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Here are some observations I made, after listening to a bunch of recordings* and comparing them, about what I think pairs well with what, in terms of wood density, djembe diameter, skin thickness, and shell/wall thickness. (*: All recordings were from Drumskull Drums YouTube channel, so same room, same mic, similar quality of shells and skins, and same team who put them together)

Disclaimer: I'm focusing on what helps the wood and skin "sing", with the widest, most flexible vocal range possible. There are tradeoffs, and people's tastes/applications vary. I like drums that defy these observations, in fact I like any well made djembe that's made of a classic djembe hardwood.

1. Skin Thickness to Djembe Diameter:

I noticed that the wider the diameter, the more "flex" the skin has.
So a thick skin, like cow has more vocal range on a wider diameter, than on a narrower diameter. Starts somewhere around 13.5-13.75", to my ear.
Conversely, the narrower the diameter, the stiffer the head, so a thinner skin works well, because it's more flexible inherently.
I have no observation about thin skins on wide djembes, I wasn't listening for that.

2. Wood Density to Djembe Diameter: (one exception*, and a caveat**)

I also noticed that a very dense wood sounded great on a wide drum (14"), and not as good on smaller drums. This isn't the only factor though, wall thickness is important also (covered in #3).
I was listening to recordings of Dugura (aka Dimba/Douki/Beng) djembes, but I would predict it to apply also to Gele. Not enough recordings to check.

And vice versa: a not so dense wood, Djalla, sounded great on a narrower drum (12 7/8"), and not as good on wider drums (~14"). (this isn't always true though because as I said wall thickness plays a role) I'm predicting this to also be true of Cassia, which has the dame density of 40lbs per Square foot(according to the wood database), but not enough recordings to check. Seems true for Hare, even though it's a little denser, it rings brightly like a light wood.

*: The exception here is Iroko, which really starts to shine on a wide drum, but again, wall thickness is a factor, and Iroko is almost always carved with thick walls.
**: The caveat is wide diameter shells that are made from less dense woods, like Iroko, Djalla, and Cassia, that are carved with thick walls, will sound better than ones carved with thin walls. I assume its because the extra mass helps. Which brings me to my third observation:

3. Wall Thickness to A) Wood Density, and B) Djembe Diameter.

A)Denser woods pair well with thinner walls. If they're carved thick they seem to reflect most of the sound, and the wood doesn't "speak" or "sing" as much.
Whereas less dense woods seem to be able to handle a thicker wall.
(It's a matter of taste though, some people like the crisp sound, and don't care about the dissonant woody tones.)

B)Wider diameter djembes can have a thick wall and still be able to get the wood to sing, whereas a narrower djembe with a thick wall might reflect most of the sound instead of resonating the wood.
Example: I noticed a narrow Lenke djembe with thick walls sounded less distinct from an Iroko djembe, whereas on a larger diameter, even with thick walls, the difference becomes more noticable. Which leads me to think a wider diameter helps the wood to sing more, similar to the effect of thinner walls. (by sing I mean the woody dissonant voice djembes can have on slaps and tones)

4. Wood densities in lb/sq ft, according to wood database: (*except for Hare and Dugura - see below for sources)
(Caveat: Wood density varies dramatically, so these are only useful in the general sense.
They're averages based on samples. Many factors affect density including age, location/height in tree, growing conditions, and of course whether it's made of heartwood or contains sapwood)

Cassia 40 (Casha/Kassa)[Cassia Siamea]
Djalla 40 (this is for the whole Khaya species, so it might be a little off) [Khaya Senegalens]
Iroko 41 (I thought it was the softest, but that's what the database lists it at.)
Hare 46.2* (Khadi, Balafon Wood, Gue'ni) [Pterocarpus Erinaceus]
Lenke 50 (Lengue)[Afzelia Africana]
Dugura 53.7* (Dimba, Douki, Beng) [Cordyla Pinnata]
Gele 59 (Ghenou, Bois De Forgeron, African Mesquite) [Prosopis Africana]

*: Hare and Dugura weren't in the database so I used other sources.This page listed Hare at 74g/cm. This page listed Dugura at 86g/cm dry. This page helped convert to lb/sq ft.

(About Iroko's density: it's reputation is as the softest, so the database may be wrong about the density, but I wonder if maybe the reason that they carve it thick is because Iroko is brittle? I noticed this when I picked at a chipped area of the carving on the base of the stem. It sounds reasonably hard when I knock on it, but it chips easily. Djalla can be brittle too, but maybe Iroko is even more brittle?)
Interesting observation. There is also one parameter that influences the sound quiet significantly in my opinion: shape and depth of of djembe bowl.
To put it in short: round shallow bowls tend to have "shorter" sounds with litlle more attack, the ones with deeper, vertical, narow bowl shape tend to have slightly more resonant, hollow, but often somewhat fuller sound.
nice compilations of information.

IME, the densities of the preferred woods falls in line with the article that Michi posted.

i have an old bele wood shell from guinea that's been sitting in my closet for years. may need to fix that one up soon.
Thanks Dugafola, and I second fixing it up, it wants to be played :)

Thanks Michi, I appreciate all the info resources on woods I can get.

Not sure I agree with all the categorizations of density/hardness entirely, but I'll get into that in another post. So much variability makes it a tricky thing to pin down.

Boromir76, thanks for pointing that out. I didn't even think of mentioning bowl depth and shape, but you're right it's a big factor in the way a drum sounds. I agree with what you described, and I generally prefer a deeper bowl, if I had to choose only one, for sound quality. They are a little slower playing, but to me it's worth it.
As for shape, I had a hard time deciding which sounded better, the projection and percussive clarity of the classic straight-sided wide-ledged short-bowled Guinean, or the tone and range of the deep, round-bowled Malian.
So I think I like somewhere in the middle best, along the lines of Ivory Coast shape (but without the extra thick walls). With a fairly parallel top half, slowly tapering round bottom half (golden ratio curve), and a little ledge. I've seen this hybrid shape come out of Mali and Guinea, so it's possible to get it in thinner shells and woods other than Iroko.
Of course there's tradeoffs with any shape, so it's a matter of taste and style.