I’ve made quite a few high speed videos since I got my Sony RX100Mk5.
I never thought that I’d use it that much, but it’s proven to be quite educational and entertaining.
There’s a clip on youtube of high speed bass strings that I often show people, but often I can’t find it when I want it, so I decided it was time to do my own.
As you can see, once plucked, the string doesn’t just vibrate as one body, side to side. If you pay attention and pick a spot to watch, (I recommend the top edge of the neck pickup) you’ll see that the string actually comes to rest for a bit as the wave travels up and down the string.
As you all should know, being bass peeps, the pickups generate a voltage from the coil windings because the magnetic field through the coils changes. In a generator, we generate a voltage by moving the magnets. With pickups, we move the magnetic field directly by disturbing it with magnetic material. This is why strings have to have magnetic content.
If the motion of the strings is proportional to the voltage that the pickups generate, can you have a guess at what the waveform might be like, given the motion of the strings over the pickups?
Unfortunately, there’s no way of recording sound in high speed mode, or I’d be able to show you directly with a data overlay.
I think the next thing to do might be to take the front off the bass cab and drag that outside too. Then I can get a shot of both string and bass driver moving at the same time in the same shot… With a dot of white paint on each, we’ll be able to track the movement. Unfortunately, the energy from the amplifier will feed back into the string giving infinite sustain, but still, nobody has shown that yet.
There are two competing theories for how electric pickups work:
One says that the vibrating strings pass through the magnetic fields generated by the pole pieces which changes the magnetic flux, inducing a voltage in the coil.
The second says that the magnetic pole pieces create magnetic fields around the strings and when a string vibrates, the field moves through the coil, inducing a voltage in the coil.
Is it likely that I’d decide to detune my string randomly or tune every string precisely before doing highspeed video? (Noting that with a stroboscopic tuner, you can easily see how much they go out of tune when you lay a bass flat.)
If you look at the very first frame of the video, you can probably see why it’s flying up and over the other strings. It’s being held against the scratch plate and then released. A standard pluck causes the same movement, but looks about half as big as when you really drag the string off the side and let it go. It also starts more horizontal, but as you pull it more out, eventually you hit the E string. This clip just had the biggest movement, so I went with it.
Interesting. As I kid armed with paper, magnets and Iron filings, I spent hours looking at magnetic fields and how they interact. If you put ferrous metal in a magnetic field, the field distorts around it. The filings follow the distortion. It’s really cool. If you feel like having a go, don’t bother trying with ball bearings. We know that disturbing the field with magnetic stuff affects the entire field, all the way back to the magnet. That’s because the magnet moves if it’s not secured.
I’m in favour of the theory where the pole pieces disturbed fields cause the coil voltage. Any magnetic fields induced in the strings are going to be significantly smaller than the pole magnets field because they’re further away.
More interesting is the roundwound strings having a ribbed magnetic disturbance which would contribute towards the higher frequency zing beloved of those type of strings.
Now I’m wondering about eddy currents induced in the strings, but I don’t have an oscilloscope… yet.
Here’s the G string. Same traveling wave, but just heaps faster. Watch what it does near the pole pieces…
thanks for the clarification…I thought as much, but because the title of the post I was wondering if you were demonstrating strings move that much all the time even though you wouldn’t think it (which would have really confused me)
In real time, it’s best guess, I’m afraid. between 3 to 5 seconds. 960 fps, then take the 50fps video and bring it to 25 fps. With those sort of distortions, I’d need a 100th of a second stopwatch in the background to work it out.
Last night I showed my girlfriend how to pump energy into the strings by just sitting the bass in front of the cab at normal volume. It takes about 15 seconds to go from totally dampened strings to clipping the amp. It blew her mind. We tried all the strings to see which were most susceptible and the E string is the champion. We couldn’t get the B string to do it at all, so it’s likely a combination of string weight and energy available, or the amp/cab doesn’t perform well at such low frequencies.
I must say that my second semester university physics course on electricity/magnetism cured me of much of my interest in that stuff… and that was 30 years ago
Turns out the school of Engineering worked out with the physics department that they both needed a weed out class. I did fine in the class (got a B I think) but getting a B on the midterm with a score of only 16% convinced me to switch to Chemistry. Sadists
And then ended up loving Physical Chemistry classes as they were all about half of what I cared about in physics anyway. The other half (astrophysics) was probably starting to go all in on the string and dark matter stuff anyway, so glad I skipped out in retrospect.
That paper on the Natural-frequency splitting of a guitar string caused by a non-uniform magnetic field is gold. It makes a lot of sense. I’ve never really thought about the effect of the string having more deceleration away from and more acceleration back towards the pole magnet. The video of the strings with and without the magnetic influence demonstrates the beating effect beautifully. My F Bass A string tends to sound similar when it’s being tuned. You can hear it experience a beating period. I’ve always wondered why it’s that string alone that does it.
For some reason I find these videos very interesting at the not so clean interruption of a magnetic field or as how I see it. I work on ESWL machines. They use a coil pulsed with a high voltage signal to produce a sound wave that is focused to a very sharp focal point which is placed over a kidney stone. The stone oscillates between rapid pulses or shocks and shatters. sound can do awesome things!
