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Physics anyone? why cover a pickup?

Discussion in 'Pickups & Electronics [BG]' started by Whil57, Sep 30, 2017.

  1. 4StringTheorist

    4StringTheorist Supporting Member

    Your thread title asks for some physics insight. I was trained as a physicist and have a degree in physics. The people telling you that the plastic cover is wholly and completely ignored by the magnetic field are correct. As far as the electromagnetic action of the pickups are concerned, everything non-metallic on the bass does not exist.

    Any metallic cover will have an influence, thanks to Lenz's Law. A ferric metal will have a bigger effect of course, as then there are direct magnetic alterations to the field as well as Lenz's Law effects. A non-ferric pickup cover like aluminum or copper or what-have-you will only have the Lenz's Law effects.

    line6man mentioned this effect on page 1, and he was correct about it, as usual. When the string vibrates and alters the magnetic field it passes through, that changing magnetic field field will induce a current in any metallic pickup cover (whose design allows for a closed loop). This induced current will create a magnetic field in opposition to the original field, not cancelling it, but definitely reducing it.

    One of the great shielding guides on this forum showed a method for shielding the insides of pickup covers with copper tape. The creator of the guide was hip to this effect because he didn't connect the tap all the way around the inside of the cover: he left a gap on one side so that the tape wouldn't form a full loop. This will greatly reduce back-EMF (Lenz's Law) effects, though I doubt it'd remove them completely.

    Bottom line: If you want to surround your pickup in metal between it and the string, prepare for tonal differences from an "invisible" plastic or wooden cover.

    Sartori, CameronJohnson and Fuzzbass like this.
  2. No real difference other than looks. May help with the poles not rusting though.
  3. Drgonzonm


    Sep 4, 2017
    American SW
    You don't want nickel its ferro magnetic.
    Try to pick up a spoon and a fork with a magnet. You will pick one but not the other.
    I don't think you want aluminium, because of its Para magnetic properties. Al has a tendency to depress magnetic fields. There some alloys which are magneticly neurtal
    Last edited: Oct 1, 2017
  4. bassdude51

    bassdude51 Supporting Member

    Nov 1, 2008
    Central Ohio
    I think that this would look cool.
    As for the physics question.....................as long as it is not brass or some other metal, plastic or wood covers over the magnet poles are not gonna affect anything.

    The metal covers on the Les Paul guitar and the chrome cover on the Fender Telecaster neck pickup did restrict signal. Plastic does not.
  5. What about those metal ashtrays that come on rickenbacker and some fender basses that go over the strings? They look cool but I've never understood what they were for.
    Whil57 likes this.
  6. reddesert


    Mar 19, 2015
    As others have said, non-metallic pickup covers make no difference.

    The effect of metal pickup covers depends on the metal. An actual ferromagnetic metal would have a large effect, but these are rarely used. Non-ferrous metals have an effect that depends on the degree to which the metal supports eddy currents at audio frequencies. These eddy currents are caused by Lenz's Law reactions, as 4StringTheorist said. The susceptibility to eddy currents depends on the resistivity and magnetic permeability of the metal and isn't simple to calculate, so there are people who have simulated it or actually measured it. It also depends on the geometry of the cover and whether there are slots in it.

    The most common alloys for metal pickup covers seem to be nickel silver (which is actually about 60/20/20 copper/nickel/zinc, no silver), and plated brass. Brass has more eddy currents and reduces the pickup's resonant peak at treble frequencies more than nickel silver. You can find some long threads about measuring this at tdpri.com, because Telecasters use a covered neck pickup. My impression was that the difference in effect on the resonant peak was less significant once they added in other real effects like the small capacitance of a long cable to the amp.
    Sartori likes this.
  7. john m

    john m Supporting Member

    Jan 15, 2006
    Right on with the shorted loop explanation--- a typical PU cover is not a shorted loop.
    I'm not sure what the frequency of any eddy currents would be, but those are usually an issue with a solid block (of ferrous metal)--- that's why transformers are constructed with insulated laminations.
  8. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    I don't believe this was covered in Physics, IIRC.
  9. reddesert


    Mar 19, 2015
    Eddy currents can flow within a pickup cover shaped piece of metal, apparently. The frequency would be audio and the length scale should be set by the scale over which the magnetic field is varying, I would guess that's roughly the amount of excursion of the vibrating string. There are people who say they can minimize the eddy currents by cutting slots in the cover so as to break up the area where a continuous loop current can form. This is not something I've ever tried, just read about on TDPRI. The skin depth is important for eddy currents, which might be one reason that transformers use laminations.

