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A bamboo marimba proposal

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-----Original Message-----
Sent: 19/03/05 2:39:16 PM
Subject: bamboo marimba
Dear Jim McCarthy,

Hey, I am multi-instrumentalist trying to build a cheap marimba, nothing fancy, out of some scrap bamboo I have for fun. I have an interest in instrument building I'm trying to cultivate with this project. I've read most of the articles on your site and found them to be extremely helpful in understanding the underlying principles of marimba sound production. I started to design my project marimba (is it even a marimba???) and I wasn't sure if I should split the bamboo or leave it whole. I opted to split it so that I might be able to use the rest of the bamboo to make resonators. Visually deconstruction a marimba (or probably vibraphone in my case having no experience with marimbas) I split the bamboo to make bars. Would you recommend splitting them in the future? It seems like if you leave them whole they might act as their own resonators, but maybe not.

I was also wondering about figuring bar length. I have a finite amount of bamboo and don't want to mess this up. Do you have any suggestions for how to approach this? This is what I was going to try. My lowest pitched piece is 29cm by 1.5cm. I was going to cut the 2 heaviest/highest pitched pieces into thirds to make 6 pieces around 6-10cm for the most upper register. Should I cut the pieces to make a linear change from the longest to the shortest and then try to turn everything or trim as needed? Or is there a formula or ratio I can follow that'll put me in the ball park. I feel like I should be able to get 2 bars per piece of bamboo if I plan well. I haven't figured out what range it is in yet or what my lowest note is but I could find out if it helps. Is it unreasonable to think I can alter the note enough just by thinning the bar or should I be focusing more on working with bar lengths?

I have one more question. Making resonators may be too time consuming for this project but I saw an interesting design:
that I was wondering about. I realize this is very much a toy, but is that kinda of all encompassing resonator possible on a larger scaler? Is there a way for me to just run another piece of bamboo with an opening running the length that will act as a resonator? The pieces I would be using are not nearly as wide as that, actually just 2.5cm diam. which probably isn't enough. If I build a bigger resonator box out of other wood that I can set the bars into/onto, might that work? I imagine it would work like a guitar body, but I wanted to get your thoughts on it before I waste a bunch of bamboo I could turn into resonator pipes, or a bamboo flute.

Your site has been the most help to me and seems to be the only page that actually explains the physics of a marimba in a useful manner. Thank you so much for your time and effort in developing the webpage. Keep up the good work!

~ mumbles

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Dear Mumbles!
(I'm rudely assuming this is not your real name!)

I'll do my best to answer your questions although my experience with bamboo is limited by never actually building bars with it myself.

Firstly - I think splitting the bars is the right thing to do, you double your material and get rid of potential circular vibration modes that might make a tuning nightmare. It also means you can be sure there are no non-hollow bits which would tend to act as nodal positions where you might not want them. A note on that: if you can avoid having any segment junctions in your bars, do so- if you don't have the lengths to plan it that way, cut so the joins are at the positions where the vibrational nodes will be anyway. The idea of having the bar as its own resonator is interesting but wouldn't work very well even in the incredibly unlikely event that you could get a tube that would vibrate longitudinally and have a single open end with correct acoustic length. The bars would have to be too long to be practical anyway and, and it wouldn't resonate properly as the direction of vibration would be perpendicular to the bar's primary mode.

If it was me, I would make a tube resonator for each note. They are WAY more efficient than a box style resonator and you will need this efficiency with bamboo as it doesn't produce a huge tone by itself. A slit length of bamboo would not be big enough for a box style resonator - you are right there - although in theory it could work ok with the right dimensions as long as you segmented the strip into a number of sub-boxes with the size of the slit getting smaller at the low end. Individuals are what you need though. A guitar is a bit different again of course as a high percentage of the sound is transferred to the body mechanically through the bridge / Post assembly.

BARS: you are best to get your highest note and lowest note first. Once you have found the ideal nodal points for these you can work out how many notes you need in between and yes, a linear relationship. Line up all the rough cut lengths in a row with the finished top and bottom bar at either end. Use a big ruler or some straight edge to rule lines for both the bar ends and the nodal points - don't drill the nodes yet though. Use the markings as a guide - you don't want to cut the ends of the bars at an angle. You should be able to lower the pitch of bars quite a bit by carving the middle of the bar underneath - not inside the hollow just the edges of your 1/2 circumference. If you want a really low bar without it being to long, you try a bit that hasn't been split - carve a small slice from the whole length of the bar so its not a tube, then continue carving in the area between the nodes. Because the area outside the nodes is thicker it will be a bit lower than with a 'sliced in 1/2' bit.

Anyway, once you have the frame built with suspension mechanism and all - then you lay you bars out exactly as they will sit and mark out the exact position / angle of the nodal holes to drill.

Hope this has been some help.
Jim McCarthy

Jim Reccomends for comprehensive blueprints and building guides to make your own marimbas.

answers by Jim MCCarthy - 19/03/2005

For more help on instrument building you can email Jim.

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