[the kid]

But the harder a wood is, generally the more brittle it is because it's less flexible.

I get that. I think it is dependent on the grain in the wood as well. Plus the quality of the wood and the thickness. You can take wood from one section of a tree and it will be different than another. Big difference between sap wood and heart wood too.

I would say the carve is also relevant. When you see chunks of wood removed you know the carver is using only a few tools or there blunt. When the drum is left grainy and with pot marks and not oiled well it get get dryer and brittler. It doesn't mean the wood has that characteristic.

From my own experience i though balafon wood was the most 'brittle', or likely to crack but it is much of a muchness with these hard woods as they are similar hardness any ways and variations exist in hardness and density. I though balafon wood was harder. It is definitely regarded as harder to carve than Dimba.

Also The wood can be damaged before it is used on a drum or have weak spots rotted dry brittle or damaged by insects. It could be sitting in the sun for a year before being made into a drum. The original choice of wood is important in making a good drum and makes a difference into how the wood ages.

[djembefeeling]

The dowel increases the surface area for the glue, and also changes the forces from lateral to round, which distributes the energy, a bit like the spokes on a bike wheel.

Sounds reasonable. I usually do this kind of repair when I see a crack developing from the bearing edge, i.e. when it runs from one side to the other (I mean those approximately 1,5 cm) and is still very small. It prevents them from growing. I don't know how much this fix can do in a serious crack.

About that dugura wood: for some reason I always had brittle dugura, but never once a brittle balaphon wood.

[batadunbata]

The Kid:
Wow I didn't take that in at first, you've had 20 Dimba Djembes! I'm taken aback.

That all makes sense. As far as I can tell, Dimba is denser than Hare/Balafon wood/Pterocarpus (by weight), but less dense than Gele/Ironwood/Prosopis. But that's on average, there's lots of variability as you mentioned.
That doesn't mean it's more or less brittle, since brittleness also has to do with growth pattern and chemistry. Tropicals tend to tear out more than split due to interlocked grain.
Chemicals that make the wood stronger can also make it brittle, such as the tannin in hard Eucalyptus woods, or the aldehyde containing quinones in "true" Ebony. I suspect Dugura contains tannins, judging by the color, but I could be mistaken.
Bala wood/Hare is in the Pterocarpus family, as is Padauk, and full of such chemicals, as shown by the rich color even before any oil is rubbed on. These chemicals make it very resonant, which is why it's so chosen for percussion instruments. (not just Balafons, but also Mbiras/thumb pianos and Ukpas/tone blocks)

Djembefeeling:
At first I thought drilling and inserting a dowel would make the situation worse, but the more I think about it, the more it makes sense for long term durability. It should prevent side to side forces. I would use a dense wood for the dowel though, to transfer resonance, if possible. There is a small crack on this Djembe which doesn't appear to be repaired, but it's very tight, and I was planning to glue it using the tapping method. (placing glue on the crack and tapping on it to pressurize it in)

[djembefeeling]

I would use a dense wood for the dowel though, to transfer resonance, if possible.

If you can find any, please tell me. I couldn't. But I guess it'll make no hearable difference, being molded by hardwood.

I was planning to glue it using the tapping method. (placing glue on the crack and tapping on it to pressurize it in)

For some reason I don't believe in that method. Can't quite picture how this effectively stops the inner forces of the wood.

[batadunbata]

I imagine you're right that the dowel provides much more reinforcement.

The tapping technique is from luthery, where the wood intstrument shells are very thin, so the only methods are glueing and stapling/bracing (mentioned above).

I imagine the strength depends on the glue used.
Luthiers use Hot Hide Glue, which is very strong (made of collagen, a long amino fiber, and the main ingredient in skin and bone). It's also flexible, and very resonant in a warm smooth way (saddle and nut of guitar is usually made of bone to transfer sound into the wood).

Natural adhesives sound better than synthetics IMO, and aren't toxic to dry in the house. But Hot Hide Glue requires heat. So I might use:

1. Tree Rosin [aged] - it's very strong, hard and resonant - dissolved in Essential Oil (Orange/Limonene based household cleaner is cheapest). But: it takes weeks to evaporate/set unless cured with heat/warmth, which is risky to apply to a drumshell.
or
2. Tree Gum [water based] and Protein Glue (Gum Arabic with Wheat Gluten) - flexible and strong, warmly resonant, easiest to apply overall, but not as hard or resonant as Rosin. Gluten also requires insect repelling additives to avoid being eaten, like herbal essential oils

[djembefeeling]

I think a lot of drumbuilders work with epoxy. It has great sound characteristics.

