Influence of the arch on decay time

It's been a while since the last post, now there should be again some time to work on the marimba. In this post, 2 topics will be discussed: A final tuning experiment, and the planned procedure for tuning the bars.

Influence of the Arch on Decay Time

The arch depicted in Bork's paper extends towards the edges beyond the areas of influence for f4. I was wondering whether removing this material in an arch like shape would influence decay time or pitch.

Before working on the arch, the decay time was measured in the following way: Strike the bar 3 times hard, and measure the time from maximum to 30dB below, as displayed by the current baudline setup. I am only interested in detecting any influence, and not in determining the actual T60 of the bars, so this should be ok. Measured this way, the average decay of the original bar was 731ms on the fundamental.

The straight-edge shows the how the arch would extend towards the end of the bar.

 Material was removed in steps with a 6mm router drill.

On both sides, the typical marimba bar arch is now present.

Then, the decay time was measured again, as shown in the images above. It was found to be 772ms after removing the material. Even after flattening the whole area to the level of the innermost step, the decay time remained approximately the same (745ms).

Results

By removing the material in an arch like shape lead to

• a small increase in decay time of the fundamental (+5%)
• a small decrease of f10
• no other effects noticed

So, I don't consider it worth the effort to finish the arch from the position of f4 outwards.

Tuning strategy

I am going to tune the bars in the following way:

• start at the C6 bar, work from highest pitch downwards
• double tune bars C6 (994Hz)  to A4 (443Hz)
• triple tune bars G#4 (418Hz) to A3 (221Hz)
• quadruple tune bars G#3 (209Hz) to C3 (131Hz)

Inspecting the archs of  a commercial marimba, I found that the thickness at the middle of the arch decreases with decreasing pitch. So starting at the top pitch, I can use the higher bar as a guide to set the router, this should speed up things a bit.

Let's see!