I have heard that rumor and understand how people would think that to be the case. I have myself gotten a pickup too close to the strings and received bad sound... but that was mainly because the strings either struck the pickup while vibrating, or the magnetic field was unbalanced and didn't handle the strings evenly.
In reality, the magnetic field produced through a pole pickup is so weak at the tip of the poles it would be almost impossible for it to negatively impact strings. The rumor that a "too strong pickup" can negatively affect strings or another equally strong rumor-- that it can diminish sustain-- was put to the test by a guitar tech on Youtube.
In his test, he took a 200 lb / sq in magnet (far stronger than any pickup magnet) and near-mounted it directly over the strings on an electric guitar. He tested the volume and sustain of the strings with and without that ultra-strong magnet in place. The result: zero discernible effect. His finding: myth busted.
I have busted the myth myself (at least to my satisfaction) with flat pickups. We're talking pickups with high-power neo magnets mounted directly beneath the strings-- often within 1/8" distance. No distant magnetic field-- we're talking direct magnetic attraction from a high-power magnet (far more powerful than just about any standard pickup magnet).
The results: everyone who has used a flat pickup can attest to the terrific sound they produce. And their magnetic pull and field is far stronger and direct than that of any standard pickup. In fact almost the entire upper magnetic field of a flat pickup surrounds the strings, unlike an inner-body pickup with pin-transferred magnetic field.
So I'll have to respectfully disagree on this one. I've not seen evidence that any magnetic field short of a high-power electromagnet could negatively affect the performance of guitar strings. And if the magnet was that powerful, it would literally suck the strings right down to the surface of the magnet.
Since I build and sell flat pickups, it makes sense I would have researched this rumor pretty extensively. Not seen any concrete evidence so far supporting pickup/string interference. But I'm not posting this carved in granite. The above are just the findings of a YouTube experimenter any my own playing... as well as the experience of flat pickup players world-wide. I still have an open mind on this subject and am interested in any documented experiments that a standard pickup (or flat pickup for that matter) can adversely affect guitar strings. My findings though: the more the magnetic field surrounds the strings, the better the sound (which is why flat pickups sound so awesome on a CBG).
No tests or data to back this up, but at some point the magnetic pull has to effect the strings ability to vibrate /oscillate i'd think, and effect volume less than sustain probably, as i say, just my thoughts.
I agree with you. The question is at what point that "some point" is. I figure if a 200 lbpi magnet can't do it, and N45 neo magnets can't do it... standard guitar pickups don't get anywhere near that point.
I'm sure it could be figured out mathematically by determining the amount of pull a magnet would need to have on an X-diameter string with Y-content ferrous metal to overcome the Z-tension factor of the string (or something like that), but I'm guessing that figure would come out pretty high-- at least well over 200 lbs/sq in magnet pull-- and way out of guitar pickup mag field power.
Considering your experimenting, I'll have to experiment more and pay close attention to a possible "choke point". In this field I've learned to discount nothing. ;D
P.S. Adding a 0.15" blade, for comparison you would have to lower the pickup 0.15", otherwise you just raise the pickup for this amount which increases the volume.
Ah I see. Yeah, if that height would bring the pickup too close to the strings, that is definitely a consideration. Fortunately most pickup heights are easy to adjust. Unless of course they're perma-installed on a solid CBG shelf, which might present an issue.
So good point.
Further experiments with my 5.4K four pole single coil alnico5 pickup with a plastic cover: I used a 10" piece of a very fine string to find the magnetically hot spots of the pickup. On the surface of the pickup hottest is the point between the middle poles, second the middle poles, third the outer poles, at a distance of 0.20" same results. With a 0.02" steel blade above the poles nearly same results, but with a better response for the outer poles.