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Building a marimba - 01 - (Discussions with Dave Woodall)

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-----Original Message-----
From: Woodall, Dave
To: Jim McCarthy
Date: Friday, 15 June 2001 10:50 PM
Subject: Marimba Info
Jim,
I have really enjoyed your marimba web pages. I also have a degree in percussion, and am interested in building some marimbas (I'd like to start a church marimba choir). I am in search of details - primarily in resonator sizing/construction. I see that the resonator should be 1/4 the wavelength, but how are elbows (bends) figured into the length? Also, how is the proper diameter of the pipe computed?

I have Jon Madin's book on marimba building, and I see how to tune the first overtones of the bars. Have you tried tuning more than the first overtone?

Any information you can point me to would be greatly appreciated! I am currently trying to assemble a spreadsheet containing all bar and resonator sizing info from C2 (65Hz) to C7 (2093Hz). Is there a lot of lift (or maybe depth is a better word) for going below C2?

Thanks in advance!

Dave.


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Hi Dave - good to hear from you, and great to hear that your so keen to build some marimbas.

On resonators:-
There is no proper resonator diameter as such, in fact if you look at the traditional marimbas of Central America, you will see that they often flare so that they are wider at the bottom. This is partially in imitation of older instruments which used hollowed our gourds and such things as resonators. A flare at the bottom also means that the pitch of the resonator is lowered, so that it can be lower without being long enough to hit the ground on a low to the ground instrument. It is much easier of course for us to use a constant diameter pipe. I have found that PVC pipe that you buy from a plumbing shop is the best solution - cheap, easy to work with, large range of sizes and fitting etc. The wider the pipe the bigger the amplitude of the wave in it. This means a louder sound from the resonator, but it also means that energy is taken from the vibrating bar faster, so the note tends to ring on for a shorter period. Most concert marimbas have bars that get quite a lot wider as they go lower and lower below middle C. The wider bars produce more energy which is done to compensate for the ear's reduced sensitivity to these lower notes. This problem becomes exponentially worse as you go lower, which is why most people tend to shy away from making marimba notes that extend much below the bass clef. A wider bar does of course need a wider resonator. Generally if you match the diameter of the resonator to the width of the bar you can't go too far wrong. It doesn't have to be exact, and you will find that it depends a bit on what pipe is available. For example you might use 60mm pipe for middle C and for the next 7 or so notes down before changing to the next size pipe up - say 70mm or whatever. Some diameter pipes might be pressure rated and others just storm water, but it doesn't really matter. The elbows don't really effect the length - just bend the tape measure around following a contour in the approximate middle of the pipe. You will find that the actual length of the pipe is slightly less than the acoustic length - by an amount we call the "end correction" The end correction is about 0.61r where r is the radius. You will find in practise however that all this is not that exact - the pitch of the resonator can drop if the bar is closer to the mouth or there are variations depending on air temperature and humidity as well. I once did a show with the State theatre that was outside in the middle of a heat wave, and Even with the resonators propped up a whole cm closer to the bars it still didn't sound that great until it got cooler in the evenings. For the lower notes, I find that the ear test is the best anyway. Get the mouth of the pipe finished and in the correct position, the work out how long the pipe SHOULD be. Cut it a little longer, then shorten it until it starts to come in range. You will hear when it starts activating well under the bar. There is a certain amount of adjustment potential in the end caps and elbow fittings, so you can go past the ideal length a little way when cutting the pipe, then slide things around till you get the best sounding length. Mark it all off with a texta - THEN you can glue the joins. For the higher notes, it can be a little more difficult to hear - I don't do those anyway, but the guy here in Adelaide who does, uses a stroboscope to tune the pipes with. He activates the pipe on the closed end by hitting it with a mallet, and takes a pitch reading, then cuts the mouth end down gradually till its correct.

You will find quite a bit of info about all this stuff in an essay of mine which is posted in the Articles section of the PCA website. The essay is "Marimbas: Exploring The Depths" This link here should take you to the section on resonators.

Overtones - Yes I always tune the first two. Most commercial marimbas have the first two tuned nowadays.
- Fundamental
- harmonic one - two octaves up
- harmonic two - three octaves plus major third

Some marimbas - especially older ones use three octaves plus minor third for second overtone.

For high notes, obviously the second harmonic is not really audible anyway, so it does not really matter. Be careful of the side tone though in those middle register notes - you will need to use wedging to avoid secondary tone clashes. Read the essay more more info on that.

I'm not sure what you mean by "lift" or "depth" in your last question about going below C2 ???
Feel free to ask though - Hope all this has been some help.

Jim McCarthy


Jim,

Thanks for the great information! Unfortunately I have more questions. Sorry if I'm being a pain!

