Designing djembe - calculations

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Designing djembe - calculations

Postby Johny11Fingers » Tue May 16, 2017 6:52 pm

Hello everyone :D

I've joined a local drum circle 3 months ago and need to practice at home. I need a djembe - but instead of buying one I want to make one.
My friend is a woodturner and can craft the actual piece.
There is also a local hardwood supplier that can give me freshly cut oak pieces for free.

I just need to design and make a technical drawing for the drum.
I measured some existing drums and made dimensional calculations (assuming a djembe is a Helmholtz oscillator).

Could anynone check if calculations are correct?

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Re: Designing djembe - calculations

Postby drtom » Tue May 23, 2017 8:42 pm

Hey Johnny,

Great for you!

I've taken a look at your numbers thinking they'd be self-explanatory, but I'm left with some questions.

Are you a doctor by any chance? I've seen prescriptions very similar to your calculations. :?: Maybe it's just that pesky extra finger getting in the way.

What exactly are you trying to solve?
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Re: Designing djembe - calculations

Postby Johny11Fingers » Wed May 24, 2017 11:55 am

Thanks for anwsering :) !

I'm not a doctor ;) , sorry for bad handwriting, I have dyslexia and I am trying to improve writing style...

Anyway, I wanted to solve the diameter of the neck in the Helmholtz oscillator.

The dimensions of djembe I'm looking for, are as follows:

Bass sound frequency: f=70 Hz
Diameter of drumhead (measured internally): R=32,5 cm
Djembe total height: H=62,5 cm
Lenght of the lower part L=46,25cm

For that assumptions I calculated that the neck diameter should be d=6,4cm (page 3 of my calcualtions).

My questions are: Is that OK? Not too small of a diameter? Will it muffle the bass too much?
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Re: Designing djembe - calculations

Postby michi » Wed May 24, 2017 2:34 pm

Diameter is OK. Height is on the tall side. Most people prefer no taller than 60 cm (61 cm as a limit).
Length of the pipe seems a little long to me.

Rather than mucking around with theoretical physics, I'd suggest to go to a few classes and/or drum circles and keep an ear out for djembes with a sound that you like. Pick two or three of them and take measurements. Once that's done, there is some data that sets limits on the various parameters, and you can go and think about making your own shell. The precise measurements are not important, as long as you are in roughly the right ballpark. I've heard djembes that you'd swear were some sort of mutant mishap that sounded great. And I've heard hundreds of djembes that looked just fine and sounded really poor.

The proportions of the shell matter, but so do lots of other things. First and foremost, the species of wood. Surface texturing on the inside is important too, as is the bearing edge.

And, once you have the perfect shell, with the perfect proportions, made out of the perfect wood, with a perfect bearing edge, and you put a skin on it, there is a good chance that it may sound like crap. Not because there is anything wrong with the shell, but because some skins just sound bad.

About 50% (or even more) of the sound quality of a djembe comes from the skin. Yes, a bad shell will never sound good, no matter what you do. But a good shell will not necessarily sound good, no matter how hard you try with a given skin. And then you go and put another skin on the drum, and suddenly it sounds great.

It's an imperfect science, I'm afraid.

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Re: Designing djembe - calculations

Postby drtom » Thu May 25, 2017 2:51 am

Johny11Fingers wrote:Thanks for anwsering :) !

I'm not a doctor ;) , sorry for bad handwriting, I have dyslexia and I am trying to improve writing style...


I was kidding you Johnny, but don't worry - no one ever gets my jokes. I AM serious about admiring your gumption for taking on such a project and for going to such lengths.

Johny11Fingers wrote:Anyway, I wanted to solve the diameter of the neck in the Helmholtz oscillator.

The dimensions of djembe I'm looking for, are as follows:

Bass sound frequency: f=70 Hz
Diameter of drumhead (measured internally): R=32,5 cm
Djembe total height: H=62,5 cm
Lenght of the lower part L=46,25cm

For that assumptions I calculated that the neck diameter should be d=6,4cm (page 3 of my calcualtions).

My questions are: Is that OK? Not too small of a diameter? Will it muffle the bass too much?


That is very much on the small side, though I can't say what effect that will have on the bass.

Michi has some good advise, though I do want to add to one of the points he makes. From my experience, the longer the trumpet, the deeper the bass. So if you're looking for lots of bass, I'd keep that length.
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Re: Designing djembe - calculations

Postby michi » Thu May 25, 2017 7:28 am

Hmmm... If the drum is 62 cm tall, and the foot is 46 cm tall, that leaves a bowl that is 18 cm deep.

Those proportions don't sound right to me. Draw it to scale and take a look. I suspect it'll be this weirdly tiny and squashed bowl sitting on an enormous foot.

Usually, the waste of a djembe is at half it's height, give or take a small margin.

Cheers,

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Re: Designing djembe - calculations

Postby korman » Thu May 25, 2017 7:55 am

Your drawing does not describe djembe, but a goblet shaped drum similar to djembe.
- the bowl is never hemispherical
- the neck is never so long
If you're primarily interested in the bass note, go ahead and try if your formula works (sorry, can't help with the calculations). However, if you want a djembe, better follow michi's advice and take the empirical route.

Also, if you plan to play in ensemble, the tone and slap are much more important, the djembe bass can't compete with a dundunba anyways:)
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Re: Designing djembe - calculations

Postby the kid » Thu May 25, 2017 12:04 pm

I would say work on drawings of the drum and look at shapes of modern drums as a guide.

