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Marimba resonators and More!

By Jim McCarthy | June 14, 2009

Hi all… well last time I left you  it was almost two weeks ago and I’d just got to the interesting stage of putting the marimba together. I’d tuned a few of the easier resonators in the middle octaves and layed the bars out on the basic frame structure.

The previous weekend was a “long weekend” with an extra public holiday, so I had a little extra time to play around with the next few steps. Here’s what I managed…

 Ok – so lets go through these things one step at a time…

What I really WANTED to do was to get on with tuning the resonators, but experience told me that for the top octave and the bottom octave, there would be difficulties. The middle three octaves are quite easy to tune, simply by hitting them on the bottom with your fingers or a mallet and using a digital tuner or stoboscope to test the pitch. I’m very quick at this these days and know the approximate length of the pipes anyway. SO what I do is simply cut them a fraction shorter, then slide the end cap off a fraction till the correct pitch is read – then mark the position of the cap on the tube with a a texta, apply the glue and make the join. It also pays to check the pitch and adjust the end cap in the ten seconds or so you have as the glue is setting to make sure it is right on.

testing resonance of marimba resonatorThe top and bottom octaves tend to be more difficult to produce measurable tones from though, so what you really need is the actual tuned marimba bar to test how it sounds over the resonator. Here is a photo of me doing this type of thing to the left using my special rubber band and open box system to suspend the bar. I can move the resonator in and out of position under the bar whilst striking it and listen for the effect in amplification and pitch.

drilling nodes in marimba barBefore I could do any of this however I needed to do the fine tuning of the bars, and before I could do THAT, I needed to drill the nodal holes for the suspension strings. You may remember I kinda mentioned this order of events in my last post. Remember I had just ruled the nodal lines over the bars? So I spent a day drilling all the bars, then fine tuning them down to the last few cents.

So now at last I could get on with my experiments for the new clustered resonator idea!

You see, the problem with normal resonators under the bars in the top octave, is that they are SO SHORT! Firstly, they have a diameter about the same as the height, so they don’t sound very well when you activate them. You cannot actually hear a pitch when you hit them on the bottom, so the only way to tune them is by testing them for effective resonance under the matching bar. This is a hit n’ miss affair which is difficult to get accurate as you are relying on the ear’s ability to detect very subtle differences in resonances. The other way which is common to get a correct resonator length, is to calculate it. Essentially this is done using the equation v=fw where v is the velocity of sound in air (at average temperature) – f is the note’s frequency – and w is the wavelength. As we are using quarter wavelength resonators the basic calculation for resonator length is: (v/f)/4. There is also a factor called the “end correction”. Essentially the effect of the confining resonator wall continues a little way past the actual end of the resonator mouth making it effectively a little longer. This means that you have to make the 1/4 wavelength resonator measurement a little shorter for it to be in tune. Officially this is calculated at 0.61r (where r is the tube radius). In practice however, it is extremely difficult to calcualate this accurately as the effect changes with proximity to the marimba bar etc.

cluster of small diameter resonatorsMy cool idea to help deal with these issues was to use a “cluster” of very small diameter tubes instead of a single large diameter tube. The idea is that by using a smaller diameter you are reducing the amount of end correction. Therefore a slight calculation error in the end correction factor accounts for a much smaller percentage of the tube length and creates less variation from the correct target pitch. Of course to achieve the same volume from the resonators, the equivalent cross-sectional area must be preserved, which is why yu will need a cluster of the smaller tubes. By using extremely thin tubes, the end correction could be cut down to a amount so small as to be not worth factoring into the calculation – BUT… the problem with this approach, is that air close to the tube walls tends to resist movement because of cohesion. In a large diameter tube this accounts for a very small percentage of the total volume of air, but in a very thin tube it is actually the greater portion of the air, and the result is a resonator which produces very little sound volume. So I reached a compromise for my main tests… I used 15mm PVC pipe which strangely actually had an internal diameter of about 19mm. By using four of these together arranged in a square, I had a cluster which fit nicely under the high marimba bars and had only slightly less cross-sectional area in total than the 40mm pvc pipe normally used.

small resonator tube insideThe PVC end caps would have been quite expensive in total considering the amount needed – and also they were quite thick and would have meant that too much space would have been needed under each bar – so clearly I had to find another way to seal off one end. Ordinary wine corks ended up providing a good solution. I sanded back a cork just a fraction so it could be force a couple of millimeters into the tube, then used a hacksaw to cut away the remaining cork and sanded the bottom flat. The cork was glued into the pipe using the normal pvc pipe glue – I also found that by dripping two drops of glue into the bottom of the tube, it sealed off the cork and made a nice hard floor to the tube.

