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Old 15th April 2015, 08:33 PM   #1
onni is offline onni  Sweden
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Default 3D-printing waveguides and phase plugs

So my workplace bought a 3D-printer. Of course my first thought was: "How can I use this in speaker building."

Click the image to open in full size.
My first speaker related print. Is it obvious what it is?

Last edited by onni; 16th April 2015 at 10:20 AM.
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Old 15th April 2015, 09:09 PM   #2
onni is offline onni  Sweden
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Default A waveguide!

Of course it's a waveguide! Inspired by Dave Pellegrenes impressive results I threw a simple CAD together and made a small print. This is a 100x80x40 mm waveguide (~4 x 3 x 2") with a 1/2" throat. Lucky for me it was a test, it was supposed to be a 1" throat

Here it is straight from the printer:
Click the image to open in full size.

Click the image to open in full size.
It took 2 hours to print and weighs 55 g, material cost about 2 USD.

Removal of support material took about 1 min:
Click the image to open in full size.
The purpose of the support material is to prevent sag around the screw holes.

Here is a close up:
Click the image to open in full size.
This is the kind of surface you can expect directly from the printer. Add som primer and matte paint and I'm guessing the finish is acceptable for most.


All I've heard when people start talking about 3D-printing speaker parts is that it's too expensive, that the surface finish is crap and that it's somehow difficult. This was none of those things. The 3D-printer costs 900 USD (Wanhao Duplicator 4S) and the material 30 USD for 1 kg. The CAD took 1 hour to make (parametric geometry and new software), next one with other dimensions takes 2 minutes.

The only part I could possibly break of with my hands is the flange with mounting holes (waveguide->baffle). It has 1.2 mm thick walls and 30 % infill (as visible in the first post).
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Old 15th April 2015, 09:20 PM   #3
xrk971 is offline xrk971  United States
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Nice work! I am at about te same point as you - the software is key to make it easy. I got the Simplify3d suite which integrates all software functions needed after one has the STL file.

Is there an STL file of a horn in the SEOS thread?
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Old 15th April 2015, 09:25 PM   #4
onni is offline onni  Sweden
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Originally Posted by xrk971 View Post
Nice work! I am at about te same point as you - the software is key to make it easy. I got the Simplify3d suite which integrates all software functions needed after one has the STL file.

Is there an STL file of a horn in the SEOS thread?
Thanks! I'm using Simplify3D as well, tried free options but they were horribly slow. I have not seen the SEOS profile as *.stl. I made a circular OS profile, that was easy. But the elliptical is slightly more difficult.

/Anton
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Old 16th April 2015, 07:53 AM   #5
onni is offline onni  Sweden
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This is a comparison of the 100x80x40 mm waveguide I printed yesterday and close to the biggest that will fit in our printer:
Click the image to open in full size.
That's 210x140x80 mm (maximum is 225x145x150 mm). I added a roundover at the mouth, not sure i should (more difficult to print, Pellegrene guides have very little roundover). Printing time is about 7 h.

Maybe the most interesting thing about 3D-printing waveguides is the possibility to incorporate phase-plugs with almost any shape. Here I need some help...

My plan is to use either the D2604/833000 after seing Dave Pellegrenes results here. Before we had the 3D-printer I was planning on buying from him, I still might if I can't get good results myself. I'm hoping I can make a deeper waveguide and thereby increase directivity, and maybe incorporate a phase-plug.

Three questions:
1. What profile should I use? Right now it's a simple conical waveguide. When looking at the Pellegrene waveguides there seems to be some kind of change in slope close to the throat. OS?

2. What mouth shape should I use. When looking at the Pellegrene guides I think the elliptical guides have best behavior in the top octave. But a rounded rectangular mouth is also possible:
Click the image to open in full size.

3. How do I design a phase-plug? Where do I start? My understanding is that the purpose of it is to make the pathlengths (more) equal. So the simplest one should be a small lump (sphere, ellipsoid?) close to the center of the dome to increase the pathlength for waves eminating from the top, correct?