Generally the bowl is more cylindrical than spherical. If you want more bass the larger the better. Wouldn't the Helmholtz theory apply more to the ratio of the bowl compared to the hole the bass passes through.

But anyways if you want a more bassy drum you go for cow skin as it is much more bassy.

The drum should be uniform thickness too. Having a thick stem is really just adding unwanted weight to the drum. But the bowl should be thick enough to deal with the pressure from the skin and ropes. Up to an inch thick is what to aim.


instrument-building-and-repair-f3/frequency-helmholtz-resonator-t3619.html

https://en.wikipedia.org/wiki/Helmholtz_resonance
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Re: Designing djembe - calculations

Postby Johny11Fingers » Thu May 25, 2017 12:35 pm

Hi :)

Thanks for your anwsers guys. You have opened my eyes. I wanted to calcualte everything to get optimal formula and mathematically designs it but I see its more complicated than that.

I think I will take your advice and measure some of djembes in our drumming shool. We have some excelent pieces crafted in Mali, some of them are used for public concerts (the sound they make is great). So I will base my drawings on the and try to imitate those djembes.

michi wrote:Those proportions don't sound right to me. Draw it to scale and take a look. I suspect it'll be this weirdly tiny and squashed bowl sitting on an enormous foot.

Usually, the waste of a djembe is at half it's height, give or take a small margin.

I will keep that in mind.

michi wrote:It's an imperfect science, I'm afraid.

That probably sums it up pretty well ;)

drtom wrote:I was kidding you Johnny, but don't worry - no one ever gets my jokes. I AM serious about admiring your gumption for taking on such a project and for going to such lengths.

No problemo, thanks for adivce.

korman wrote:However, if you want a djembe, better follow michi's advice and take the empirical route.

Also, if you plan to play in ensemble, the tone and slap are much more important, the djembe bass can't compete with a dundunba anyways:)

You are right, thanks.

the kid wrote:But anyways if you want a more bassy drum you go for cow skin as it is much more bassy.

I am planning to use cow skin since goat skin available in my area is too weak. We have a cow skinned djembe and it has a deeper sound to it.
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Re: Designing djembe - calculations

Postby the kid » Thu May 25, 2017 1:59 pm

Or you could try to emulate what the pros make. Basically an deep bowl on top of a sturdy stem.

https://wuladrum.com/collections/shells
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Re: Designing djembe - calculations

Postby drtom » Thu May 25, 2017 6:41 pm

After a second look I see that your bowl is relatively shallow because it's spherical. (The depth of most djembe bowls is greater than their radius - more of an oval.) I'm sure this would affect the sound but don't know at what point the effect would be undesirable.

I've skinned three or four djembe shells with long trumpets and shallow bowls, and they sounded fine. I remember them because djembes of that proportion are unusual.

A long trumpet is only somewhat unusual, but yours IS on the extreme side. Trumpet length to bowl depth is almost 3 to 1. I don't know anyone who knows for a fact whether this is good or bad. Most djembes are closer to 1 to 1.

The safe thing to do would be to emulate a "typical" "good" djembe.

Johny11Fingers wrote:Hi :)
Thanks for your anwsers guys. You have opened my eyes. I wanted to calcualte everything to get optimal formula and mathematically designs it but I see its more complicated than that.


If we knew the optimal formula it would greatly simplify things. What's optimal for you is not optimal for me, but with the optimal "optimal formula" we'd simply plug in our customized values.

It would be great if you could systematically experiment with a great variety of designs, including your current design. Eventually, you might just work out that optimal formula. It would be great if you could do that and report back the results.

Any chance you might do that Johny?
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Re: Designing djembe - calculations

Postby ternarizator » Fri May 26, 2017 3:52 pm

Hello, here's my idea of an optimal djembé, if it can help...

From my experience, a height of 62 cm is optimal to play sitting on a chair.

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I may be wrong and you may be right, and by an effort, we may get nearer to the truth. (Popper)
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Re: Designing djembe - calculations

Postby drtom » Fri May 26, 2017 4:52 pm

ternarizator wrote:Hello, here's my idea of an optimal djembé, if it can help...

From my experience, a height of 62 cm is optimal to play sitting on a chair.


OK, if we're really going to discuss this in earnest, let's define our terms. What do you mean by optimal?

Is your version "optimal" only in height? If, so then I believe I understand what you mean by "optimal", but I also believe the original post encompassed other factors.
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Re: Designing djembe - calculations

Postby ternarizator » Fri May 26, 2017 5:50 pm

As I already told, 62 cm is optimal (for most people) in height when you're sitting on a chair of typical dimensions, for comfort of playing, in order to avoid problems with your shoulder muscles. For the rest it is a combination between sound efficiency and aesthetic criteria of my own and it doesn't need a deep discussion... I just mentionned it as an example, for information, to help Johny in his search.

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Last edited by ternarizator on Sat Jun 03, 2017 9:27 am, edited 1 time in total.
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Re: Designing djembe - calculations

Postby michi » Sat May 27, 2017 1:34 pm

ternarizator wrote:Hello, here's my idea of an optimal djembé, if it can help...


Looks damn close to my idea of an "optimal" djembe, too!

Those are sensible proportions. Pick the right wood, texture the inside surface, and you'll have a good drum.

62 cm is at the upper end of the height spectrum, I think. Not that it's wrong, just that it's "tall and fine". Once you get to 63+ cm, most people start to think that the drum is is too tall because they have to "reach up". Anything shorter than 58 cm, and people start to feel that they have to "reach down".

So, to me, the sweet range if 59–61 cm. That's a good height for almost everyone, except for really short or really tall people.

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