I have this pair of Dipole AMTs I’m working with. They sound great but they needed a bit of directionality to them. I built this (waveguide?) out of foam. The goal is to force the wave forward and keep it from reflecting off the lower half of the baffle where the woofer is as it’s creating some boosts in 6k and 8k octaves. After some trial and error I found an angle and distance that was manageable but now is like to understand how to model this so I can build a more calculated version of these guides out of foam. Anyone have advice on how to do that?
You ask a big question. Consider taking the walls out to be a portion of a wavelength at a frequency you want to control. Then consider adding a roundover that is a portion of a wavelength in radius to ease out these frequencies that you have controlled. Also consider baffling at this point to reduce the big jump after the radius. You might also think about angling the top and bottom.
Thanks Allen.
So my portion of wavelength do you mean 1/4 the length? Example, 6khz wave...
13503in/s / 6000
= 2.25”
1/4 of 2.25 = 0.5625”
Or am I using 1/2
Let me know thanks!
You could start with 1/2 across, then add 1/4 radius. You should then measure.
You should decide on a frequency to control. Also a frequency to cross. Consider the directivity of the woofer at your cross.
You should decide on a frequency to control. Also a frequency to cross. Consider the directivity of the woofer at your cross.
Thanks, so I'm crossing at 1800 @ -18db buttersworth. That blends nicely with my woofer and is flat at the xover point. Also, with the tweeter 3" back I get the proper timing, so I think what I'll do is put 3 pieces of 1" foam together and carve it out into one huge round-over and give that a shot, or maybe 3 different chamfer angles.
Let me know what you think or if you have any advice. If going the angle route, any tips on what the first needs to be?
Cheers,
Joey
I'd like to say try a 2" radius. This takes about 1/2" of the depth. Maybe try 45 degrees, or 60 degrees (120 total) for the other 2.5" of depth. This would be at least 9" wide, you could optionally baffle outside that.
It would be necessary to listen to this arrangement, maybe measure front and to the sides, consider where the woofer is, and all that.
It would be necessary to listen to this arrangement, maybe measure front and to the sides, consider where the woofer is, and all that.
The next step is to make the vertical dimension wind out to the baffle as it makes a square with the sides, or maybe a little more.
Nice work with the pink XPS. Adding a secondary expansion angle helps a lot.
Just use Bwaslo’s Synergy spreadsheet. Make a rectangular to square throat smooth adapter. That works very well. Also, the tractrix profile generator in HR works well too. For the Bwaslo spreadsheet, I just simulate in Akabak as if it were a piston driver with the same Sd.
Just use Bwaslo’s Synergy spreadsheet. Make a rectangular to square throat smooth adapter. That works very well. Also, the tractrix profile generator in HR works well too. For the Bwaslo spreadsheet, I just simulate in Akabak as if it were a piston driver with the same Sd.
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The best round over is to transition to a half cylinder on the sides. There is an old article of a guy testing a long planar driver with different edge treatments. Wish i could find the link. Anyway transitioning the horn smoothly to half of a large pvc tube would be ideal.
Thanks for the tip. What would you consider a large PVC tube? 3" 4"? I have some foam so I could very quickly carve some out and run at test with this tonight.
Cheers,
Joey
Much larger, large enough to join the front of the horn back to a smooth transition to the rear horn or faceplate
Here's a waveguide I made for a ribbon:
Ribbon Unity Horn
Your ribbon will not work well. You need a narrow one.
Ribbon Unity Horn
Your ribbon will not work well. You need a narrow one.
The width of your diaphragm is going to set a limit on how high your waveguide will work.
For instance, if you have a tweeter that's one inch in diameter, the waveguide that it is attached to will control directivity up to 13,500khz. (One inch.)
So when you have a diaphragm that's large, you run into an issue where the directivity is uncontrolled. For instance, a diaphragm that's two inches in diameter that's mated to a waveguide, the waveguide will have no effect below 6750hz. (two inches.)
So that's the challenge you have there. Now, if you don't care that your ribbon is going to beam above 6khz-ish, go right ahead. But if you DO care, consider a different tweeter.
Also, this is why compression drivers use phase plugs. The phase plug is there to take a large diaphragm and change the wavefront shape, and also shrink the aperture down to an inch in diameter. (This assumes a compression driver with a 1" throat.)
The BG ribbons are fairly wide, but that's why they're offered in a "PDR" and "non-PDR" version. The PDR version uses felt to change the wavefront shape.
I'm not saying your project is hopeless, just warning that directivity is going to be uncontrolled above about 6khz-ish.
Also, if you'd like to read more about this phenomenon, check out some of my threads on ribbon tweeter phase plugs, and phase plugs in general. Fun stuff!
The width of your diaphragm is going to set a limit on how high your waveguide will work.
