2 way waveguide speaker build ABEC modelling

[fluid]

Originally Posted by AllenB

So I see this was a shape in the Ath library, one that reminds me of the JBL M2. IIRC Earl once suggested that the goal with this would have been to create diffraction in consideration of the precedence effect with a pseudo random spread in time and space.

I can see why you might think that but no this was not a shape that was included in the Ath demo files. There is some similarity sure but there are some key differences.

This is the JBL Style file included with Ath 4.5. It has "knuckles" that protrude out. I haven't run the simulation for this demo properly to show what it looks like but the same sort of thing in previous versions of the tool was a little ragged. Where and how the bumps stick out make quite a difference,





My waveguide doesn't do that, it's hard to see exactly what is going on without being able to revolve the 3D model. There will be diffraction but it simulates as well controlled to improve the outcome.

Originally Posted by AllenB

I could see the benefit of running diagonal polars, and I could also see the spinorama data here as not being on point. What you may be trying to show isn't easy or straight forward as I've watched others attempt to do the same.

I have ran diagonal polars and they are very similar to the horizontal and vertical, there is nothing horrible hiding there. I am much further along than I have shown so far but I am trying to post it in some semblance of a chronological order. So when I get to that you can tell me what you see.

In what way is the spinorama data not on point? What do you think I am trying to show and why is it not easy or straightforward?

[fluid]

I went back to 1/4 symmetry to speed things up while trying to work out the kinks in the simulation. Don helped me to understand that the waveguide mesh needed to be used to cut the baffle in the cad model to make sure that there were no gaps and that the mesh and baffle were conformal.

I exported the mesh from ABEC as an STL, imported this to Fusion, converted the mesh to a BRep surface and used that to cut the baffle. This made the corners line up to the mesh points and made the baffle fit tight to the waveguide mesh.





I was still trying to save some elements by making the baffle mesh less dense.

This is the simulated output, not bad till 5K but not right either.

[fluid]

Don recreated the CAD files and meshes and ran a simulation showing much improvement. He used a flat interface instead of the one created from Ath and made it so that every waveguide and baffle vertex were aligned with each other. The vertical edge is rounded so the vertical looks smoother than the horizontal.





This is the result from that simulation, much better. I am using the Horizontal curves as a comparison because they are the most representative and show the differences between the different tries. I have all of the different graphs and outputs but it becomes tiresome to keep snipping them and attaching when the differences are all relative.

[mabat]

I see there's a lot of struggle you are going through with these models... It will be possible to create an enclosure like that for a free-standing situation with the Ath tool automatically. I've just been quite busy last weeks. With Gmsh library it is easy to make the mesh element sizes vary so it can be denser at the front and get progressively less dense on the back sides (which is what I'd consider optimal). I only can't promise any deadline.

[mabat]

Originally Posted by fluid

... He used a flat interface instead of the one created from Ath and made it so that every waveguide and baffle vertex were aligned with each other.

I've read this many times (about the flat interface) but it still doesn't make any sense to me. The whole purpose of the extruded interface is to avoid too sharp angles between boundaries at the mouth egde, i.e. too narrow spaces - this is even recommended by the ABEC author and I can see the point in that. Otherwise the shape of the interface should be just irrelevant.

Your second result is more stable at higher frequencies because the mesh elements around the mouth are smaller. And of course it will be better because the egde is rounded. Good job!

[fluid]

I wanted to try and get a similar result using my own CAD process. I spent a lot of time getting Rhino to Duplicate the mesh edge as a polyline where there was a point at every vertex. I was able to export that as a DXF and import it into Fusion to cut my baffle. Fusion still joins all the straight lines into one so it was no different to using the STL to BRep.

I created my own flat interface and meshed the interface and baffle only at 10mm as the edge of the waveguide used that resolution. This caused all the vertices where the waveguide baffle and interface joined to be very well aligned.