/Anton
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Old 16th April 2015, 09:05 AM   #6
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Quote:
Originally Posted by onni View Post
1. What profile should I use? Right now it's a simple conical waveguide. When looking at the Pellegrene waveguides there seems to be some kind of change in slope close to the throat. OS?
The profile that is best suited to the shape of your tweeter dome and surround. Simulating the propagation of the sound in 3D should help you determine this.

Quote:
Originally Posted by onni View Post
2. What mouth shape should I use. When looking at the Pellegrene guides I think the elliptical guides have best behavior in the top octave. But a rounded rectangular mouth is also possible:
The shape that provides the directivity that the speaker requires to match the other drivers and the room it will be used in. This is very unlikely to be a round waveguide but it is also likely to change from speaker to speaker.

Quote:
Originally Posted by onni View Post
3. How do I design a phase-plug? Where do I start? My understanding is that the purpose of it is to make the pathlengths (more) equal. So the simplest one should be a small lump (sphere, ellipsoid?) close to the center of the dome to increase the pathlength for waves eminating from the top, correct?
Assuming a standard dome tweeter then the KEF phase plugs they use on their coaxial tweeters is likely to be a reasonable starting point.

It is an interesting project you have set yourself but it strikes me that you probably need to learn a bit more about the physics of what is going on before you can create a successful design of your own. An alternative might be to team up with someone with knowledge in the area and work on a joint design.
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Old 16th April 2015, 10:08 AM   #7
onni is offline onni  Sweden
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Originally Posted by andy19191 View Post
The profile that is best suited to the shape of your tweeter dome and surround. Simulating the propagation of the sound in 3D should help you determine this.



The shape that provides the directivity that the speaker requires to match the other drivers and the room it will be used in. This is very unlikely to be a round waveguide but it is also likely to change from speaker to speaker.



Assuming a standard dome tweeter then the KEF phase plugs they use on their coaxial tweeters is likely to be a reasonable starting point.

It is an interesting project you have set yourself but it strikes me that you probably need to learn a bit more about the physics of what is going on before you can create a successful design of your own. An alternative might be to team up with someone with knowledge in the area and work on a joint design.
You are completely correct in your last part, I don't know enough to do this by myself. That's why I'm here! I'm not in a hurry, I want to learn and I think other people in this community could be interested/benefit from this. 3D-printers are getting cheap and increasingly common at work places.

Is it something like this I should try?
Click the image to open in full size.

I've started doing some simulations in COMSOL Multiphysics to look at how the waveguide shape affects the directivity. I'll post some pics later so we can discuss whether the model is reasonable or not.


A little background
My living room looks like this:
Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
The media console contains one subwoofer at each end and all the electronics in the middle.

My plan is to build a speaker that looks like this:
Click the image to open in full size.

or this:
Click the image to open in full size.

I don't have any drivers yet, but the plan is to match the directivity of the waveguide by rear mounting the midbass and chamfer the (thick) baffle, like seen in the last pic. As my chamfer bit is 45 degrees I'm starting with that, i.e. a horisontal directivity of about 90 degrees, I'm guessing a little higher due to the shallow "waveguide" that the chamfering will make. Vertical directivity should be narrower and I'll need to do that by careful selection of ctc distance and crossover.

/Anton
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Old 16th April 2015, 12:06 PM   #8
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Quote:
Originally Posted by onni View Post
Is it something like this I should try?
Perhaps. My only experience with cheap "3d printers" was in the mid 90s when they were called desktop rapid prototype machines. We were offered one for free by the manufacturing department if our department took over the maintenance contract. A bit of investigation determined that both the surface finish and the sagging made it unsuitable for the small aerodynamic parts we wanted to make which looked quite a lot like that KEF phase plug in size and shape except the blades were at an angle to the air. We stayed with the remote and expensive rapid prototype machine that we had been using.

