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  #76  
Old 10-11-2023, 02:48 PM
Alan Carruth Alan Carruth is offline
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davidd wrote:
"So we are back to people believing whatever they want to believe."

Exactly, at least until we can come up with some objective data that everybody will accept. So far there's not much of that out there.

I did an experiment on the effects of vibrating a wood strip years ago. I used a piece of spruce trimmed from the edge of a top blank, about 50 cm long and a couple of cm wide. I glued a small magnet to each end of the strip, and suspended it with rubber bands at the two 'node' points for the fundamental mode. The magnets were positioned near small coils that were attached to the 'input' and 'output' ends of a 1W amplifier. When the strip was tapped the vibration at one end caused a signal at the other that pushed on it, and when the phase was right it was easy to adjust the power to get it to keep going.

I tested the Young's modulus and damping of the strip without the magnets on it, set it up, and left it running in a humidity controlled room at my violin making teacher's house for a month. When I came back I removed the magnets and re-tested the strip. The only change was that the damping factor had increased by about 25%. That's one test, on one piece of wood.

An effort to repeat the experiment with a number of strips that had been mass loaded to have the same pitch failed to get usable results when the wood strips absorbed oil from the clay used to load them. My teacher's humidity control was wrecked when a plumbing leak over the following summer flooded the basement room, and I have not had the time since to re-do the experiment.
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  #77  
Old 10-13-2023, 01:35 PM
YamaYairi YamaYairi is offline
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It seems simple enough to set up a controlled test using a microphone and measuring the maximum output for each string (maximum output should remove the variability of the plucking force by measuring the maximum sound it's possible to produce.) for before and after treatment. I wish I had thought of this as I have the necessary test equipment. Now the guitar has already been treated.
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  #78  
Old 10-15-2023, 01:02 PM
Alan Carruth Alan Carruth is offline
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One of the best ways to drive a guitar for such tests is the 'wire break' method for plucking the strings. You loop a short length of fine copper magnet wire ~#44 seems to work well) under the string and pull upward on the ends until the magnet wire breaks. The quality control required to produce such fine wire is so tight that it always breaks at the same force, within 2% or so. You can control the location and direction of the 'pluck' very exactly, and a set of four to six plucks on the same string will show significant variation if you're careful. The main thing to watch for is that the wire breaks right where it goes behind the string; if it breaks someplace else it can leave the string moving 'sideways' to some extent, and change the output. You have to visually inspect the wire after each pluck.

The big advantage of this method is that it doesn't add any mass or stiffness to the guitar, as a mechanical or electric driver would. It takes a few minutes to set up the plucker for a 'shot' though, so you can't 'play' the guitar.

I made a simple solenoid operated 'plucker' for this to do a study on break angle and string height off the top as influences on the sound.
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