Post by antigua on Sept 20, 2017 18:19:52 GMT -5
In this testing guitarnuts2.proboards.com/post/82388/thread , I found that most all magnetic pull upon the strings (save for the types of magnetic pull which my testing might not have accounted for) causes an increased oscillation, which can be clearly seen in the waveform:
The graphic above depicts the effect of moving the pickup closer to the string, but this happens with the introduction of any reasonably strong magnet.
Aside from the harmonics of moving string, which is constantly producing audible frequencies, it can be seen above that the pickup's magnet and the guitar string form a secondary harmonic oscillator, one which oscillates at a very low frequency. The plucks in waveform above are 15 seconds apart, so you can see that the individual cycles are never shorter in duration than about one second, which would be 1 Hz. Most of the oscillations seen are well below that.
Also note that you don't need a waveform analysis like the one above to see this. If you have a neodymium or other power magnet on hand, and you pluck the low E or A string on your electric guitar (as acoustic guitars might not be strung up with magnetically permeable strings), and bring the magnet really close to the string, you will be able to both see and hear this increasing oscillation as your magnet comes into close proximity of the moving string.
Conventional wisdom is that the magnet is "damping" the string, but thinking about what's happening in a spring-mass-damper context, adding damping would not increase the oscillation, as seen above, it would merely reduce the oscillation. What does increase the oscillation rate is increasing the tension on the spring.
Here is a Wolfram Alpha demo. Here is the neutral state of the demo. There are input values, mass and spring stiffness, but no damping, so you get an oscillation:
Now if the damping is increased, you see the oscillation die off, but not gain frequency:
Overall, this resembles what happens with the guitar waveform, because there is damping, in the form of air resistance and mechanical stresses, causing energy to be lost to heat. This decay happens regardless of whether a magnet is anywhere near the string. When a magnet is near the string, we can see that the oscillation increases in frequency, while the overall amplitude decay is about the same, no matter what.
Here's what happens when you increase the spring tension, the oscillation frequency increases, essentially identical to what happens when magnetic pull is imposed upon the guitar string.
If the magnet pull of guitar pickups is contributing to the damping of the guitar, this is a very minimal effect. The much more dominant effect that is observed, the increased oscillation, relates to spring stiffness.
Of course, the "spring mass damper" is only an analogy, and there is no "spring" in a guitar string, but this model effectively describes many real world resonating systems, so the questions is, what do the spring, mass and damper represent in this situation. In this case, the tension between the magnet and the string is the "spring", and the strong the magnet, the "stiffer" the spring, while the whole of the guitar string is the weight, and there is no particular damping agent, aside from the same air and mechanical stress that dampens the overall string movement.
Therefore, rather than talk about "magnetic damping", it would be more accurate to talk about "magnetic springing".
This can be demonstrated with a thought experiment, too. The ultimate damper would be to drop a bed pillow on top of a vibrating string. That would stop the string movement completely and immediately. But what would happen to the string's vibration if a powerful electromagnet was direction above the string? The string's vibration would probably dissipate more quickly that if there were no magnet at all, but it would still not stop vibrating as swiftly as it would if you were to simply drop a pillow on the string.
The graphic above depicts the effect of moving the pickup closer to the string, but this happens with the introduction of any reasonably strong magnet.
Aside from the harmonics of moving string, which is constantly producing audible frequencies, it can be seen above that the pickup's magnet and the guitar string form a secondary harmonic oscillator, one which oscillates at a very low frequency. The plucks in waveform above are 15 seconds apart, so you can see that the individual cycles are never shorter in duration than about one second, which would be 1 Hz. Most of the oscillations seen are well below that.
Also note that you don't need a waveform analysis like the one above to see this. If you have a neodymium or other power magnet on hand, and you pluck the low E or A string on your electric guitar (as acoustic guitars might not be strung up with magnetically permeable strings), and bring the magnet really close to the string, you will be able to both see and hear this increasing oscillation as your magnet comes into close proximity of the moving string.
Conventional wisdom is that the magnet is "damping" the string, but thinking about what's happening in a spring-mass-damper context, adding damping would not increase the oscillation, as seen above, it would merely reduce the oscillation. What does increase the oscillation rate is increasing the tension on the spring.
Here is a Wolfram Alpha demo. Here is the neutral state of the demo. There are input values, mass and spring stiffness, but no damping, so you get an oscillation:
Now if the damping is increased, you see the oscillation die off, but not gain frequency:
Overall, this resembles what happens with the guitar waveform, because there is damping, in the form of air resistance and mechanical stresses, causing energy to be lost to heat. This decay happens regardless of whether a magnet is anywhere near the string. When a magnet is near the string, we can see that the oscillation increases in frequency, while the overall amplitude decay is about the same, no matter what.
Here's what happens when you increase the spring tension, the oscillation frequency increases, essentially identical to what happens when magnetic pull is imposed upon the guitar string.
If the magnet pull of guitar pickups is contributing to the damping of the guitar, this is a very minimal effect. The much more dominant effect that is observed, the increased oscillation, relates to spring stiffness.
Of course, the "spring mass damper" is only an analogy, and there is no "spring" in a guitar string, but this model effectively describes many real world resonating systems, so the questions is, what do the spring, mass and damper represent in this situation. In this case, the tension between the magnet and the string is the "spring", and the strong the magnet, the "stiffer" the spring, while the whole of the guitar string is the weight, and there is no particular damping agent, aside from the same air and mechanical stress that dampens the overall string movement.
Therefore, rather than talk about "magnetic damping", it would be more accurate to talk about "magnetic springing".
This can be demonstrated with a thought experiment, too. The ultimate damper would be to drop a bed pillow on top of a vibrating string. That would stop the string movement completely and immediately. But what would happen to the string's vibration if a powerful electromagnet was direction above the string? The string's vibration would probably dissipate more quickly that if there were no magnet at all, but it would still not stop vibrating as swiftly as it would if you were to simply drop a pillow on the string.