Now after a fair bit of playing around with this idea, I did in fact get some of these clustered resonators sounding really good – probably even a small fraction better than a regular resonator! The problem was that to get four of these smaller tubes tuned was taking just as long as the single bigger tube. There were still problems with tuning the smaller ones, and there were other problems as well which I considered would come up when building them into the resonator banks themselves. I came up with various solutions for these, and I may pursue this further at a later date on a different instrument – perhaps a super high pitch instrument???? – but in the end I realised that for this marimba, the standard single-tube approach despite being troublesome in the top octave, was still the all round most practical method. So I fiddled for ages with short 40mm tubes, testing them for relative resonance under the actual bars they would amplify – and eventually came up with the set for the top octave.

middle tuned resonators
high tuned resonators

So now I had all the resonators tuned except the lowest few. So it was a matter of tuning the bottom octave of resonators which are the longest and actually require some design because they are too long to fit under a normal height instrument without putting some bends in the tube. Fitting them all in and having them suspended properly can be an issue, especially if you want to have resonator tubes on the wider side like myself.

bottom tuned resonatorsI came up with a design and tuned all the lowest tubes. The bottom C and D I tuned, but did not glue all the joints as I wanted to leave some room for later adjustments. So with all the tubes tuned, it came time to actually assemble them. Before that though I needed to decide how the resonator banks would be split – if each row of notes had one single long bank of resonators, it would be terribly difficult to dissemble and transport the instrument, so it is common to make the resonator banks in sections. For a four octave plus sixth instrument or smallet it is usually quite acceptable to simply divide each row into halves. For a full five octave instrument with wide bars like this however, that would make for sections which would not easily fit in your average family car – something I feel very strongly that a design should allow. So I split each row of tubes into three sections. Where to make the splits ended up being quite easy as the diameter of the resonator tubes changed at the points roughly a third of the instrument length anyway, so it was obvious to make them there. This has the advantage that it cuts down on bending the aluminium strips which hold the tubes together.

resonator bank support strutsI measured the distance between the timber struts at these points that I’d decided on, and cut lengths of 50mm wide aluminium flat to these lengths. I also came up with a cool way to bolt them to the timber so that there was no bits protruding up towards the bars, or more than 19mm from the timber of the struts. Also so there is no bare metal attached to the timber struts, which is a design flaw which means that during transport your nice estapol finish to the timber tends to get scratched up by the metal bits unless each piece is individually wrapped in a blanket! The design is extra good I think because in one simple structure these pieces perform both the task of providing the resonator bank support, as well as providing structural support between the timber struts and keeping their distance fixed – a needed part of any marimba design with long spans like this. You can see from the photo here that I’ve also cut the 25mm wide aluminium strips which will rivet to the top of the resonator tubes and hold them up.

aluminium string holderWhilst I had the aluminium grinding wheel on the angle grinder, I also tried my hand at producing a few of these string holders. As I said in my last post… In the past I’ve simply had these produced by a third party business with computer controlled milling machines – which is certainly a time saver although of course at a cost! I really wanted to show that you CAN actually make these quite easily however with nothing more than a drill and angle grinder (with a special wheel for aluminium – a normal metal wheel will NOT do it!) The photo here is my FIRST effort… NOT my best! However you can see that it will work just fine and actually look ok too. The slighly messy bit at the bottom will be embedded into the timber strut and unseen, and there will be some rubber tube over most of the rest of it anyway, so even if these are a little scratched and imperfect, it will not really be visible.

Well that was pretty much the result of that long weekend and some bits from the week. Since then I’ve had almost another week and I’ll post about that pretty soon, so keep watching!