    An example of eddy currents at work is the resistive magnetic unit in a bike trainer or exercise bike. If you take one apart, you'll typically find stationary permanent magnets around a rotating metal disk (aluminum). The magnetic field threads through the disk, and as the disk rotates through the field, each part of the disk sees a variation in the magnetic field, causing eddy currents. These, by Lenz's law, tend to oppose the motion, so there is a braking force against the rotation.
    4StringTheorist likes this.
  10. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    so exercise bikes are mostly just un-hooked-up generators? good to know.

    as for metal covers, the explanation that i can wrap my head around is that they should be totally non-"magnetic" (so no iron or carbon steel) and conductive but not too conductive (so no copper or aluminum, and brass is a tone-duller). "nickel silver" is used by gibson and others because it's not as conductive as brass and so doesn't create as much eddy current to block the tone.
  11. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    here is a fantastic article explaining the whole thing, complete with experiments with cutting slots to stop eddy currents and a reveal of why the gretsch filtertron has that gapped cover.
    4StringTheorist likes this.
  12. Badwater


    Jan 12, 2017
    Pickup covers protect the fragile wire that could be damaged by the pickguard or the pick.
  13. 4StringTheorist

    4StringTheorist Supporting Member

    I had little hand crank generators in my physics classes that I taught. Turnign them was easy, until you hooked up a load, then you had to work at it. So those bikes are pobably just pushing current through a resistor (with a heat wink I'd hope) to provide resistsance.

    Metals are going to be pretty darn conductive no matter what. I'd avoid metals entirely (if it were up to me)
  14. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    the tone loss difference between brass and "nickel silver" covers is well-established.
    Sartori likes this.
  15. Bob T

    Bob T

    May 26, 2016
    Rhode Island
    As someone who began playing in the mid 70's I can tell you that everything was covered and that was considered "farty" - so off came the covers (bridges included). Some did think it was a "sound" thing (apparently the guitar company engineers knew nothing)!
  16. Stonetown Lows

    Stonetown Lows Commercial User

    Jan 23, 2014
    Ringwood NJ
    Owner, M&V Guitars &Pickups
    All of the foregoing is true, for sure. One thing I didn't see yet was that, without that .040" to .062" of plastic or metal cover over the pickup, you can get the pole piece just that much closer to the string bottom. Since the field strength from the pole varies somewhere between the square and the cube root of the distance to the string, moving the string to or away from the pole greatly affects the field strength and the pickup response. So while it is true that there is a sort of magnetic 'bubble' and that any disturbance will excite it, the pickup response changes a lot the closer the string is to the pole. Of course there is that clicking aspect to avoid, depending on how you pluck.

    A steel pickup cover will just spread out and weaken the field somewhat, creating a flatter, weaker 'bubble', losing you some highs.
  17. Whil57


    Aug 7, 2013
    Long Island
    Yeah, i took off covers, like you said. And now, they will graciously supply you with those covers again for a mere 4k. I'm like what? i took those off because i didnt like them.
  18. Had a MusicMan Stingray 5 and had the pickup very close to the strings. I got a little clicking if the string hit a pole piece. So I just put electricians tape over the pole pieces.

    No change in tone except I got rid of the clicks.

    I guess I did a good job cutting the tape to shape. A couple people came up and asked what brand of replacement PU I was using. They liked the no pole piece look and were sure the tone was "better" than the regular Stingray PU. After that I started having fun carefully cutting different color electricians tape in strange shapes just to mess with people;-)

    Eventually lowered the PU slightly and got better technique so no need for tape. Took awhile to get all the residue off.
  19. alder


    Feb 17, 2012
    A plastic cover has no effect on tone or noise , but will protect the delicate wire coils from corrosives like sweat and also mechanical damage. A properly grounded brass or alloy cover may also shield the pickup from electrical noise, depending on how completely it isolates the coils ( see Faraday Cage. ) Probably the best example is the lipstick tube pickup. Open-backed metal covers have little effect on noise.

    I don't think any pickup covers are made from steel, forming the shape would be difficult and expensive compared to say soft brass. So as far as magnetic effects or shielding, I do not think there are any - all the common materials are non-magnetic.

    Exposed or hidden pole pieces makes no difference, nor really does spacing - it is the overall magnetic field that matters. The type of magnet makes no measurable difference, except in as much as a modern permanent ceramic magnet will likely have a much stronger field that an old worn-out AlNiCo. Field strength will certainly make a difference, but the idea that different kinds of magnet produce different kinds of field is nonsense, a magnetic field is a magnetic field, regardless of how it is produced. AlNiCos are not 'warmer' than ceramics, they're just weaker.

    There can also be mechanical effects - the cover is one more thing that can vibrate, and that can be transduced by the pickup. However, good modern pickups are wax-potted, so that such microphonic effects are prevented. The pickup cover provides a handy container for the potting material. Finally, the pickup cover often serves as the mounting for the pickup inside. Pickups are made from fairly fragile fiberboard that can be easily broken if used for the actual mounting.

    This is all physics and engineering. However, guitars are magical, and defy the laws of physics for many people, who's extraordinary hearing can detect differences that no oscilloscope can. These same people hear differences between different fretboards and even finishes. Don't argue with them - they are right. It is magic.

    To the previous poster who brought up Lenz's Law - look at the size of the power supply used in the demonstration. Yes, the effect exists, but for the amount of current generated by a guitar pickup, it will be negligible, and probably not even measurable. Also, a properly designed shield ( like a lipstick tube with a gap in the middle ) defeats the effect. But again, through magic, all things become audible to some people.
    Last edited: Oct 10, 2017
    Bob T, Sartori and ctmullins like this.
  20. basrelief


    Mar 8, 2014
    I put electrical tape over the uncovered pole pieces to prevent my right hand pushing the srings too low and making a loud "pop" sound. I noticed my bass player does that, too. Muddy Waters made the "pop" sound part of his technique but I don't like it.

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