3D printed waveguides

[nareshseep]

Good night all, does anyone know where I can get a round 6-8" waveguide that is around 1" deep? I plan to utilize this for the SB26STAC tweeter. I got suggestions for the SEOS8 already, but from this thread I saw benefit for the round shallow waveguides.

[augerpro]

Big update! This will be my last as the money to go on has run out. This is a big data dump as I haven't gone over it for trends, so I could use members help here to tease out best practices. I have several variants of the 5" guide including a shallower 1/2" deep version. I end with two 8" waveguides.



Above I had mentioned that a waveguide between "A" and "C" should be ideal. Here is that waveguide "E" with and without phase shield:





Early on I had discovered the terminals on the tweeter sometimes kept it from sealing against the back of the waveguide. So I wanted to try a thin shim to get a good seal. Here is that result with a .03" thick shim:




I wasn't sure if the difference was due to sealing or because the ever-critical throat geometry changed, but figured trying a thicker shim might shed some light on that. Here results using a .06" and .09" shim:




Now the phase shield is moving away from the dome as increase the shim, so in the spirit of good scientific method I felt I needed to measure with the phase shield back closer to the dome. Now this is not back to default distance since I had to cut the phase shield off and then using some bluetac moving it closer to the dome about 1/16".So it is close to default but not exact:




On the no-phase-shield version I did only one measurement with the .06" shim, I'll try to get others if I have time:



My attempts with an occluded surround have been poor but the last version I made the throat smaller, terminating above teh former of the diaphragm where the earlier version terminated above the surround. I went back to that larger, more promising throat here:




Here is the shallower 1/2" deep guides. The first is simply a shallower version of the earlier A,C,E type of guide:




As I go shallower, or much wider -like the 8" guides- the horn curve radius gets larger and/or the throat entry angle gets wider. Smaller radius and narrower (@80 degree) throat angles seemed better from earlier experiments. To accomplish that I tried the elliptical profile of the horn wall again. Here is that result for the 5 x .5" guide, I have not had the 8" version printed unfortunately. Both with and without phase shield:




And finally the 8"-ers! One is 1" deep, the other 1.5". With and without phase shield, and tried a few shims.



Here is the 1" deep guide:






And the 1.5" deep one:






The shims have a clear result worth investigating. Phase shields pretty much always have an advantage, but this has narrowed since going strictly to elliptical waveguides. Other things worth studying are how some of these features effect the directivity in the top octave, how the resonant peak is knocked down or amplified (I'm talking 20-23khz, at 24khz the soundcard cutoff causes all sorts of spurious spikes so ignore that), and the effect on mid-band smoothness (for example look at the 40 degree measurement across designs, some are straight and quite smooth, others very curved and/or bumpy). I hope to keep going, I think some designs are already the equal, if not better, than the Kef and Revel posted above.

[ani_101]

Thanks Brandon, very detailed analysis...

[augerpro]

Should have a Satori TW29RN soon for testing. Hopefully the diaphragm size is similar and I can reuse some of these guides.

[mememe]

Hi Brandon.
Any updates?
Paul.

[augerpro]

Well I bought a house and I'm in the middle of remodeling the main floor. Then in March we have a major plant outage at work, so I probably won't be able to get back to this until early summer. But I'm excited to get back to it, I was pretty happy with some of the last results (even though no else seems to be), and now have a Satori TW29RN to play with too. I may try a smaller 19mm tweeter too.

[Ro808]

Brandon, thank you for all your efforts. It has been a very educational journey so far. Would you mind sharing the CAD files for the 8" ellipticals?

[augerpro]

Yeah let me look them over and I'll see what is worth posting.

[xmax]

I recently CNC'd some .75" deep 6" diameter waveguides from mdf for the compact SB ring tweeter and so far it seems
to be doing exactly what it needed to (increase output below 4K and better time align the drivers) seemingly without
causing any ill side effects. I'm not going to post any pics or measurements because I would have to endure a millions
posts of how I should have done things from the trolls and I'm still experimenting with depths, diameter, shape and
profile. PM if interested.

[Ro808]

Due to differences in wave propagation between dome tweeters and compression drivers, the (science behind) waveguides for dome tweeters tend(s) to be fairly straightforward and generic.
Harman uses state of the art technology for development of their horns and waveguides. Still, the 5th generation waveguides for their Revel brand are less complex in geometry compared to previous versions that featured tiny slots near the throat. These WG's are basically gently (tangetial to the dome) curved ellipticals with a phase shield. For the JBL professional monitors advanced custom dome/WG combinations have been developed. Cloning these for DIY applications is useless in my opinion.

This idea is consistent with the COMSOL literature on this topic and other examples of FEM/FEA optimized waveguides. The shape of the dome and (position/dimensions of its) surround appear to be decisive for the initial shape/geometry at the throat. This seems the most difficult and least generic aspect in the design for a tweeter waveguide. A deep waveguide with a narrow angle at the throat will evidentally yield the highest boost at lower freqs, but if there is something wrong with the area around the dome + surround you'll most likely end up with a rather miserable response from 8-10KHz and up. Presumably, this is caused by interference from :
- misalignment between the dome shape and geometry of the WG
- reflections from the WG against the surround etc.

