How does an electric bass work?

Discussion in 'Basses [BG]' started by Jim Ingraham, Feb 12, 2005.

  1. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    I understand the principles of an acoustic instruments strings vibrating and and being amplified by the sound chamber, but how about on an electric. Ive been playing 35 years and i dont understand how a pickup actually works. I think that somehow the string vibrating in the magnetic field produces something that the pickup amplifies and turns into something else which then goes to the speaker and vibrates it somehow... :confused: :confused: :confused:

    Can someone explain this whole process in understandable terms for me. The main reason im curious is because i want to understand how body and neck wood choices affect tone. If an electric guitar uses electrical and magnetic forces to generate sound, where do the physical vibrations of the neck and body slab enter the equation???
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    It's mostly in the "character" of the vibrations. For instance, if you use a bone nut and a brass bridge, neither one of those witness points will be "perfect", so there will be some "influence" on the way the strings vibrate. The same is true for woods, in terms of the "resonances" they contribute to the sound. And the same is certainly true for pickups and the way they're built. The whole instrument ends up being a "system", from an engineering/physical standpoint. It's almost impossible to pick out the detailed contributions of each of the parts, from a theoretical point of view. The instrument just "is what it is", most of the time. There's certainly "engineering" that goes into designing a good bass, as I'm sure any good luthier will tell you. But, the end results of that are sometimes very difficult to program "precisely".

    But, I should probably shut up and let some of the luthiers chime in. :)
  3. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002

    Ok ... but what is it exactly that the pickup is doing?? How does that metal string vibrating in the magnetic field get turned into the electricity that then can go thru wires??

    (Maybe this should be in a different forum??)
  4. Toasted


    May 26, 2003
    Leeds, UK
  5. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002

    Thanks Toasted...that helps a lot :) how do the vibrations of non-magnetic wood enter in??? :confused:

    Is a pick-up like a microphone then, thats amplifying the acoustic sound of strings and wood together??
  6. Toasted


    May 26, 2003
    Leeds, UK
    The woods alter the way the string vibrates. I think, because all materials have a different resonant qualities. I think.
  7. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    That's pretty much it.
  8. paintandsk8

    paintandsk8 Pushin' my soul through the wire...

    May 12, 2003
    West Lafayette, IN

    No, all the pickups "pick up" is vibrations of the metal strings (unless you want to get into piezo's, but we'll let you figure out mags first). The wood and the construction of the instrument enter into the equation inderectly. The vibrations of wood are not directly amplified, but the vibrations and/or tonal properties of the wood effect how the string vibrates.

    A sound is made up of a collection of many different frequencies all at different volumes. A piece of wood will in effect absorb these frequences at different rates. Leaving some untouched at almost completely removing others.

    Or, if you prefer, lets think about it from a physics stand point. The first law of thermodynamics says that energy (in our case vibrations) can not be created or destroyed, it can only change forms. So when you pluck a string on your bass you are transforming the motion in your finger into vibrations in the string. So then the instrument starts vibrating too, so where did it get it's energy? It got it from the string, but since energy cannot be created, in order for it to start vibrating, it had to take some of the energy out of the string. The kicker is that it doesn't do this at the same rate at all different frequencies. So, as soon as a piece of wood, or anything else starts vibrating, it has essentially changed the way the string is vibrating, by stealing some of it's vibrations.

    So, tone is based on the way in which different woods, different constructions, and different hardware all absorb energy. And some factors cause a big difference, while others are hardly noticeable.

    That's not a perfectly correct scientific explanation, but you should get the idea. Hopefully I didn't lose anyone in there.
  9. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    We always remembered the the first law as "you can't get something for nothing." Of course, I am an electrical engineer and those ME required courses never thrilled me when I was in college.

    Anyway, I think everyone here has given the correct answer. The wood and constructions has an effect on the vibration of the string. The pickup creates an electrical current because of the steel string moving through the magnetic field.

    The funny thing is that the original idea to go electric with guitars was do in attempt to make the wood unimportant. Guitar makers had to careful choose woods in acoustic guitars to get the right sounds. They thought that an electric would be unaffected by the wood choices. Perhaps they were right to some extent, tonal differences in an electric are much more subtle than they are in an acoustic. However, they are there.
  10. cosmicevan


    Feb 1, 2003
    New York
    i understood it this way...simple math.

    remember when you attached a nail to a battery with a copper wire and created a magnet? well that looks like this:

    electrical charge + steel(nail) + copper wire = magnet

    now, based on math for a pickup you get this:

    magnet + copper wire wrapped around it (pickup) + steel (string) = charge

    that charge is amplified and that's how the pickup works in most basic form. i found a lot of info at stew mac's website a while ago.
  11. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    Thank you... i think thats what i was looking for... the wood is acting sort of like a filter taking away certain parts and perhaps accentuating other parts of the vibrations :bassist: ....
    i'll be able to sleep tonite now
  12. vene-nemesis

    vene-nemesis Banned

    Jul 17, 2003
    Bilbao España
    Also think that when the string vibrates it makes the wood vibrate, the wood is the place where pickups rest so the pickup also vibrates and that can change the sound a little enchancing some sounds and dampening others (in neckthru basses specially). If you would like to you can bite your bass and play a string to feel the vibration.

