|MadSci Network: Physics|
I did some digging around to try and find out why guitarists might think magnetic fields interact with wood. I wasn't able to find any specific references that directly connected these two things (magnetic fields and wood), but I did read enough about magnetic pickups to make me think that this might be where the confusion came in. In particular, I read this internet article by David Lamkins, On guitars... a block of wood? Though he doesn't get into the physics of what's happening, he did seem to accurately capture the complicated nature of the physical system that is the guitar.
So do magnetic fields interact with different wood or not? Magnetic fields do not interact with wood because wood is an insulator. In order for a magnetic field to interact with a material, the material has to have loose electrons. This Ask A Scientist page discusses how magnets (and magnetic fields) work in more detail. But wood does not have lots of loose electrons, which is why it makes a good insulator and why it doesn't interact with magnetic fields.
That being said, the magnetic pickup on a guitar does interact with wood. The pickup is designed to detect the actual physical vibrations of your guitar and the vibrations will obviously vary depending on the wood it's made out of. For example, a denser, hard wood would vibrate less than a soft wood such as pine. But there are also many variables such as thickness and what you coat the wood with that will affect the amount of and the quality of the vibration. This is why many people feel that guitar making is more of an art than it is a science.
Hope that answers your question,
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