Quote:
Originally Posted by onni View Post
I've started doing some simulations in COMSOL Multiphysics to look at how the waveguide shape affects the directivity. I'll post some pics later so we can discuss whether the model is reasonable or not.
That looks like good software to get the answers subject to asking the right questions (and having the appropriate modules and access to a big enough computer). I look forward to seeing some of the results from the simulations.

Quote:
Originally Posted by onni View Post
My plan is to build a speaker that looks like this:
Close to a wall like that will cause significant issues with the strong reflection off the back wall causing cancellation and reinforcement with the direct sound. It may be wise to consider a speaker designed to work flat against a wall and, optionally, placed on a surface. A waveguide on the tweeter directing the high frequencies is not going to help unfortunately because the most problematic frequencies tend to be at a few hundred Hz.
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Old 16th April 2015, 02:19 PM   #9
onni is offline onni  Sweden
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Quote:
Originally Posted by andy19191 View Post
Perhaps. My only experience with cheap "3d printers" was in the mid 90s when they were called desktop rapid prototype machines. We were offered one for free by the manufacturing department if our department took over the maintenance contract. A bit of investigation determined that both the surface finish and the sagging made it unsuitable for the small aerodynamic parts we wanted to make which looked quite a lot like that KEF phase plug in size and shape except the blades were at an angle to the air. We stayed with the remote and expensive rapid prototype machine that we had been using.



That looks like good software to get the answers subject to asking the right questions (and having the appropriate modules and access to a big enough computer). I look forward to seeing some of the results from the simulations.



Close to a wall like that will cause significant issues with the strong reflection off the back wall causing cancellation and reinforcement with the direct sound. It may be wise to consider a speaker designed to work flat against a wall and, optionally, placed on a surface. A waveguide on the tweeter directing the high frequencies is not going to help unfortunately because the most problematic frequencies tend to be at a few hundred Hz.
I think a lot has happened in the last 20 years when it comes to 3D-printers. I have the possibility to print supportmaterial in water soluble PVA which suposedly increases print quality considerably for difficult geometries. The nozzle is 0.4 mm in diameter which sets some kind of smallest dimension when it comes to size.

I used my lunch break to go to my local speaker shop to buy a D2604/8330000, they sold the last one yesterday :/ They did however have a R2604/8330000, so I bought one for testing. It seems to be the same as the XT25BG60, which is the same as the XT25TG30 with an additional magnet (and larger back chamber). The latter one behaves like this in an 8x5 waveguide:
Click the image to open in full size.

This is the D2604/8330000 in the same waveguide:
Click the image to open in full size.
Quite similar, except the XT25TG30 looses some output above ~12 kHz. Probably fixable in my nanoAVR.


The proximity to the wall is indeed a problem, I'm hoping it is (partly) solvable by adding a damping panel behind the speakers, maybe something like this:
Click the image to open in full size.
(Ignore the speaker, old rendering)
I can't move the speaker further than a feet from the wall due to how the room looks.

/Anton
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Old 16th April 2015, 05:50 PM   #10
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I think a lot has happened in the last 20 years when it comes to 3D-printers.
Perhaps but the same basic problem of surface finish and sag remain. It will be interesting to see by how much they have improved.

Quote:
Originally Posted by onni View Post
Probably fixable in my nanoAVR.
My guess would be throat geometry. Your Comsol simulations should hopefully tell you a lot about this sort of thing. You are aware of the Comsol studies by Harman on waveguides like this. I think there are more publications on the subject if you dig around.

Quote:
Originally Posted by onni View Post
The proximity to the wall is indeed a problem, I'm hoping it is (partly) solvable by adding a damping panel behind the speakers, maybe something like this:

(Ignore the speaker, old rendering)
I can't move the speaker further than a feet from the wall due to how the room looks.
Absorption will not work. At low frequencies in order to be effective you will need absorption that is as deep as the distance from the speaker baffle to the rear wall. Even then you do not want normal room absorption but to almost completely remove the rear wave. I think more realistic options might be to ignore it or design a speaker to work flat against the wall.
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