3D printed waveguides

[augerpro]

Update! I'll post results over the next few days as I progress. This post will be all SB Acoustics: SB26ADC, SB21SDC, and SB29SDAC. I also will have results for the CSS LD22, Dayton RST28 A & F, and SB19ST over the next few days.

Couple reminders, all of these have elliptical mouths. Measurement conditions are MLS with 7ms of reflection free signal, NO smoothing. The spikes you see around 24khz are artifacts of the sound card cutoff causing some crazy FFT math. This isn't my old monster baffle, so baffle effects start to show around 3khz and below. The most obvious sign is the waviness of response and ripples and dips that flip around according to angle. Anechoic response would be smoother and SPL would drop as you move off axis.

First up, version G for the SB26:



I would say we're done here, but for the heck of it I designed one with an oval throat, vertically aligned. I don't have measurements yet of that.

Unless you can design a phase plug similar to a compression driver, the key to a smooth top octave is to make the throat small, the closer you can get to an inch or so the better. That is why I was excited to see SB's new 21mm tweeter with fabric dome (no phase shield needed) and copper shorting ring! Results were very satisfying:






I have one last waveguide to test with SB19ST but with the above results and the fact that the SB21 has a shorting ring, makes me think spending more time on the SB19ST is unneeded.

Next is the big fabric dome SB29SADC. I reused the 8" waveguide from the TW29 tests I did. The SB29 looks a lot better, but still that big throat causes some issue. Still the directivity would probably match an 8" woofer well 1-1.5khz and you would have a smooth upper midrange and lower treble.

[JRT]

Brandon, Your efforts and results in this are worthy of high praise. Very nicely done!

[augerpro]

If anyone wants to play with the SB21 and SB26 designs: https://www.dropbox.com/s/ws5u5ovfj7...%20v5.stl?dl=0 and https://www.dropbox.com/s/ts535mtgou...%20v5.stl?dl=0

[augerpro]

Brandon, Your efforts and results in this are worthy of high praise. Very nicely done!

Much appreciated. I'd like to give a huge thanks to DanP at CSS for printing these. If I had to print these at retail prices, this project would be dead.

[augerpro]

Today's update. First up the CSS LD22. When I was given the throat diameter I knew there would be top octave trouble, and this is exactly what we see. The good news is now that I have this in hand, I can tighten up the throat quite a bit, at least to 1.31" used in version G of the SB26 guide which is looking quite good now.



Next is the new Dayton RST28A and F. Lot's more to experiment with: using the standard faceplate the throat is huge, and I have my doubts about the phase shield being so far away from the diaphragm. Improvements in both areas in future versions.




Up next is the SB26 version G, but I made the throat slightly oval. Imagine it being about a 3/32" longer in the vertical dimension. Forgot to do no phase shield measurements.



Earlier in my experiments I played with phase shield shape and size. Eventually settled on a round .25" diameter phase shield. But since those waveguides were so different -all were circular mouths- I thought I'd revisit phase shields to see if the earlier results held with these current, more optimized, designs.



And last, the SB26 version G with no phase shield (and with and without a shim at the throat):




6" and 8" guides for SB26 and RST28 coming soon.

[andy19191]

Much appreciated. I'd like to give a huge thanks to DanP at CSS for printing these. If I had to print these at retail prices, this project would be dead.

Do you know the printer type and material? Is the deviation from smooth in the photo above due mainly to the printer or the resolution of the STL triangles?

[Navy Guy]

None of the images are visible for me

[augerpro]

Do you know the printer type and material? Is the deviation from smooth in the photo above due mainly to the printer or the resolution of the STL triangles?

It's laser-sintered nylon, that's about all I know about the machine. The resolution of the .stl is Fusion360's "High" setting. The lines are visible but I think the lighting is exaggerating the effect in this photo.

Anyone else having problems with seeing the pics? Navy, are you at a work computer?

[andy19191]

It's laser-sintered nylon, that's about all I know about the machine. The resolution of the .stl is Fusion360's "High" setting. The lines are visible but I think the lighting is exaggerating the effect in this photo.

Thanks. That machine is from the high quality end of the spectrum which I suspected. I am toying with the idea of playing with one from the low quality end of the spectrum and possibly reproducing one of your builds. Could you provide something like an IGES or STEP file of the smooth surfaces rather than a "bag of triangles" STL file?

When I briefly played with generating waveguide STL files a few years ago I initially had a problem with the edges lining up in ovals due to the way the triangles were generated. Changing the tessellation algorithm for a more random one broke this structure resulting in a smoother looking surface for the same number of triangles. Not sure if your commercial software will offer the choice but it might.

[TN Allen]

You might try sanding low end 3D printed guides for a better finish. This worked for me with MakerBot printed guides a few years ago. I don't recall the filament I used.

[bwaslo]

keep in mind that wavelengths that these handle are 3/4" and longer. Lack of smoothness of the levels discussed here isn't going to have much effect at audio wavelengths.

[augerpro]

Last update! Moving now to the 8" waveguides. (note: 8" is the width, as these are all elliptical mouths based on the golden ellipse, the height will be .618 x width).

First up is the SB26ADC. W/ and w/o phase shield, and w/ and w/o throat shim:






RST28F on 8" waveguide:



RST28A on 8" waveguide:



Can't weight to redesign with the smaller throat and no faceplate...

Next are the 6" waveguides. Starting to get enough variation to identify trends accross size. SB26 first, in .75" and 1" depth:










And finally RST28's on 6" waveguide:

[augerpro]

Going forward I need to get my Presonus 22VSL working to get measurements out to 48khz. These plots hint that some subtle tweaking of the phase shield may allow us to notch out the dome resonance of the metal tweeters. That would benefit both linear and non-linear distortion!

[augerpro]

To get an idea of what these could look like after simple crossover/eq is implemented, I've taken the SB26 on the 8" waveguide C above and applied a HP and high shelf. Shown is on axis, and 40 degrees off-axis which I've found is a good proxy for the "early reflections" as defined by Olive @ Harman:

[jhollander]

That is impressive. I'd like to hear a TM with that tweeter and guide.