Overtones - Yes I always tune the first two. Most commercial marimbas have the first two tuned nowadays.
- Fundamental
- harmonic one - two octaves up
- harmonic two - three octaves plus major third

I've read that you tune the first partial by holding the bar in the center and striking the ends. How do you tune the second partial - try to divide the bar into thirds? Is the overtone loud enough that you can use a tuner, or is it done by ear?

I'm not sure what you mean by "lift" or "depth" in your last question about going below C2 ???

That didn't make much sense, did it? In an ensemble, is there a lot of benefit (musically) from having a marimba that can go below C2 (65Hz)? Is it worth the trouble of building/storing/moving such an instrument?

Thanks in advance!

Dave


No problems at all Dave - you'd be surprised at the amount of silly questions I get asked, so its refreshing to answer some sensible ones for a change.

I think you've got the right idea with getting the bar to sound the upper partials. The fundamental is produced when the bar vibrates by flexing in a single arc with the string holes as the nodal positions. Of course when played normally, this is the strongest mode of vibration, but the others are also simultaneously present. In order to make them relatively louder you have to hold or clamp the bar in a spot that effectively dampens the tones you don't want to hear, but not the one you do. To hear the first harmonic holding the bar in the centre works well as the centre of the bar is the exact antinode of the fundamental and also the exact node of the first harmonic. By holding the bar here you force the bar to vibrate in its second mode. The third mode - you guessed it - divide the bar roughly into thirds - it wont be exact and will depend a little on the pitch of the bar and the shape of the arch as to where the ACOUSTIC nodal point will be. You may have to hold both points 1/3rd of the bar length along, as sometimes the fundamental remains strong with just one. The real tip here is not just to hold the bar at the correct point, but also to strike it with a fairly hard mallet and NOT in the middle. You need to strike it almost where the string hole will be - just outside the spot you are holding it.

How you actually tune the note will depend on the pitch of the bar, which overtone you are trying to tune, and your own ability or that of your equipment. For really low notes the fundamentals become difficult to place by ear, but the vibrations from the bar are strong and last a long time, so a digital tuner is usually a good option as long as the note falls within its range. The harmonics of these bars are pretty much in good hearing register so that is an option, but the tuner is more accurate if you can get the note to sound strong and long enough. As you get into the mid to high register things change. The fundamentals are in good ear range and maybe the first harmonic, but the tuner is still better although the 1st harmonics will probably not vibrate for long enough to get an accurate reading. High range - all the notes, even the fundamentals tend to be too short to get a good reading from the tuner - the harmonics are really tricky to even isolate let alone getting any accuracy reading it. This is really getting into the territory where you really need a stroboscope. This is in fact the best instrument for tuning everything especially seeing as in most cases you don't even have to clamp the bar to measure the upper partials.

All this said - how fussy are you? I have heard plenty of non-commercial marimbas that just have the fundamentals tuned and even those are not that accurate if you test them with good equipment. They can still sound ok though - its all a matter of degrees I guess.

Most concert marimbas today are 4 octaves plus either a third, sixth or five octaves. I feel that the 4 1/6th is the most practical solo instrument, as there is not much call for a five octave. The 4 1/3 goes down to A(110Hz) - the 41/6 down to the E and the Five octave down to C(64Hz) - the C below the bass stave. This is the same as the bottom string of a cello, so it is fine for most just about any ensemble works - plenty of range. Some ensembles use a bass marimba which is separate to the other full range instruments - This has the advantage of being shorter, and keeping one person to an instrument. There is no real standard for this, but the most common range for a bass marimba is from A(55Hz) up to C(512Hz) These instruments are even considered by some a bit of a luxury, and can be more trouble than they are worth - not in my opinion of course, but I'm a bit obsessed. Getting down to that A can be quite tricky if you want to maintain a good sound. The bar really needs to be wide and also pretty long. No point trying to make it shorter and really really thin, as it will just crack when the player tries to get more sound from it as they inevitably do.

C(32)??!! Well you would be getting into my territory there indeed! Making a whole instrument that includes notes this low would me a real challenge if you wanted it to sound good and fat. Even just making that note alone. There really isn't a lot of call for that kind of range - I mean that's a major third below a double bass! The kind of thing you build - drag out once or twice for a specially written composition - then it sits in your garage taking up as much space as a car for several years. Maybe you are in a situation where you could use it regularly though. That would be great!

Good luck, and be sure to keep me posted - I'm keen to see how you go and maybe even have a listen when you get some finished results.

Jim

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

answers by Jim MCCarthy - 21/06/01

For more help on marimba building you can email Jim.

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