube?

The lower bit isn't in active use anyway. It houses the crossover and there is plenty of room to add some mass.

My very general idea was to add some volume to the shape, very smoothly transitioned, so it looks sort of like part of a KEF Blade on top of a pole. Not sure if that makes sense. Basically just kind of an aerodynamic shape mostly swept towards the back.

You're only talking about the top plate, right? That is actually a very good idea, and cut-wise should be manageable. Not sure if I’m that good at modeling.. I’ll try

 

[voodooless]

Looking at yout hornresp simulation results, a couple of comments.

1. If you enter a "Path" length, the linear distance between the driver center and the open end center, many of the severe peaks and dips will disappear.

What field is that? Doesn’t it already know this number?

2. If you offset the driver to about 1/3 of the length the 3/4 , 9/4, 15/4, ... standing waves will not be excited so fewer peaks and dips.

It’s at about 1/5 now, which should already help. At 1/3 it will need some creativity to keep the visuals working nicely. I’ll try some sims, see if it’s better. If so, I’ll try to make it work visually and practically.

3. You will still need to address the baffle step, your low frequency response will be lower than shown in the simulation. Try increasing the cross-sectional areas leaving the length and taper the same to see if it improves/increases the bass, your TL might be too small so your actual bass response may be disappointing.

Well, bass response was always a bonus anyway. I really don’t want to make the tube diameter any bigger (slightly longer should be okay).I tried, and it’s better, but this is a compromise that was consciously made.

As to baffle step: yes! The room correction will need to fix that.

Once, a long, long time ago, I created a small bookshelf with a small 3” driver with a long heavily folded line with a low SD ratio output port. For its size it, it really had very excellent bass response. Obviously it didn’t go very loud, but that was also never the goal.

This real speaker will probably be crossed actively at 80 Hz, maybe slightly lower if it plays cleanly. Nevertheless, it’s fun to push the boundaries of a design concept and see where it could end. That’s also how that bookshelf got made.

4. While adding damping will help control the higher frequency standing waves, the harmonics, it will also attenuate the fundamental which produces bass output. Do as much as you can with the geometry before trying to use damping materials to fix remaining problems.

I think the simulation screenshot already includes some damping. And yes, it does lower output! Will double check. In the end, measurements will need to be the final arbiter on where the damping works best.

http://www.quarter-wave.com/TLs/Part_5_Inside_the_Box_and_Outside_the_Box.pdf

Hope that helps.

It does! Thanks!

I was wonder the following: people (and also companies) have experimented with reflex tubes with holes in them to lower resonances very effectively. Did anyone ever try this in these kind of folded transmission lines? Tuned holes at specific distances might also work to lower resonances?

 

[MJKing]

"What field is that? Doesn’t it already know this number?"

After calculating the response follow Tools ---> Output ---> Combined and enter the "Acoustic Path Length" value.

"I was wonder the following: people (and also companies) have experimented with reflex tubes with holes in them to lower resonances very effectively. Did anyone ever try this in these kind of folded transmission lines? Tuned holes at specific distances might also work to lower resonances?"

I have never looked at holes, I want to keep as much of the fundamental resonance as possible. There are other tricks that can be used in the geometry to tame the harmonics.

 

[kemmler3D]

It should be fine; it doesn't have to be that shape on the inside Off axis might be better. The standard crossover should be ballpark anyway. EQ will have to fix the details.

The lower bit isn't in active use anyway. It houses the crossover and there is plenty of room to add some mass.


You're only talking about the top plate, right? That is actually a very good idea, and cut-wise should be manageable. Not sure if I’m that good at modeling.. I’ll try

I think maybe you could combine the top cap and driver fitting into a smooth shape... probably multiple prints glued together though, which may complicate the build process, as then you're into a fill / sand / paint cycle.

 

[voodooless]

I think maybe you could combine the top cap and driver fitting into a smooth shape... probably multiple prints glued together though, which may complicate the build process, as then you're into a fill / sand / paint cycle.

Yeah, I think the distance is a bit large to do that. Will be a lot of printing, and will go away from the tube esthetic. But the blade-like sloping is doable for sure

 

[kemmler3D]

I'm hopefully going to share a project soon that will shed some light on what I consider too much printing. the slicer says about 2.5 days for 2 plates... Luckily no painting though.