Maeshowe Build

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As noted in a few other related threads, I’ve been working on building a pair of Woden Maeshowes. I’ve lusted after Silburys for years, but I’ve got some Pluvia 7.2HDs so Maeshowe it is.

I’m including the optional baffle braces, because I think they are essential. Material choice was very limited in my location so it is just decent quality 15mm ply. Not void free, sadly, but not really full of voids either. It’ll do.

They sell it in (roughly) 3’x6’ sheets here, so none of the cut sheets were appropriate. A quick google search got me to https://www.opticutter.com/cut-list-optimizer#form which proved to be the perfect tool for the job.

I invested in an inexpensive table saw to do the bulk of the cutting and I am glad I did. So easy to get all pieces with identical width, in order to ensure a perfect fit when that final side panel gets glued.

I was intimidated by the curve on the back of this design, and brainstormed a bit about how to draw that curve at the correct radius. Another google search brought me to https://www.blocklayer.com/arc-layout , which again worked perfectly for the task at hand. It was a bit tedious, but I only had to do it once and then use the first piece as a template. Jig saw did a decent, if not perfect, job.

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Progress is slow due to “never too many clamps” and I am allowing extra clamp time because it is pretty chilly in the house.
 
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If either is used, you move the nominal rear face of the chamber rearward to compensate for the lost volume / space. Personally, I find this approach better suited to designs with larger chamber volumes, though it's still functional here. I just don't find the gains sufficient for this enclosure; some do, so the option is there for people to try should they wish.
 
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Hi Scott! Yes, indeed, move the rear face rearward (that was factored in), but every time you do that, you have to displace that added volume with more material (no?).

So in the two examples above, if I move the rear face rearward another 15mm, then I need to cut a new piece of 15mm material with identical dimensions of the original face, dice it up, and fit it into the new (deeper) chamber.

For every 15mm that I move that rear face rearward, I am stuck with the same predicament - in order to maintain the correct chamber volume, especially keeping in mind I need space for 1/2” felt.
 
In other words, I approached this by assuming that I would move the rear wall back 15mm. The logical next step was to cut another back wall of 15mm material and dice it or slice it to simultaneously create a variable depth wall and displace the added volume. The two methods I tried both resulted in insufficient room for the driver, forget felt + driver.
 
Well, the point is to maintain the target volume; you move the panel rearward sufficiently to account for how much volume would otherwise be taken up with the additional material. So if, for instance, the additional material takes up, say, about 400ml, you'd move the panel rearward by about 10mm. How deep you make the individual blocks & their dimensions / form factor & therefore the nominal chamber is up to the individual.
 
Of course. You can’t be expected to spoon feed everything, Scott.

I know where I went wrong. It is a matter of factoring in the dimensions of the driver. Namely, how deep is the driver going to sit in the chamber and, therefore, how much “headroom” is there between the driver magnet and the back wall of the chamber. Damping material thickness also has to be considered. I was so focused on maintaining the driver chamber volume that I passively assumed I would have plenty of headroom for the driver.

Failing twice taught me what I need to know in order to get it right, but I had wasted enough time and material so I simply let go of the notion and moved on. When I build Silburys, I’ll do it right.

I have learned other things through making mistakes on this build. You can spot them if you look closely!
 
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Moving on…

I’ve started to glue up the second box. I feel more productive because I can work on other aspects of the first box during clamp time on the second box.

I’ve chosen 10mm PET felt for lining/damping/absorption inside the chamber and the first two horn folds. I plan to create an air gap between the chamber rear wall and the PET felt, and I have room for another layer if I need/want to after testing.
 
Question for @Scottmoose and/or @planet10 - At what approximate frequency does this BLH transition (or cross over) from primarily direct forward-firing cone energy to primarily back-loaded horn energy?

In other words, what is the ideal frequency for physical low-pass? I can guess about 200-300Hz, but I don't want to guess.
 
Gluing is nearly complete (except for final side panels, of course) and I am working on lining/damping. I am following the recommendations in the plan set, and also implementing the tweaks recommended by @Scottmoose in another thread.

I am a bit unsure to what extent I should line the corners, as per that advice. This is what I have now at the first corner:
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I am wondering if I should wrap the felt around the corner a bit, like this:
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Still curious about post #12 https://www.diyaudio.com/community/threads/maeshowe-build.422722/post-7904721

It will help me determine which of the following configurations to use:
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PET felt (probably all felt) performs differently when it is used directly on a surface vs. leaving a void between the felt and the surface. The void makes it more effective overall, but specifically at 1khz, 500hz, and a bit at 250hz.

Pages 16 and 17 of this doc, as reference https://www.refelt.com/wp-content/uploads/2019/02/Peutz-rapport-PET-Felt-Panels-ENG.pdf (although the void is 50mm in that testing, whereas it would be 10mm in my example).

This PET felt is sort of “board-like” and does emit a dull, low sound when tapped, albeit brief, so giving it a void might actually result in resonance. I dunno. In the second configuration pictured, the driver magnet will sit against the PET felt and actually dish it in a bit. Perhaps that will mitigate any possible resonance.

The obvious answer is to try both once everything is sealed up, and I might just have to do that. I think I can manage to do some tinkering with this through the driver hole, if I must.
 
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Considering how long you've been around and the number of times I've posted the T/S theory, I'm surprised you haven't figured it out, but I can see why they may be reluctant to post it.
 
Hi @GM ! I have seen you post the T/S theory. All I can say is I don't have a snowball's chance in hell of understanding it. I was terrible at math in school. Barely passed. Hence my philosophy degree and my 30 year career in networking and systems admin.

I would have loved to be an engineer, but without math...
 
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