The Science of marimba keys
1. Establishing the location of the nodes....
It is helpful to understand at least a little of the the science behind the production of sound by a marimba key.
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When struck, a marimba key will vibrate in a number of ways. The most significant of these vibrations sees the marimba flex in the middle of the bar, with both the ends moving up when the middle goes down, and both ends going down when the middle goes up.
As the bar flexes it generates sound waves which we hear as a note. The notes are not very long lasting as the vibrations quickly diminish. However, the bar also flexes by twisting, and by bending in an s-shape, and in other complex patterns that each generate smaller waves that we hope will produce notes that are in harmony with the fundamental note produced by the main vibration shown here. Controlling these vibrations is important for concert marimba manufacturers, but for the level of precision demanded by a set of African marimbas we can just work with the fundamental - at least, with the higher notes. With the lower register notes the harmonics become a little more important as the vibrations go on longer and the harmonics come more into play.
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If you want to try triple tuning a marimba bar to include the fundamental, the first harmonic (4 x the fundamental frequency) and the third harmonic (which is between 9 and 10 x the fundamental frequency) you can follow the demonstration by Peter Frey by following the link from the button below or by using the link on the links page.
The dotted lines on the diagram above correspond to points along the bar where the bar doesn't move up or down during the cycle of vibrations shown in the picture. These points are called the nodes and ideally you need to support your marimba bar at these points to prevent dampening the vibrations elsewhere along the bar. In the diagram, holes have been drilled at the nodes to suspend the bar on cords that run through the bars, but it should be noted there are a number of alternative methods in use by various marimba builders that utilise other suspension mechanisms.
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The nodes can be found using an elaborate technique called salting. This involves suspending the bar on tight cords or elastic bands, adding salt or sawdust to the surface and then tapping the bar to make it vibrate. The salt vibrates with the bar and begins to settle at points on the bar where there is less vibration. In this way a line of salt accumulates at each node. There are a few videos on YouTube about this, like this one....
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This is fun to do, but there is an easier way to establish the location of the nodes if you want to make more rapid progress. Basically the nodes appear at 22.5% from the ends of the bar. This means that the portion of the bar between the nodes is 55%. So, for a marimba bar that you intend to be 40cm in length, the nodes will be located at 9cm from each end, and 22cm apart. (You might notice that the nodes on the diagram above are not accurately drawn as they are closer than 22.5% from the ends. Sorry about that - but my computer drawing has limitations in terms of time and effort!). Again this is a video by RockFreakinSolid on Youtube that explains this quite nicely...
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