[batadunbata]

I'm sure they do, but I shudder at the thought of it for multiple reasons.

1. It' toxic. To produce (chemical plant pollution) and use (solvents evaporating) and dispose (lasts thousands of years, and the "empty" container is contaminated/toxic waste)
2. It doesn't sound as good as natural adhesives (less resonance) (if it did we could use fiberglass djembes and they'd sound just as good)
and
3. Wood Djembes are a very natural creation, so it doesn't seem like a proper solution to pour synthetic chemicals into them, when it isn't necessary. I'd do it if I had no other choice, but I'd rather use tree rosin or protein based glue. It will sound better and then the shell is still fully natural.

[michi]

An epoxy repair won't affect the sound. Unless it is gigantic (say ⅓ the surface area of the drum), a patch of epoxy somewhere is not going to be audible, so I wouldn't worry about that part of it.

Michi.

[batadunbata]

I'm sure you're right in practical sense, it won't make a noticable significant acoustic difference to most people.

But the rim does ring like a bell, and if I get a better tap tone with a particular glue, I'll use it, unless it's too difficult for some reason.

If epoxy was an pleasing transmitter of sound, they'd use fiberglass (made with epoxy and glass fiber) for acoustic guitar saddles and bridges. But they choose bone, rosewood and african blackwood, because they sound better.

Ok, a guitar saddle is a far cry from filling a crack in a rim, but my instinct is to use the best materials possible for anything that is meant to make music and transmit sound.
The rim is possibly the single most important part of the drum, so I'd prefer to ere on the side of doing more than I need to there, than doing less than I could.

Also, I can't leave instruments outside for epoxy fumes to evaporate, where I live. So I use nontoxics for repairs, and I have them on hand. I realize it would be a bit of doing for most people to aquire and take on learning to use new materials, but for me it's the obvious choice.

[djembefeeling]

It is obvious that you really don't want to use epoxy and for very good reasons. I don't want to talk you into it. But the sound cannot be one of the reasons. A friend of mine does teach, sell, and repair didgeridoos, and he is using a certain kind of epoxy for it. He says he loves doing it, for the sound actually improves. I have made similar experiences with my instruments.

[michi]

Sure, I respect your reasons. Hey, it's your drum, and you can choose to fix it any way you like!

I was simply pointing out that sound is unlikely to be a good justification for not using epoxy. That doesn't mean you can't or shouldn't use something else!

Michi.

[drtom]

But the rim does ring like a bell, and if I get a better tap tone with a particular glue, I'll use it, unless it's too difficult for some reason.

But how will you ever make the comparison if you don't try epoxy?

If epoxy was an pleasing transmitter of sound, they'd use fiberglass (made with epoxy and glass fiber) for acoustic guitar saddles and bridges. But they choose bone, rosewood and african blackwood, because they sound better.

Really? They conducted controlled experiments with bone, rosewood, African blackwood and fiberglass?

My instinct is that organic materials produce better sounding guitars than synthetic materials, but my instinct could be wrong.

Ok, a guitar saddle is a far cry from filling a crack in a rim, but my instinct is to use the best materials possible for anything that is meant to make music and transmit sound.
The rim is possibly the single most important part of the drum, so I'd prefer to ere on the side of doing more than I need to there, than doing less than I could.

That seems pretty wisdomy. Can't argue with wisdomy.

Also, I can't leave instruments outside for epoxy fumes to evaporate, where I live. So I use nontoxics for repairs, and I have them on hand. I realize it would be a bit of doing for most people to aquire and take on learning to use new materials, but for me it's the obvious choice.

More wisdomy stuff. I'm all for nontoxics. However, there is a qualification: I have used epoxy often and for many years and have never even noticed an odor - possibly because I use such relatively small amounts. You're not laying down a floor here. (Maybe I CAN argue with wisdomy.)

[the kid]

It would be more traditional why those materials are used in guitar making. Plus those materials have stood the test of time. Carbon fiber is respected as having good sound.

Sound wise drums are so different though. My 27 year old djala drum has something like concrete in a hole in the stem. It is simply fix and doesn't contribute to sound at all.

