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Plans for a LARGE outdoor marimba

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-----Original Message-----
From: michele wilkins
Sent: 1/02/08 1:56 AM
Subject: Plans for a LARGE outdoor marimba
Hello Jim,

We are an Americana museum on Cape Cod set amidst 100 acres of botanical gardens. We are building a "Nature Explorer Classroom" as a new attraction for our family audience in an area of our property that is a large kettle hole. This new feature will provide us an opportunity to interpret this geologic feature and provide an area for families to explore the gardens.

Our plans will include Ares to run, climb, crawl, build and yes, make music!

Therefore I was wondering If you had plans for a very large marimba - say 4'wide by 20' long? Simple construction using wood and pvc as it will be outdoors.

Any guidance that you can provide in this area would be appreciated.

Thank you in advance for your attention!


Dear Michele...

My sincere apologies for taking so long to get back to you about this. The truth is I've been putting it off because your question is a tricky one to answer well without writing a whole essay on marimba design and construction. Really you need a complete original design which is no small thing. For an instrument builder with experience it would be quite easy to make it up during the building process which is what you may end up doing, as clearly the result you seek is not a concert instrument! If you do this, I am very happy to help you with questions and advice along the way.

I would suggest a non-chromatic instrument ("white" notes only) that way there is just a single row of bars and a much simpler frame design. You can always add an extra row of black notes later if desired.

I'm assuming this instrument is outside and exposed to the weather, so you will want a timber for the bars that is very weather resistant - in particular one that is hard and close grained because you don't want moisture to soak in. This tends to change the bar's pitch. I would also make the bars bigger all over than a concert instrument. Better for the weather - over enthusiastic kids and also it means your finished instrument will be bigger and more impressive with less notes to build. At 20 foot long there will be plenty even with over size bars!

The higher pitch bars will be fine with just the fundamental note tuned, but notes from roughly middle c down will be worth tuning the 2nd transverse mode as well - it should be tuned 2 octaves above the fundamental. I will give you a video of me tuning a low c for a concert instrument to help you with bar tuning.

The key to choosing the size for marimba bars is this... You can easily lower the pitch of a bar by sanding/carving the arch underneath - But the only way to significantly raise the pitch is to cut it shorter. So if your short bars for the high notes are sounding too low before you start, then you need to either cut them shorter or choose a thicker plank size.

My suggestion for your instrument would be to aim at a top note of f an octave above the treble stave or even a few notes higher if you like. Try bar thicknesses about 2cm or preferably thicker - they certainly will need to be thicker for the lower notes. Maybe even as thick as 4-5cm for the lowest. A concert instrument uses about 2cm for the whole range, but they need a uniform thickness so low notes don't sit higher on the frame - I don't think this will be a huge issue for you if you want to vary the thickness. Thinner low bars also means that they are quite fragile hence thicker means more "overenthusiasm" proof! Try a highest note with a bar about 5cm or more wide, and cut the length so that it is just a little above the target pitch. (- it can then be tuned down by shaping the arch.)

Next job to select the low bar - If it was me, I would aim at a low C (below the bass clef) making the entire range between 4 and 5 octaves. Suggest a minimum of 3cm thick. The wider the bar and resonator, the louder the sound, and this is more important at the lower notes, so here is the opportunity to make big impressive notes that use a big space. The longer the bar is, the less thin it will have to be in the middle to get down to pitch, so I would suggest a decent bit longer than a concert instrument - say about a meter long? Minimum width 10cm - I would go much wider if possible, like 20cm or even a little more. Carve the arch to tune it, and this will give you an idea about whether your dimensions are giving a suitable result in terms of durability etc.

With the top and bottom notes done you can simply cut the rest of the bars to be evenly graduated in length between. The widths should also be graduated, but it does not have to be perfect - a few bars at one width, then step to another width etc is ok.

Find the nodal points on the bars, and mark them with a pencil -they should be about 2/9ths of the bar length from either end. To be really precise you might try sprinkling some salt over the bar and tapping it to get it vibrating. It will need to be roughly held or suspended to allow normal vibration. The salt will begin to accumulate over the nodes.

PVC is great for the resonators - good call there. You will need a variety of diameters so that the diameter of each resonator roughly matches the width of each bar. The length of each resonator will be a little less than 1/4 of the wavelength of the note it is tuned to. It will still need tuning though so in general cut to 1/4 wavelength then tune by gradually cutting down and hitting the end cap to activate the pipe with a tuner at the open end to measure pitch. Do not tune the resonators in direct sunlight or temperatures too different to the estimated playing conditions as this can considerably alter the result . A side note here: it is probably a good idea to position the finished instrument where there is some permanent shade as well.

The resonators can have bends in them without at all affecting the pitch - in fact you might even find that that flexible agricultural pipe with the corrugations works ok and it can be bent any way you like! The plastic in this pipe tends to be a bit thin though, so strength might be an issue for an instrument of this kind. As long as the pipes are correct length and sealed at the end they should work.

A traditional marimba has two long struts of wood to hold up the bars - one under each nodal point of the bars. The bars are held suspended above these struts by string which is threaded through holes running sideways through the bars. Little thin metal contraptions are nailed into the struts between each bar to hold the string off the struts - in this way the bars do not actually touch the struts that hold their weight.

This method of suspending the bars on string is certainly the best - in your case you might use something a bit thicker - various grades of plaited nylon rope are usually available from most hardware stores. As to how to hold the bars at the correct height ... Well the possibilities are endless. You could build a standard type of timber frame but at 20 feet long you would probably need impractically substantial struts. Seeing as spacing of the bars etc is not likely to be crucial like that of a concert instrument you might use a method that only supports the bars every few notes or so. Example - a post in the ground with the rope attached on top - runs through a bar - is knotted (to keep the space from the next bar) runs through another bar or three also with knots between - then another post in the ground. This type of system might work well for big low bars. But perhaps not so well for smaller ones - too many posts. Don't forget that the resonators also have to be positioned under the middle of each bar about an inch under. The crucial thing is that the string holds the bars at their nodal points or at least extremely close to them.

As far as the framework is concerned it all depends whether the instrument is a permanent installation, and if so, what else is around the site etc. I'm sure there are many ways a system could be devised such that the instrument becomes part of the landscape or theme.

As I said before - feel free to email any time - I promise I'll be quicker with the replies when I don't have to write as much.
It really sounds like a fun project, and If I lived a whole lot closer I'd offer to be involved in person - such is life!
All the very best...
Jim McCarthy

answers by Jim MCCarthy - 13/02/2008

For more help on instrument building you can email Jim.

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