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If you search on eddy current demonstration you can find quite a few. Here is one good web page and here is [Invalid or Expired Link Removed]. They solve the problem of how to see where the magnet is in a metal tube in two very different ways! Eddy currents are quite real and the effect is proportional to the field strength and material characteristics just like the bass signal from a pickup so it is wrong to believe it will be too small to be important. It will only be too small to be important if you understand the physics and design to make that happen, as Walter's article link does. The bicycle trainer brakes work just like some of the demonstrations in that Russian video. They are just magnets placed in proximity to a rotating disk or cylinder of an electrically conductive metal. I have an Ohaus triple beam balance that I used to weigh components back when I flew competition model rockets and it has a magnetic damper. An aluminum blade moves vertically between two magnets in the frame of the balance to damp motion.
When I started playing many light years ago, it was a P, & always w/the covers on, same w/the bridge ashtray. The bridge & foam went 1st. I experi-listening to the sound w/the p/u cover off, then on again. Remember liking it better w/the cover off, & that's how it stayed. Also a tad lighter, & easier access up/down the rt. hand area. Once again; results may vary..........
I wish Nordstrand made covered pickups. I have two J basses with their pickups and both of them buzz when I touch some pole pieces. (not all of them) So I returned one set and received a replacement and that did not fix it . I had my repair guy rewire the bass and it's still doing it. So I ended up making a cover plate for the pickups but because of the different height of the pole pieces I still had to drill holes. But at least some of the pole pieces are sunk in deep enough that I don't have any troubles. The pickups sound great but a solid cover would be nice to have.
I think some people can hear the difference and some can't. Maybe I'm more of a critical listener due to my studio work?
Maybe there's a 'Golden Ears Guy' who can chime in.
I can speak from recent direct experience. I design and build custom pickups for basses. We added thin (0.040") aluminum decorative cap plates on the special pickups that we made for Keith's Marvin 10-string bass project. With and without the plates, we couldn't hear any loss of high end or other change in sound. And these are wide-range pickups with big magnets. I understand all the theory about eddy currents, but I couldn't hear any difference.
It's easy to test. Take any bass that you have and slip a small rectangle of aluminum (or whatever material you are curious about) in on top of the pickup. See if you can hear any change.
It's easy to test. Take any bass that you have and slip a small rectangle of aluminum (or whatever material you are curious about) in on top of the pickup. See if you can hear any change.
Looks mostly. It probably has some small effect on your sound, but there's no substitution for the sound of a good pickup, even slightly altered by plastic, than by a crappy pickup that is going to sound like crap regardless. lol 
The pickup covers on Rickenbackers are made of plastic, except in the case of the now-pretty-much-defunct "horseshoe pickup", where the "pickup cover" is actually an extension of the magnet, itself...
Yes, that cover serves no real purpose and was put there to emulate the horseshoe pickup. Some players who might have used the horseshoe as a handrest would have liked the new cover, but most people take it right off, as it is in the way. The "ashtray" covers on Fenders are basically the same.
The "horseshoe magnet" is actually two magnets arranged to enclose the strings. Rickenbacker basically stole the idea from a guy named Tutmarc who invented it in the 1930s. Look him up, even here on TB.
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i see massive difference between mechanical waves - sound passing through mechanical barrier - grill cloth and electromagnetical field passing through negligeable magnetic resistance eg. plastic cover . as exercise put your mobile phone into plastic box or steel box You will see difference
The point I was trying to make was that the plastic pickup cover is essentially transparent to the EM field in the same way that good grill cloth strives to be acoustically transparent, not to equate mechanical waves and EM waves.
I'd actually be more likely to buy them. I've never liked the look of EMGs.
I don't think all that woo about finishes having an effect on sound is real. I think it's a bunch of superstitious nonsense that guitarists like to think up.
Maybe stop touching the pole pieces?
