3D printed waveguides

[TN Allen]

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.

I was thinking of appearance, the filament surface is sometimes irregular. It might be possible to paint the surface too, but I haven't tried that.

[andy19191]

I was thinking of appearance, the filament surface is sometimes irregular. It might be possible to paint the surface too, but I haven't tried that.

When examining some of the examples from a low cost 3d printer I was concerned about the likely structural integrity of a thin structure like a waveguide. Is this a recognised problem?

[TN Allen]

It would depend upon how thick the guide is, and how dense the print. I've used .1" thickness, and chosen a dense matrix (?). The inside is a honeycomb matrix, so to speak. The guides seem quite strong. I doubt I could break them using my hands.

It probably would not be difficult to find published data for the post printing hardened plastic/filament.

I think one instructor where I teach may have tested some pieces she printed in a materials class she taught. I'll ask if any of the data is available.

I milled one after printing, it wasn't a heavy cut, but the plastic held up well and milled nicely as I recall. All of the above pertains to guides printed on a MakerBot.

[Mike220]

You can 3d print to whatever in fill you wish. You can do almost no infill and make it like a vase or do 100% and make it solid. Depends how much filament and time you want to consume to make them. I can try printing one at some point if i find a good match for a project.

[augerpro]

Could you provide something like an IGES or STEP file of the smooth surfaces rather than a "bag of triangles" STL file?

Looks like I can export to both of those. What are the advantages of each? Which would a 3D printer operator prefer? Or CNC operator?

[augerpro]

I want to take a moment to demonstrate how different measurement techniques impact the results. I use a 7ms gate that is FREE of reflections, and no smoothing. The gate itself actually has a smoothing effect once you see what is going on in the math. For example, at 7ms, the cutoff frequency is 143hz. This is also the first valid data point. The next valid data point is double that: 286hz. Each following data point is 143hz from the last and everything in between is interpolated. So if the valid data points are wider apart, with everything in between interpolated, you can see that effectively you lose resolution, or "smooth" the response. How would the data points be further apart? A smaller gate. For example at 3ms the first valid data point is 333hz, and all of the next data points are simply every 333hz after that.

The reason I bring this up is that I want people to understand that my plots show as close to reality as practical. Because of that, some people may say, oh look at that dip, or that peak, I don't want to use that driver, speaker, etc. I'll use that one over their that is oh so smooth. But the smooth plot is probably not realistic. If it looks like it was drawn by a child with a crayon, I have my doubts on the accuracy Below is a plot where I play with gate and smoothing to give you an idea of the effects. This is why knowing the measurement conditions is critical when evaluating, and especially comparing, different plots.

[augerpro]

Quick summary on the designs that I'm really liking at this point. I also need to do some additional analysis, like CSD, waterfall, etc. Any preferences?

[augerpro]

When I first started this project, I didn't want to make generically sized waveguides, but ones driven by application. The first application was: crossing to a 12-15cm woofer at 2.5khz. So let's pick that apart. First, we want the directivity to be about the same at the crossover point. Looking at some manufacturers' plots of the 0 and 60 degree responses, they were typically 2.5-5dB apart at 2.5khz, with a sweet spot of 3.5dB. So this is the first criteria: 0/60 degree plots 3.5dB apart at 2.5khz. Second, I wanted to match the acoustic center of the woofer, typically about .75" - 1" deep for 12 - 15cm woofers. I picked .75" and moved on. I should mention though that now that I have some good designs, I need to verify this assumption. Some plots to illustrate:






Regarding directivity, I think this design hits its target, in fact it might even be able to be a bit smaller. Acoustic center is an assumption to be verified, but should be pretty close.

So what are some other applications? I think matching to a 18 - 22cm is an obvious one and one reason why I jumped from 5" to 8" waveguides. I still have to define directivity @ XO, and acoustic center targets. So work continues there. Any thoughts here appreciated.

Now most of the time people are wanting to use a waveguide to push lower, because you now have control of dispersion. But I can also see value in using it to push higher. Think a small 3" midrange where the designer wants it to carry most of the vocal range, so no crossover until 3-4khz. Like this Scan Disco 10F:



The waveguide allows us to push higher because now, even though we have some roll off of the 3" driver response, we can now match that response for the tweeter. So the off axis response will be falling, but still smooth. So we could define that application as -3.5dB at 3.5khz, and maybe .5" - .75" deep to match the acoustic center.

Anyway, that's how I'm approaching this. What typical apps would you guys like to see?

[andy19191]

It would depend upon how thick the guide is, and how dense the print. I've used .1" thickness, and chosen a dense matrix (?). The inside is a honeycomb matrix, so to speak. The guides seem quite strong. I doubt I could break them using my hands.

It probably would not be difficult to find published data for the post printing hardened plastic/filament.

I think one instructor where I teach may have tested some pieces she printed in a materials class she taught. I'll ask if any of the data is available.

I milled one after printing, it wasn't a heavy cut, but the plastic held up well and milled nicely as I recall. All of the above pertains to guides printed on a MakerBot.

You can 3d print to whatever in fill you wish. You can do almost no infill and make it like a vase or do 100% and make it solid. Depends how much filament and time you want to consume to make them. I can try printing one at some point if i find a good match for a project.

It was less the strength of the material and more that some runs of material seemed to be laid next to each other without being fused. If I recall correctly, this seemed to occur most at the boundaries between light and heavy density of material. It was hard to resist the urge to twist the structure to see if it would snap. Perhaps one needs to learn how to avoid this sort of thing?

[andy19191]

Looks like I can export to both of those. What are the advantages of each? Which would a 3D printer operator prefer? Or CNC operator?

They are the old and current open standard for exchanging CAD data. They will contain the solid model as a set of smooth shells, faces, edges and points. To create an STL file for a 3D printer or CNC machine this exact solid model is approximated by covering the surfaces in triangles. I wanted to avoid the triangles because the algorithms used to generate them can sometimes perform poorly with smooth surfaces like waveguides leaving visible edges. Alternatively if you can tell me mathematically how the surface was defined that would be good but a often a CAD user has let the software do some blending for them and so doesn't know mathematically but as a sequence of CAD operations.

[augerpro]

SB26ADC in 5" G and SB21SDC in 5" A in .STEP format, which should be easier for the CNC people:

https://www.dropbox.com/s/3ft1m04i22srqqz/5x.75%20G%20SB26%20%20v5.step?dl=0

https://www.dropbox.com/s/b90bd6sv6ajpk8s/5x.75%20A%20SB21%20v5.step?dl=0

And SB26ADC in 8" C, .stl and .step:

https://www.dropbox.com/s/ayihv0yx83svama/8x1.5%20C%20SB26%20v3.step?dl=0

https://www.dropbox.com/s/44jnjw4i7obh8ew/8x1.5%20C%20SB26%20v3.stl?dl=0

[andy19191]

Thanks for the step files which seem to load without problem.

[Mike220]

Aspire doesnt read step files but does STL. I think aspire is what most us are using. I'm not trying to cause excess work for you or be a nuisance.

http://support.vectric.com/aspire-questions/item/what-3d-file-formats-can-be-imported

[augerpro]

I'll do both .stl and .step since the needs of CNC and 3D printer operators are different.

[TN Allen]

Aspire won't import/read .step/.stp files, seems odd? .stl files are primarily 3D print or stereolithography files. Does it read .dxf, .dwg? No reason a .stl file couldn't be converted to another format, or read as a part file, but it seems limiting for exchanging designs.