    A vibrating string transmits cinetic energy thats also vibration to the place where it rests. When 2 structures that are in contact vibrate the vibration of each affects the vibration of the other one, so more dense the wood more powerfull and least longer vibration and also slower wave pattern this echances Bass sound, and viceversa yuo enchance treeble sound.
  13. geshel


    Oct 2, 2001
    cosmicevan: it's not quite like that.

    edit: I don't think what I said here is quite correct anymore - see the discussion in later posts.

    The link posted by Toasted is good but it skips over one part: why the moving string affects the magnetic field.

    A wire moving through a magnetic field has a current induced in it. The funny part is, this current creates it's own magnetic field. It's smaller than the existing field, but in the opposite direction. The net effect is that the overall field fluctuates as the wire moves around. So:

    A current is induced in the wire whether or not it's the wire moving or the field is moving / changing.

    0. magnetic field exists, with a coil of wire and a metal string in it
    1. the string vibrates, and the magnetic field induces an fluctuating current in it
    2. this current's own magnetic field causes the overall field to fluctuate
    3. the coil is sitting in this fluctuating field, so a current gets induced in it

    Now, if you "read ahead" you'll realize that the current in the coil also affects the magnetic field, and so on and so forth. The reality is that each effect is smaller than the previous one, and the sum total of all those smaller interacting fields is what actually happens (more or less simultaneously).

    I've glossed over a few details - namely, the orientation of the field and the direction that the string is moving in, and how those two interact (moving in some directions wouldn't induce any current). Likewise for the coil.
  14. vene-nemesis

    vene-nemesis Banned

    Jul 17, 2003
    Bilbao España

    Moving the string in any direction will induce current, but the string must vibrate within a speed parameter so the coil can be affected by the cinetic change.
  15. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    geshel -

    Is that really the primary source of flux change in the coil - the field change due to the induced currents in the string? Wow, I had always thought that the change in magnetic flux in the coil was due to the warping of the stationary magnet field, as the article describes. Live and learn. So, this then implies that if you (theoretically) made a string out of discrete metal particles that could not form a long linear curent, the result would be gone, or at least greatly diminished. It also implies that since the induced current in the pup coil is primarily due to the induced current in the string, and not due to distortion of the magnetic field, that copper strings could be used, since there is no need for a ferromagnetic string to distort the field produced by the stationary magnet.

    -But then again, there can be no longitudinal current in any string, since it isn't a closed circuit- so there can only be small eddy currents -

    geshel, are you sure about this? I always thought you had to have some iron in the string to make a magnetic pickup work.

    vene-n -- if the generation mechanism geshel describes is correct, then a movement along a path of constant field strength - such as possibly side-to-side - would produce no induced currents in the string, and hence in the rest of the process. Further, even if the other description is true, then a side-to-side motion on a blade-type pickup would give little or no response.

    geshel - any idea about how a bartolini works, since it's original name of "Hi-A" was meant to indicate something about being able to pick up motion in all directions, unlike an acoustic guitar?
  16. geshel


    Oct 2, 2001
    You know what Peter, I could definitely be wrong. I see what you mean, that since a ferromagnetic string will concentrate some of the field lines through it, its movement could cause the field lines through the pickups to fluctuate.

    My gut tells me that the string doesn't have to be a closed circuit for the EM feedback effect I described to happen. I mean - it's a conductor, so if you move it through the field, the electrons *will* move -- charge would build up at either end of the string, oscillating back and forth. Whether or not there's enough current to affect the field, I don't know.

    It's anecdotal evidence (I haven't tinkered with it myself), but the fact that I've read a) bronze strings (which have a steel core, yes?) work with magnetic pickups, but poorly, and b) high-nickel content strings create more output, leads me to believe that the mechanism you describe is the predominant one.
  17. geshel


    Oct 2, 2001
    P.S. I don't know why/if Bartolinis would have better response to string movement in all directions. I think Lane Poor attributed this to his pickups to (eg giving the same response to slapping as to plucking). The width / composition of the pole pieces will mostly determine the resulting field shape. . .but without knowing if it's the induced current or the flux density change, it's hard to guess what field shapes would have what effects.
  18. glnflwrs


    Jan 25, 2005
    Hesperia, CA
    The metallic string vibrating in the pups mag field causes the pup's field to fluctuate in strength and shape. The strength = amplitude or volume and the shape = envelope or tone. When the string comes to the center of its motion there is no field fluctuation which = silence. This is where the 60Hz hum gets in. Humbuckers do their magic because there are two coils, in series, but in opposite polarity to each other. The outcome is, basically one pickup where the string is never centered because of the different location of each coil. When it is centered on one coil it is off center on the other. Thus, no 60 Hz hum.
  19. geshel


    Oct 2, 2001
    I don't think that description of a humbucker is quite correct. It doesn't have to do with the "center". The key part is that, not only are the coils in opposite polarity to one another, but the magnets underneath them are also in opposite polarity.

    So, as the string moves, the changes in magnetic flux it causes are of the opposite polarity through one coil as through the other. So, the current goes in the opposite direction (say, clockwise instead of counter-clockwise). So, the leads from that coil are wired up in the other direction to counter-act this.

    The physical center isn't the reason - in fact, there are "stacked" humbucking designs (with one coil above the magnet and one below it) where both coils are under the exact same "spot" on the string.
  20. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Quite so. +1