This is what the mesh looked like





This is the mesh before ABEC adds the symmetry





This shows the alignment between the three surfaces

[mabat]

Yeah, the nice thing about using the Gmsh library (when used alone) is that it aligns all edges automatically - you just don't have to care about that. It "simply" creates all the vertices from defined geometry and mesh densities (at each control point) and joints them into one cohesive structure.

[fluid]

Here are the simulated results for the baffle with a 36mm roundover on the horizontal as seen before. Left to right : Horizontal polar 20 deg normalized, Horizontal Curves, Vertical, Diagonal and curves exported from the normalized polar.

What is nice is that the numerical instabilities that showed up in some of the other graphs are virtually gone and my conclusion is that it had a lot to do with the alignment of the vertices and having the important objects meshed at the same resolution to enable that to happen.

[fluid]

Thanks for your input mabat, without your tool I would not be undertaking this at all. I would have just used the XT1464 Horns I have sitting in the shed. I do have hope that all this effort will give me a better result.

Originally Posted by mabat

I see there's a lot of struggle you are going through with these models... It will be possible to create an enclosure like that for a free-standing situation with the Ath tool automatically. I've just been quite busy last weeks. With Gmsh library it is easy to make the mesh element sizes vary so it can be denser at the front and get progressively less dense on the back sides (which is what I'd consider optimal). I only can't promise any deadline.

I would say the struggle was to understand how to go about it, some of that was down to not being a CAD expert and having to try and figure it out as you go. Once that part is over then it is much less effort to do it all over again. That was my motivation for posting my journey from start to finish in the hopes that it might help someone else who is trying to do the same thing.

I look forward to the day that Ath can do all of this without my intervention. As that day has not yet come and me not liking to wait for a smarter person to do it all for me I got stuck in

Originally Posted by mabat

I've read this many times (about the flat interface) but it still doesn't make any sense to me. The whole purpose of the extruded interface is to avoid too sharp angles between boundaries at the mouth egde, i.e. too narrow spaces - this is even recommended by the ABEC author and I can see the point in that. Otherwise the shape of the interface should be just irrelevant.

Your second result is more stable at higher frequencies because the mesh elements around the mouth are smaller. And of course it will be better because the egde is rounded. Good job!

I can offer no good explanation to say why the flat interface worked better with a finite baffle, all I can say is that I tried it with the interface produced by Ath and a flat interface at 10mm exactly and the latter gave the output seen above with very little numerical issues.

Don used a flat interface that had higher density in some places and that did not work as well in this particular case as the one that was at 10mm.

I am now more convinced that the spikes seen in some simulations that we discussed in your thread are somehow mesh alignment/density related and not non uniqueness issues.

Originally Posted by mabat

Yeah, the nice thing about using the Gmsh library (when used alone) is that it aligns all edges automatically - you just don't have to care about that. It "simply" creates all the vertices from defined geometry and mesh densities (at each control point) and joints them into one cohesive structure.

Very true, I can't reconcile that against what I just said above though as some gmsh library produced files have suffered from spikes.

[fluid]

This post should just about catch up to where I am now. I have compared a flat baffle to one with a 36mm roundover on the horizontal edges. I won't rehash all the curve graphs but you can see the diffraction off the sharp baffle edge as a less smooth response overall. It is not horrendous as the waveguide has a very gentle transition to the baffle itself but it is there.

The best way to show it is with the observation fields. I have put them all in one image side by side. Left side is the flat baffle right side is the rounded edge. The image is pretty big to keep decent resolution so I have not put it in line. The pattern can clearly be seen to be smoother overall with less raggedness to it in the rounded edge version. It is visible from about 100Hz upwards but more pronounced at high frequencies.

It seems that the back corners might also benefit from being rounded although the difference is likely small in reality. I might draw up some more observation fields to show the effect of that more clearly.

I do have a mitre rounded baffle and fully rounded baffle and carcass created in CAD but not put into ABEC projects yet. Time will tell if I think it is worth the effort to see what they show.

The big image doesn't seem to have saved very well so I added some full size comparisons.