Can someone please explain pickups

[The13thFret]

Firstly, sorry for all the pics, I asked this question in another thread, and it got kind of buried. A couple of people tried to answer, but maybe I wasn't making myself clear so I have added pics.
I have changed lots of bass pickups over the years, I still have spares that were not quite what I was looking for, but may use one day (read as compulsive hoarder)

I thought I understood the theory of operation (I must have too much time on my hands now, because I am thinking past more than what they sound like). I can't get my head around why they pick up vibration that's not a ferrous metal.
If it really is a pickup, then something non ferrous should not produce a voltage?
It was suggested that the poles were moving within the coil, and so it was working like a microphone. All the pickups I have looked at are solid and the coil does not move, many are potted with wax, resin or conformal coating.

So why, when I wire them up with no controls to my little desk amp.

And check them.

If I tap it with my finger, I can clearly hear the tapping through the amp.

any suggestions?

 

[Crater]

I'm guessing the capacitance in your finger causes a little current bump in current, which is then amplified.

Touch screen devices work by sensing the capacitance in your fingers. Try repeating your 'experiment' but with a non-conductive object like a pen or pencil. There might be some voltage leaking from the amp's input jack, and when you touch the pickup, your finger is 'grounding' the voltage and causing the noise. Again, this is just a guess.

Suggestions? Don't worry about it. If a pickup is microphonic, it will be come obvious at high volume.

 

[Warpeg]

Pickup (music technology) - Wikipedia and Invalid Link Removed

 

[Killed_by_Death]

Your body carries electrical charge, believe it or not.

 

[Coolhandjjl]

You are experiencing the piezoelectric effect.

Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat.

 

[megafiddle]

How much gain is being used for this experiment? With enough gain, you will always hear something from tapping on a pickup, even if the coil and magnets are completely potted against relative movement. It could even be due to movement of the pickup within the earth's magnetic field.

A good test would be to first set up the amp gain for a normal volume with an actual bass with the bass's own controls at maximum output. Then compare the bare pickup microphonic volume without changing the amp gain. It should be relatively quiet.

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[The13thFret]

Sincere thanks to everyone for your input. Crater, Warpeg, Killed_By_Death, coolhandjjl & megafiddle some good info, and its nice to be able to bounce some ideas around.
This is all just for a side hobby (time I should be spending practicing or looking for a gig). I'm going to try winding some commercially available pickup kits just for fun & thought I understood the theory of operation, but this kind of goes against the theory, or what I thought I knew, and it interests me (and threw me a bit).

I'm sure its microphonic. What I am hearing when I tap on the side of the pickup is a replication of the vibration caused by the tapping. Its not that pop of discharging capacitance, or equalising a potential difference in voltage (I know what that sounds like from every time I have touched my strings in a club with some earth fault in the building). Piezoelectric effect?, I'm not sure, maybe, but I don't think I am applying enough pressure, and between PVC, coated copper, and whatever the poles are made of? hmm.

Gain is certainly an issue, the microphonic effect I estimate to be somewhere between 10 and 20% (with my small sample pool) of what a string sounds like on that amp, but its there and increases proportionally with gain.

Next test will be to earth myself and try tapping with a non conductive (chopstick?) trying my quiet passive BG before and after.

Maybe I'm chasing an earth fault in my house, or maybe I should just leave pickups to the pro's and go practice....or watch Netflix... that Umbrella Academy seems pretty good, and has an awesome soundtrack

 

[megafiddle]

I like to experiment myself.

If you are testing the pickups as shown in the photos, you have other pickups (and their magnets) in close proximity. You would want to keep any magnet or magnetic metal far away from the pickup being tested.

10 or 20 percent microphonic noise may not be bad if you are testing the pickups loose on a tabletop. In that case, you are giving the pickups every opportunity to move (vibrate). For a more practical and realistic test, you might want to mount the pickup on a slap of wood, just as it might be mounted in a bass.

I don't believe copper is piezoelectric. Not sure about the other pickup materials, but they are all insulated from the circuit. If there was any piezoelectric noise, I would expect it to be a higher frequency "click", as it would need to be capacitively coupled into the circuit. So I would agree that it's not piezoelectric.

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