Marimba Bar Tuning Problem - (from Brad Waldron)
-----Original Message----- From: Brad Waldron Sent: Tuesday, 24 August 2004 10:47 AM To: firstname.lastname@example.org Subject: marimbasG'day Jim,
I only found your marimba pages a few weeks ago and have been reading with great interest, over the years I have built a few small instruments and always seem to get one or two notes, in the high end, that are just dull, no resonance, the pitch is right, resonator length ok, but, sort of clunk in "F" compared to the others, these keys do, however, ring quite nicely if placed in another position, essentially moving the nodal point inward. I've tried re-configuring the keys, changing the length and depth of the cut-out against the length of the key, as well as trying different resonator sizes, with little or no improvement, this particular key is an "F" it is 310mm long and 22.5mm thick. I was wondering if you could shed any light on the matter. Thankyou for your time..
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Hi brad - many thanks for your email,...
Resonator width will only effect the correct length of tube required by a very little amount because of that thing referred to as "end correction" - primarily it only effects the amplitude of the wave inside the tube and therefore how quickly it can take energy away from the bar. The effect this variance has on the bar is close to negligible. Usually you just make the diameter roughly the same as the width of the bar, then tune to suit.
I'm not 100% sure what you mean by "these keys do, however, ring quite nicely if placed in another position, essentially moving the nodal point inward".
Do you have the bars suspended on a string going through the bars, or when you say "placed in another position" do you mean just sitting on top of the frame? Either way it seems to me that you've gone a long way to answering your own question. If the bar sounds as it should with the nodal points further in, then the nodal points are what need to be corrected. Try the well used technique of sprinkling a little fine sawdust or talc or similar over the nodal areas of the problem bar. Just place the bar sitting loosely on top of the framework where it sounds good and gently tap it in the middle a number of times. The dust will start to congregate on the natural nodal points in the bar, as they are the only bits of wood not vibrating (by definition!) Mark them with a pencil. Your pencil marks should line up pretty much exactly over the frame supports. If you place the bar in a different position where the frame does not sit directly under your nodal points, then of course it will not vibrate properly, and sound gets choked.
Ok - given that you can't just move the bar you need to actually change the bar's nodal positions. THE ONLY WAY TO DO THIS (that I know of) is to CHANGE THE LENGTH OF THE BAR! If your nodal points need to be further apart - throw it away and start again - you can't make the bar longer. If the nodal points need to be closer together then you cut the bar shorter till they are correct. Of course you will need to re-tune the bar afterwards to get the fundamental and upper partials back down to correct pitch. That may be difficult as shortening the bar tends to effect the partials by different amounts. You may find that you have to sharpen the bar a second time by removing timber from the underside of the bar at the very end, before you have enough to play with. Don't worry - unless you mess with the top of your bar the nodal points shouldn't be effected.
Hopefully this helps you - feel free to email again if it doesn't, or if I've misinterpreted your problem. As usual, the caveat is that I can't be 100% sure of anything without being there and seeing/doing for myself, so I only accept responsibility for successes, not for failures. Try these things at your own risk etc. etc.
answers by Jim MCCarthy - 11/10/2004
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