On tuning overtones in marimba bars - (Discussions with Leticia Latrónico)

-----Original Message-----
From: "Leticia Latrónico"
To: jim@percussionclinic.com
Sent: 20/02/03 7:01:53 AM
Subject: Marimba-tuning

Thanks in advance!

Leticia

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I find it surprising that you would be able to hear the second overtone so easily; usually this is too soft, and too high (3 octaves + major 3rd above the fundamental) to too fussy about. I will tell you about tuning that overtone but it maybe that what you are actually hearing is what we call a secondary tone, which is cause by wave interference between one of the overtones and the side tone. (The side tone is that made from vibrations moving sideways through the bar rather that along it, and is usually too weak to be heard on its own.) One of my articles on the website (I forget which one - I think its "exploring the depths") tells you a bit about this problem, and how to get around it by a process called "wedging".

The best way to tune all the tones on the bar is to tune them all together as the bar is made. It can be a bit more difficult to tune just the 2nd overtone - you usually end up sharpening the whole bar first so can then flatten the group of tones together in the correct ratio.

If you think about the bars not just in terms of their notes, but in terms of their vibrational modes, it's easy to understand how all vibrational modes are tuned. You could even tune the 3rd or 8th overtone in theory! With the bar vibrating in its fundamental mode, the strings are at the nodal points (the points of no vibration) - and the middle and ends of the bar are the antinodes. (Points of maximum vibration.) You will probably already be familiar with the idea of carving wood out from the centre of the bar to lower the pitch of the fundamental - well this works because you are making the timber thinner and therefore more elastic at the antinodal point. To lower the pitch of the tone produced by any vibrational mode, you carve timber from its antinodal point. For the fundamental, this is in the centre of the bar.

For the first overtone, make the centre another nodal point, which means the antinodes are at 1/4 and 3/4 along the length. For the next overtone, divide the bar length into 3rds - these are the nodal points, and the antinodes are 1/2 way between them. For the next - divide the bar into quarters etc...

The trick is to remember that the carving points for all the overtones are all also in the "fundamental" area. You need to keep measuring all the tones as you lower them, to make sure they all arrive at the correct point together.

Hope this helps - feel free to ask if you need more.

Jim McCarthy.

Dear Jim,

Thank you very much for your answer.
My problem now is that I find in the bass bar's tuning (they get me very worry) is that in the second overtone (3 octaves + major 3rd above the fundamental) the sound is double, I mean, there are two notes that are sounding in the same time, with one intervalic difference very small; as a result of that, I can hear a dirty attack... and this disturb me to much.

I red already your article "exploring the depths", was really very interesting, and also I had in my mind your suggestion about the tuning of the bars whit the process called "wedging". But this process, you suggest for the middle range and not for the bass bars.

So, my question is: How I can do for eliminate from the second overtone, in the bass bars, this two tones that sound at the same time and how I can get a clear and good sound from them? .

Jim, I think that you are the best person that knows about the marimbas' constructions, and I hope that you have the answer of my problem, because I'm really worry for that... I think that if you cannot help me, so, nobody can do it.

Again I want to say thank you for your attention and I will be waiting your answer with anxiety .

Best regards

Leticia

Dear Leticia..

There is one thing I can think of that may be causing your problem.
With some of the low bars I made that were also particularly wide, I had a similar problem.
I am not completely sure about this, as I never made enough of those bars to research this particular aspect properly - but I suspect the problem is caused by there being too much difference in the arch shape on one side of the bar to the other. I think what was happening, was that I was getting two different versions of the second overtone travelling longitudinally along the bar at the same time. If this is the case, obviously the solution is to even them up as soon as you start to hear it in the tuning process - identification will likely be the hard part. I guess you just have to get your ear right up to each side of the bar to see if you can hear a difference. Come to think of it - it may even be not so much side-to-side as a difference between the antinode in the middle of the bar, and those at 1/4 and 3/4. To help isolating which note is coming from which you are best to try some sort of clamping of the bar where the suspected antinode is - If you stop the note, then you no you got it right and that gives you a clue as to where to tune it from. If clamping is tricky you might like to try putting bits of soft putty or chewing gum on the bar - this will be a little imprecise, but the theory is that by adding a small amount of mass to the bar you can lower its frequency. I guess adding it in different spots will give you some freedom to experiment without making permanent changes.

If you are making these lower pitch bars, then I guess with the right sort of timber (particularly if its a bit softer) you might well actually have the transverse vibrational mode sounding well in your aural range. (as apposed to just creating a secondary tone with the 2nd overtone) If this is the case, then I guess you just have to apply the same principles across the bar as those used along the bar to tune it to an acceptable pitch. You may like to try for 2 octaves above the fundamental - be careful of how much the resonator will amplify the note you choose.

The other less likely possibility is that you are getting a torsional mode of vibration. I don't even want to think about how to get around that! Try the other possibilities first.. ;-)

Good luck.

Jim McCarthy

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