This behaviour may be attributed to the spherical wavefront from a dome as opposed to the wavefront from a compression driver. The (annular or whatever) dome and/or phase plug from the latter yields an optimized (equalized) wavefront across the band (for subsequent shaping by a suitable/matched horn), whereas a bare tweeter dome lacks such a feature which may cause inference when an improper waveguide is added.

As Brendons guides have shown and according to Revel, a phase shield may improve the response in the upper octaves.

The image below shows a FEA optimized WG from Arendal. It's quite deep compared to the Revel WG's and the space between the dome/surround and the actual WG sidewall is clearly visible.

[TN Allen]

Playing the devil's advocate;

I'm wondering how many people are actually listening to the effect of WGs on the sound and the music?

There seemes to be a strong focus on testing and producing graphs, but not much listening to music played through WGs in cabinets, and comparing the sound of drivers in WGs to the same drivers in flat baffles in similar cabinets, as well as experimenting with cross over networks to explore shaping the effect with and without WGs?

To go a bit further out on this limb, wouldn't it be good to build a data bank of subjective, empirical data to collate with graphs from different testing devices?

[DanP]

Playing the devil's advocate;

I'm wondering how many people are actually listening to the effect of WGs on the sound and the music?

There seemes to be a strong focus on testing and producing graphs, but not much listening to music played through WGs in cabinets, and comparing the sound of drivers in WGs to the same drivers in flat baffles in similar cabinets, as well as experimenting with cross over networks to explore shaping the effect with and without WGs?

To go a bit further out on this limb, wouldn't it be good to build a data bank of subjective, empirical data to collate with graphs from different testing devices?

The problem with subjective data is, well, it's subjective. It's hard to tell which direction you should personally take based on the opinions of others, but once you can link your own preferences with objective, measured data, it is a valuable tool for guidance.

Dan

[DanP]

Due to differences in wave propagation between dome tweeters and compression drivers, the (science behind) waveguides for dome tweeters tend(s) to be fairly straightforward and generic.
Harman uses state of the art technology for development of their horns and waveguides. Still, the 5th generation waveguides for their Revel brand are less complex in geometry compared to previous versions that featured tiny slots near the throat. These WG's are basically gently (tangetial to the dome) curved ellipticals with a phase shield. For the JBL professional monitors advanced custom dome/WG combinations have been developed. Cloning these for DIY applications is useless in my opinion.

This idea is consistent with the COMSOL literature on this topic and other examples of FEM/FEA optimized waveguides. The shape of the dome and (position/dimensions of its) surround appear to be decisive for the initial shape/geometry at the throat. This seems the most difficult and least generic aspect in the design for a tweeter waveguide. A deep waveguide with a narrow angle at the throat will evidentally yield the highest boost at lower freqs, but if there is something wrong with the area around the dome + surround you'll most likely end up with a rather miserable response from 8-10KHz and up. Presumably, this is caused by interference from :
- misalignment between the dome shape and geometry of the WG
- reflections from the WG against the surround etc.

This behaviour may be attributed to the spherical wavefront from a dome as opposed to the wavefront from a compression driver. The (annular or whatever) dome and/or phase plug from the latter yields an optimized (equalized) wavefront across the band (for subsequent shaping by a suitable/matched horn), whereas a bare tweeter dome lacks such a feature which may cause inference when an improper waveguide is added.

As Brendons guides have shown and according to Revel, a phase shield may improve the response in the upper octaves.

The image below shows a FEA optimized WG from Arendal. It's quite deep compared to the Revel WG's and the space between the dome/surround and the actual WG sidewall is clearly visible. [ATTACH=CONFIG]n1352814[/ATTACH]

What is your source for this info? I ask because I have not found this viewpoint expressed much in literature, but it mirrors my empirical data almost exactly, specifically with regards to the generic nature of all aspects besides the dome/waveguide interface.

Dan

[TN Allen]

..., but once you can link your own preferences with objective, measured data, it is a valuable tool for guidance.

Dan

But, is anyone linking personal/subjective preferences to test results/data, by listening to left & right completed cabinets, drivers in WGs vs. drivers in flat baffles?

And, perhaps there are ways to define a set of criteria to relate "subjective" impressions based upon careful listening to test data? Not perfect, but better than graphs generated from one driver in one WG.

[augerpro]

Ra808> do you have any links to the Comsol Lit or the waveguide you posted? I agree that everything important happens due to the particular combination of the throat dimensions vs the dome/surround dimensions, and for the DIY'er there isn't any predictive way that I can see to help. How could anyone predict I would have so much trouble with the first designs and the SB26, while just about any horn people slap on a RS28A looks ok (Vifa XT25's too)? BTW when you say tangent to the dome, do you mean perpendicular (normal)? Since these are only partial spheres I don't see a way for the mouth opening to actually meet tangent to the dome.

Tim> *when* I get some final designs, I will definitely compare vs the same basic design but without a wg. I've done it once before with Dave P's wg's on some Vifa tweets and it was really ear opening. As far as metrics to quantify, what are you thinking of?