Strange trebly interference picked up by single-coils

[jamesmafyew]

Not sure if this belongs here, but I'm hoping that some people on this forum might have experience dealing with odd EMI/RFI (especially from railroads). I live next to one of Amtrak's lines that runs on 12kV 25Hz AC traction power. We have 12kV catenary behind our house, but no 138kV transmission line. The signal power is supposedly 91Hz or 100Hz.

I get the weirdest intermittent squealy noise in single-coil pickups, but it's up in the low treble area. There's a cluster of frequencies 2.4-3kHz, and then more 4.5-5kHz (presumably 2nd harmonic of some of the former). There's also a constant tone at 8kHz with partials going up to at least 40kHz. The noise can be nulled if I strap on the guitar and face my body parallel to the train tracks, i.e. the neck is pointed perpendicularly away from the tracks. If I tilt the guitar up or down even slightly, I lose the null. Turning the top to face the tracks (neck parallel to tracks) makes the noise louder. Laying the guitar on its back on a horizontal surface makes the noise much, much louder, regardless of where the neck points. This is true in every room in my house.

The guitar cavity is already shielded with copper foil, and the noise is not present if I unplug the cable and leave it hanging unterminated. Humbuckers do exhibit a similar noise, but the orientation of the bar magnet (vs polepieces) means that everything is 90 degrees different in orientation.

Question is, can I do anything about this? The null is so small that it's preposterous to stand that still while trying to record guitar. Any guesses on what it is? RFI intermodulation products? Magnetic fields? I guess a B field associated with these catenary wires would have its lines of force running essentially up and down through my house. But those are only supposed to be carrying a 25Hz sine wave. This noise happens whether there are trains passing or not.

Here are some analysis visuals of a recording of the noise out of my Tele neck pickup (Lollar Vintage T):

[Helmholtz]

As the noise depends on PU orientation it's definitely magnetic interference.
Higher frequencies might be harmonics and/or added control signals (line signalling).

Unfortunately there's not much you can do if even humbuckers are noisy. Electrical shielding doesn't help against magnetic fields.
You would rather need a steel cage.

[jamesmafyew]

I do recall now that the local commuter trains using this line have regenerative braking systems. Not knowing how they work, I would guess that they dump lots of harmonic garbage into the neutral return (the rails themselves), and that might explain why some of these frequencies seem randomly intermittent.

[pdf64]

My band room backs on to the UK west coast main line rail line, and yeah, single coil pickups are unusable in there.
Guitars can be screened with copper tape as perfectly as is feasible, to no avail.
Thankfully humbuckers generally cope ok, though a bit of higher frequency buzz can break through if a particularly noisy train comes past. Kinman pickups have been a godsend for my strat and p90 les paul, but perhaps lining the walls with sheet steel would have been a better solution.

[Chuck H]

If you have two strats and two inputs on the amp you could just take the strings off one strat set the pickups accordingly for phase and plug it into the unused amp input

[jamesmafyew]

If you have two strats and two inputs on the amp you could just take the strings off one strat set the pickups accordingly for phase and plug it into the unused amp input

It's almost like it would literally............ buck the hum

Actually, this makes me curious (I'm sure the pickup makers forum would know) whether the single coil + dummy coil idea has been done with each feeding an active stage, and then having the mixer stage happen at low impedance, post buffering.

[Helmholtz]

Wondering how you did the measurements.
Do your PUs actually put out signal up to and above 40kHz?

[jamesmafyew]

Wondering how you did the measurements.
Do your PUs actually put out signal up to and above 40kHz?

This is the guitar with no strings on it, laying on its back (noisiest position), neck pickup recorded straight into a DI input on a rack audio interface at 24-bit/96kHz (so, recording bandwidth of 48kHz). I can't say whether the pickups actually produce *signal* up that high or if that interference was introduced in the wiring or some other way. But that's what was recorded. Recording a synth into the same DI input with the same instrument cable doesn't show those harmonics, so I have to assume they're coming from the pickup or the guitar's wiring.

[Helmholtz]

Point is that PUs act as a steep 40dB/decade low pass filter above their resonance. Means that actual field levels around 40kHz might be larger by a factor of 100.

Question is if that large EMI fields exceed EMI emission limits in your country.

[jamesmafyew]

Point is that PUs act as a steep 40dB/decade low pass filter above their resonance. Means that actual field levels around 40kHz might be larger by a factor of 100.

Question is if that large EMI fields exceed EMI emission limits in your country.

That did occur to me, but unfortunately the USA has no national legal limits on magnetic field intensity (that I know of, anyway).

[Chuck H]

That did occur to me, but unfortunately the USA has no national legal limits on magnetic field intensity (that I know of, anyway).

I looked it up. Some states do. Pennsylvania isn't one of them. As a result there are many services in PA that cater to the problem. Though I'll guess their service is expensive and their target is industry and not band practice spaces.

Maybe one big piece of sheet metal or that shielding fabric strategically located could give you an acceptable size space to play in.?.

[Chuck H]

Maybe something like this.?.

https://www.amazon.com/Protection-Sh...08CZ8SX91?th=1

As strong as the interference seems to be I'd hate to spend fifty bucks for nothing though. I might get the biggest piece of metal I could find for free and see if I were able to block the EMI at all relative to it's location. Like move it near and around the pickup trying to find a sweet spot trajectory. If it worked at all I might spring for a roll of that Faraday fabric or a thin piece of sheet metal and cover an area of wall (and maybe a partial floor or ceiling).

[Helmholtz]

Audio frequency magnetic fields cannot effectively be shielded by non-ferrous foils.

[jamesmafyew]

Yeah I was under the impression that it would require something on the order of 1cm thickness to block fields like this. The thinner stuff is probably more suitable for RF.

[Chuck H]

Audio frequency magnetic fields cannot effectively be shielded by non-ferrous foils.

Thank you. I did not know that.

EDIT: What is the copper banding seen on some power transformers there for?