Topics: Uncategorized | 8 Comments »

8 Responses to “Marimba resonators and More!”

  1. Sam Di Maggio Says:
    August 17th, 2009 at 4:19 pm

    Hi Jim, well I have now signed up and am able to get the updates on your post. As you know, I am building the 3 octave marimba. I just notice that you are making the string supports from, what I think looks like, 1/8″ aluminum stock. I would much prefer to use these than the eye hooks described in your instructions for the project 3. Is it possible to get the details from you about them, such as, exactly what size material are you using, and how tall are they, and how does that flat stock stay put in a round hole?
    Thanks Sam

  2. Jim McCarthy Says:
    August 17th, 2009 at 5:21 pm

    Sure thing Sam – yep it’s 3mm thick ally which is 1/8 inch in imperial. 10 or 12mm wide is what you need – or 3/8 to 7/16. The string slot needs to be deep enough to hold the string well but less than 1/2 the thickness of your bars. I made mine a 4mm diameter hole (5/32) with center 7mm (9/32) from the top – so the bottom of the slot is 9mm from the top. The whole piece is 47mm long (1in7/8) and the distance from the top to the thinner bit at the bottom is28mm (1in1/8). The width of this is not absolutely crucial but about 6mm – approx double the thickness of your stock which would therefore be 1/4inch in imperial. The idea is that you drill two holes 3mm (1/8) right next to each other in the timber strut. Pump a little glue in, then tap the ally part in with a hammer. There is a little resistence as the holes are round – not square- but actually that is a good thing, and makes for a firm fit. You can correct any slight errors in angles etc with pliers afterwards.

  3. Randy Says:
    December 1st, 2009 at 4:52 am

    Hi, I am building resonators for a five octave marimba.

    where did you get your PVC pipe for your tubes?

    what is the length and diameter of each tube for each note?



  4. Jim McCarthy Says:
    December 1st, 2009 at 8:00 am

    HI Randy… Well I purchased my marimba resonator materials all from ocal general hardware stores like Bunnings and Mitre 10 – these are common large chain we have here in Australia. I have also purchased resonator materials from other major plumbing stores like Reece Plumbing. In general the diameter of each tube should be pretty close to the width of the bar it is to sit under. For the P524, most of the top two octaves use 40mm and 50mm tube then the graduations wer 60mm, 75mm, 90mm and 100mm for the low C. The length of the tube can in general be calculated at 1/4 of the wavelength of the note it is to resonate fore, minus 0.61 x radius of the tube. This will get you pretty close, but if you want your resonators to be truly effective, you really need to tune them just like you would a bar. There is a whole bunch of information about exactly this process in the building guide which is now available at: http://www.makeamarimba.com/buildamarimba/get_marimba_plans.html

  5. Rusty Roestorf Says:
    January 5th, 2010 at 5:27 pm

    Dear Mr McCarthy.
    I am a member of Lions international and live on the east coast of South Africa in a village called Scottburgh.

    One of the continuing projects of our club is to assist local communities to become self sustainable
    We are currently investigating the building of marimbas to supply to the poor and underprivileged members of our community as a project to assit them in earning a sustainable income.
    These people are mostly totally uneducated but do have exceptional natural musical rhythm and talent.
    Marimbas seem to be a logical selection as instruments.
    Although I appreciate that supplying construction plans is your own means of earning a livelyhood we would greatly appreciate your donation of working plans for our project.
    Full media recognition would of course be given to your contribution but only if it suits you.

    Thanking you in anticipation

    Rusty Roestorf

  6. Randy Says:
    January 27th, 2010 at 6:33 am

    Rusty, thanks for the helpful response.

    I teach on a very economically challenged native american reservation.

    Is it possible that we may just purchase only resonator instructions from you?



  7. Jim McCarthy Says:
    January 27th, 2010 at 3:02 pm

    Hi Randy – I also got your email on this so I will answer you in detail there – but yes – I am happy to help you out.

  8. Carlos Carrillo Says:
    March 9th, 2011 at 6:21 am

    Could you make the slot shallower if the top were
    slightly narrower? The string would have to be pushed in,
    but wouldn’t bounce out.

    Has anyone stateside, used Black Locust?( In Europe its an invasive
    species they call “European Acacia”) Guitar makers here
    say it compares favorably with Brazilian Rosewood. It is
    harder than White Oak and has an excellent Young”s Modulus.
    Another wood that might be good is Osage Orange.

    I got interested in Marimbas because a lot of Chiapans and Guatamalans
    have moved to our area. I want to create some small marimbas for
    Conquistador clubs to use in percussion competitions. It may be a
    crazy idea to divide into an octave or less and support them like marching drums.
    They would be played a little like Chimes. The clubs can’t afford
    drums sets for competition, but I think real music would be even more fun.

    I think I will probably make the first ones out of pallet hardwood just
    to see how low I can go on cost!