A lenge drum i have had a thin wall in a spot and split when i mounted a really thick and tight goat skin. I tuned it down and filled the gap with bee wax and it actually sounded good. Ok that needs a real fix but the wax simply to contain the air inside the drum worked fine for a quick fix. When the air was allowed out of the hole there was no bass and duller tones and slaps, but when plugged with wax back to normal djembe sound.

You'd have to really consider the percentage of fix area or volume compared to the overall area and volume of a drum to consider would plugging a hole even in the rim make any difference due to the material used to plug it.

I'd try not be to conceptual about the sound out of a drum. It is what it is. It's rough not refined like a Stradivarius or whatever.

[batadunbata]

Thanks everyone.
I have tapped the rim all the way around, and the repaired crack spot does not ring quite as clearly. It's noticeably dampened, and has fewer overtones/less richness. It still has a resonance, but not nearly as beautiful. BUT the area on either side of the crack (about 1" away from it) sounds fine, so it seems very minor, unless I was playing right over the crack. It may not be epoxy, but it's very hard, and looks synthetic. If it was rosin, I'm fairly confident that it would ring better, with more clarity, but c'est la vi.\

In chronoligical order:

Djembefeeling:
I can't argue with experience, obviously epoxy has acoustic resonance. Hard things generally do.
But I'm very sensitive to differences in material. For example, I can tell when a Didgeridoo is coated on the inside with sealant, vs uncoated. I can hear the sound it adds, and the sounds it takes away.
That's unusual, most people wouldn't notice a difference at all.

Michi:
I agree, it's not going to make a noticeable difference to the overall sound. I don't agree that which glue is used would make no difference at all. Imperceptible to 99.99% of people, almost certainly. But I pick up extremely tiny changes. Repaired instruments (glued cracks) rarely sound exactly like unrepaired ones to me, but I admit a Djembe is very different from guitars etc, so how much difference I don't know, perhaps imperceptible, perhaps extremely minor difference in that spot.

Dr Tom:
I appreciate the feedback. Mostly those choices have been made over time by trial and error.
This is why Irish flutes are usually made of African Blackwood. Occasionally other very hard woods, but it's the top chosen one, due to sound quality/range. It also makes the best guitar back&sides (wide spectrum response, clarity, beauty of sound) but it's hard to work with (cracks easily when bent) so AB guitars are expensive, even though the wood isn't particularly pricey.
As for experiments, I did read about some informal ones by luthiers online.
Bone, hardwoods, carbon(epoxy+ carbon fiber) and metals were tested for saddles.
Bone came out on top because it transmits sound well, but it also transforms it and filters it, so it softens harsh highs and merges dissonant overtones into warm unified sounds.
Metals were by comparison harshly bright.
African Blackwood works for saddles, a tiny bit less warm and filtered than bone, but it's mainly a top choice for bridges, followed by Rosewood which is less dense so trasmits bass less effectively, selecting for a brighter sound spectrum. The saddle needs to dampen the strings a little, whereas the bridge needs to transmit very clearly from the saddle to the guitar top/membrane (almost always made of spruce due to it's flexibility and sound texture - acts like a skin in a way)
Ebony did not perform so well as a saddle, despite good resonance, it lacks the filtering properties of Bone and Blackwood, transmitting too much "noise", a bit too "crisp" and clear.
Carbon (with epoxy binder) lacked richness of sound, and has poor bass response. Very simple, dry, bright, clear sound, even more so than Ebony. This is why carbon flutes, guitars, etc aren't used widely. They're extremely durable however, so they do make them.

The Kid:
I have a hole plugged in a stem too and I'm sure it contributes almost nothing to the sound overall.
The stem really only resonates with the bass tones, so as long as the material is dense and hard, it should be fine. (assuming, like you said, the area is relatively small)
But the stem is way less important than the rim.
What you said makes perfect sense about the size of the repair on the rim determining the amount of effect it has. I've heard the rule of thumb is a crack of 4" or less isn't a significant issue.
As for beeswax, yea it plugs a hole, but depending on where it is (bowl? stem?) it might make a noticable difference to me if it was replaced with a hard material. How big a diff I don't know)
You're right, it's not a Stradivarius, it's way less sensitive to minor changes. But everything adds up.
And if we don't pay attention to minor changes, we miss the opportunity to refine the instrument and the music. Highly skilled Djembefolas can play any drum, but they generally choose the best they can get because it makes a difference.
I like to respect the instrument, and bring out the best I can in it, so it brings out the best in my playing. That said, I'm not going to worry about something if it can't be fixed.