I found some pictures of an installed alma, and a description of how that alma is attached to its drum. The writer does seem to think his example is unconventional, but I can't speak on that. Look pretty far down in the blog and you'll find the pictures and descriptions.
http://congadr.blogspot.com/2008_07_01_archive.htmlI can find you some literature about Don Quixote if you want to go fight some windmills too my friend.
Now don't take offense, I see the O.P. wants to set the alma debate aside and just get some info on mechanics. I couldn't help myself. I did bring some content to the table so please accept my Don Quixote remarks with humor as I intended.
The only alma I have has buckled a bit inside the drum, it has a separated kink area that points inward to the center and away from the shell. The bonding material (looks like glue or epoxy) has clearly failed almost all the way around the drum, and so now only little wood screws keep the alma attached to the shell. The alma in this case is a simple band of metal, looks like cold rolled steel, about 1/8 inch thick and about 1/2 inch wide.
Although I think almas aren't needed, I too am interested in all things related to conga mechanics, so I'm bumping this thread and hope to see more content.
Specifically, does anyone have any thoughts on the thermal expansion property differences between wood and metal? If the alma is to support the wood in adverse conditions, isn't any bonding material likely to fail under thermal cycling?
And those bloody dumb looking annular wood almas ... what's up with that? They look like plywood disks with holes in the centers, like flat doughnuts, and are attached at their outer peripheries near the upper ends of their drums. Are they attached at the edge level or just below? I can't tell from photos looking up into the drums. If such an alma is attached below the edge it forms an extra chamber within the conga just below the head ... what does this do to the sound of the drum? If such an alma is attached at the edge, the skin is gonna slap against the alma as the drum is played. Are these annular almas found only on el-cheapo ornamental drums made for tourists? That's what I suspect. I think they're basically internal assembly jigs that hold the staves in place as the drum is assembled. Uh-oh, I've just now formed a theory that all almas might just be assembly jigs that aren't needed once the glue is dry ... but that's another debate. I will say that when I've assembled my own hand made staved drums, final assembly has been tricky and an internal support ring (shhh ... "alma") would have made things a little easier.
In any event, as the O.P. requests, let's discuss alma mechanics and such here ... we all have our views on how silly almas are already ... we're past that.
Cheers,
BMac