For instance, if you have a tweeter that's one inch in diameter, the waveguide that it is attached to will control directivity up to 13,500khz. (One inch.)
So when you have a diaphragm that's large, you run into an issue where the directivity is uncontrolled. For instance, a diaphragm that's two inches in diameter that's mated to a waveguide, the waveguide will have no effect below 6750hz. (two inches.)
So that's the challenge you have there. Now, if you don't care that your ribbon is going to beam above 6khz-ish, go right ahead. But if you DO care, consider a different tweeter.
Also, this is why compression drivers use phase plugs. The phase plug is there to take a large diaphragm and change the wavefront shape, and also shrink the aperture down to an inch in diameter. (This assumes a compression driver with a 1" throat.)
The BG ribbons are fairly wide, but that's why they're offered in a "PDR" and "non-PDR" version. The PDR version uses felt to change the wavefront shape.
I'm not saying your project is hopeless, just warning that directivity is going to be uncontrolled above about 6khz-ish.
Also, if you'd like to read more about this phenomenon, check out some of my threads on ribbon tweeter phase plugs, and phase plugs in general. Fun stuff!
I’ll check it out. I’m willing to test some ribbon drivers if you have some suggestions on what to look at. These AMTs I have don’t sound beamy to me but that does not mean I am married to them.
With that said I don’t need to necessarily control the dispersion too much But i do want to smooth off the response.
Let me know your additional thoughts.
If by the looks of them, those are Mundorf's, I'd be married to them! lol
I mated up a modified Dayton H110 horn to a Hygeia RT-5002 and it worked out great, in my preliminary findings
1st iteration using fiberglass+resin to shape the throat for matching up dimensions:
Before and after frequency response:
One thing I noticed is that the tighter the throat matching to the AMT aperture, the smoother the response ends up being. My 2nd iteration will have a little bit of actual compression as I plan to make the throat just slightly smaller than the AMT aperture dimensions. I'm hoping this boosts up the lower-midrange even more. so. In fact, I liked the results so much, that I'm planning on chopping-up-then-gluing-together a couple H110 horns and reconstructing a vertical-double-RT-5002 setup to see what kind of gains I can get in the midrange.
Haha Oscar. You are in fact correct they are Mundorf AMT27D1.1 and they sound incredible despite their peaks and dips. They are also very efficient and it's insane how loud they go with no distortion.
Oscar, did I ever post the one where I made a phase plug for a ribbon and a waveguide?
I actually took a square ribbon, compressed the wavefront, with a circular exit.
There's definitely a video on YouTube, but I can't recall if I ever posted a thread or any measurements. It worked better than I expected it to!
If you're going to compress the wavefront of a ribbon, you might consider lining the compression chamber with a little bit of foam or polyfill. The reason, is because it will roll off the highs. This is a necessity in this type of design, because the pathlengths are difference.
This is one of those things where I really wish I had time to post a picture, but let me see if I can describe the dilemna:
With a waveguide, we want a flat wavefront at the throat. A ribbon is already flat. This creates a conundrum, if you're trying to make the throat smaller: some of the pathlengths are going to be unequal. The addition of foam or poly fill, on the edges of the chamber, will help reduce the impact of those unequal pathlengths. Basically the pathlengths which are slightly longer are attenuated by the polyfill.
Again, the BG ribbons use the same idea in their PDR ribbon. That's why the felt is there; it makes be beamwidth wider.
Having said all of that, I'm sloooowly moving into the Dan Wiggins camp of "why use a ribbon when a dome works so much better?" The only real advantage that a ribbon has, that I can see, is some added efficiency at very high frequencies.
I actually took a square ribbon, compressed the wavefront, with a circular exit.
There's definitely a video on YouTube, but I can't recall if I ever posted a thread or any measurements. It worked better than I expected it to!
If you're going to compress the wavefront of a ribbon, you might consider lining the compression chamber with a little bit of foam or polyfill. The reason, is because it will roll off the highs. This is a necessity in this type of design, because the pathlengths are difference.
This is one of those things where I really wish I had time to post a picture, but let me see if I can describe the dilemna:
With a waveguide, we want a flat wavefront at the throat. A ribbon is already flat. This creates a conundrum, if you're trying to make the throat smaller: some of the pathlengths are going to be unequal. The addition of foam or poly fill, on the edges of the chamber, will help reduce the impact of those unequal pathlengths. Basically the pathlengths which are slightly longer are attenuated by the polyfill.
Again, the BG ribbons use the same idea in their PDR ribbon. That's why the felt is there; it makes be beamwidth wider.
Having said all of that, I'm sloooowly moving into the Dan Wiggins camp of "why use a ribbon when a dome works so much better?" The only real advantage that a ribbon has, that I can see, is some added efficiency at very high frequencies.
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