Sound coming from mono litz coil

[fo0bar]

I've recently got a couple of Surf PI boards going and I've wound up a few coils large and small.

My favourite coil so far is an 8 inch flat spiral made with 0.05*50 litz wire. Just a pancake - no basket. Despite a relatively high resistance of around 4 ohm, I'm picking up ridiculously small targets far better than any of my other coils - a strong reaction to 0.05 gram of sterling silver at around 5cm.

What I noticed today however is that within around 30cm of the coil, I can actually hear the coil with my bare ears. The sound is the same frequency as receiving the signals as interference through headphones, however the noise is actually being emitted form the coil itself. It's not loud like an old CRT TV or anything, but it's certainly apparent when the coil is closer to my ears.

..is this normal behaviour? ...Or is this a sign that I've got something set sub-optimally such as my damping resistor? Using the variable resistor circuit I've got it at the upper extreme with no ringing, and I'm contemplating placing higher value R's in the circuit to raise the baseline for further testing.

Could it be an indication that I should be shielding the coil or something? I can't seem to find any reference either on geotech or google to noise being emitted from the coil... Apologies if my google-fu has failed me again

Thanks for any insight!

[Chiv]

I had built a sniper coil for barra and it emitted a good harmonically constant tone which was quite harsh on the ears, but once a got it dampened right is was quiet. hope that helps

[waltr]

If this is Audio, you can hear it, then this is common due to the Pulsed current through the coil.
Basic physics- Current through a conductor induces a magnetic field- Two adjacent conductors with current flowing in the same direction each has a magnetic field that Opposes each other.
Therefore when the Coil is pulsed, Current flows, Each wind of the coils tries to move away from the other winds in the coil. Then the pulse is turned off and the winds relax.
This physical movement induces vibration into the air (coil housing) which you then hear.
I am betting the sound you hear is the exact same frequency of the PPS rate.

This is common in cheap AC mains transformers. Better transformers vacuum impregnate the coils with varnish (or other sealer) to keep the winds in the coils from moving.
The audio hum is not the problem, it is the wires moving against each other which will rub through the insulation and short that causes transformer (coil) failure.

[fo0bar]

I had built a sniper coil for barra and it emitted a good harmonically constant tone which was quite harsh on the ears, but once a got it dampened right is was quiet. hope that helps

Moving the variable damper didn't seem to have any effect at all, however I'm also considering trying a higher range of resistance there as well.

If this is Audio, you can hear it, then this is common due to the Pulsed current through the coil.
Basic physics- Current through a conductor induces a magnetic field- Two adjacent conductors with current flowing in the same direction each has a magnetic field that Opposes each other.
Therefore when the Coil is pulsed, Current flows, Each wind of the coils tries to move away from the other winds in the coil. Then the pulse is turned off and the winds relax.
This physical movement induces vibration into the air (coil housing) which you then hear.
I am betting the sound you hear is the exact same frequency of the PPS rate.

This is common in cheap AC mains transformers. Better transformers vacuum impregnate the coils with varnish (or other sealer) to keep the winds in the coils from moving.
The audio hum is not the problem, it is the wires moving against each other which will rub through the insulation and short that causes transformer (coil) failure.

Thanks waltr. Makes perfect sense now. The coil's still sitting on its form, packing tape reversed on paper loosely taped over a piece of chipboard - so maybe the paper is also helping amplify the vibrations.

I haven't yet applied a resin coating to it, as I wanted to make sure everything was OK before I did so. From your comments I'd assume this noise (and associated wear) would be reduced/eliminated once I've applied the resin (and of course housed the coil).

And you're spot on, it's the exact frequency as the PPS rate. The noise isn't loud, and doesn't annoy or worry me, but just puzzled me as my other coils don't do it and I hadn't noticed any mention of this kind of behaviour over MD forums.

Thanks again for the clarification

[waltr]

Ok. The paper the coil is taped to is probably acting like a speaker cone and making the sound louder.

Applying resin will keep the winds from moving BUT
be careful to NOT increase the C between winding due to increasing the dielectric constant- Air =1, G10 = 4.5 and many resins are even higher.

Wearing through the insulation may not be a problem with the coil design (winds shouldn't rub since they are spiral wound) and Litz (cloth insulation).

I did see some details of newest high tech coil construction but can not find it now. I think it was Detech coils.
they use spiral wound Litz, sandwiched between foam then over molded.

[fo0bar]

Ok. The paper the coil is taped to is probably acting like a speaker cone and making the sound louder.

Applying resin will keep the winds from moving BUT
be careful to NOT increase the C between winding due to increasing the dielectric constant- Air =1, G10 = 4.5 and many resins are even higher.

Wearing through the insulation may not be a problem with the coil design (winds shouldn't rub since they are spiral wound) and Litz (cloth insulation).

I did see some details of newest high tech coil construction but can not find it now. I think it was Detech coils.
they use spiral wound Litz, sandwiched between foam then over molded.

The coil will be coming off the paper, so that won't be a worry. I'm also hoping the wear won't be too much of an issue as it's extremely low amplitude vibrations and along with the polysilk serving it should have a relatively long service life provided it's well housed...

After reading your reply reiterating the potential of raising interwinding C by applying resin (i was hoping litz would help reduce those effects somewhat), I've been thinking up ideas to get the coil secured flat, in a manner that won't allow the windings to loosen, nor allow any possible bonus capacitance. Best I've come up with is to cover the coil in tape to secure the windings and then lightly glue and sandwich it in a thin dense foam like you described above - I was thinking thin yoga mat/wetsuit/bottlecooler material - lined on one side with a thin plastic to add rigidity and then housed, or coated in fiberglass.

[waltr]

Sounds like a plan.
Please keep us posted on how you do this (pictures please) and the results.

Are you adding any type of shield or static drain?
Check the "PI coil, shielded or unshielded" thread for discussions.

[Ferric Toes]

I have heard this effect many times, particularly with high pulse currents. I put it down to the interaction of the pulsed magnetic field from the coil and the earth's magnetic field. This makes the coil vibrate and depending on the type of shell it is in and the type of encapsulation, you get a sounding board effect. Changing the electrical damping of the coil will have little or no effect: neither will shielding. Try orienting the coil to a different plane. The sound should be at a minimum when the earth's field is in the horizontal plane of the coil.

Eric.

[fo0bar]

Sounds like a plan.
Please keep us posted on how you do this (pictures please) and the results.

Are you adding any type of shield or static drain?
Check the "PI coil, shielded or unshielded" thread for discussions.

Will certainly keep posted, with pictures I wasn't planning on adding a shield as it seems to be performing quite reliably without it, provided I'm away from interference. I haven't taken this coil outside for testing as yet, but indoors it doesn't react to my hand or a brick. I don't really want to add extra weight and/or points of failure that don't need to be there, and right now the performance is quite good - I've now pushed this little coil to ~7cm on a 0.05 gram lump of sterling in free-air without any shield so I don't feel it's really necessary.

That said, I do get a strong warbling when the delay is right down at minimum. The warbling shifts frequencies and amplitude, kind of like two fans resonating harmonically, going wawawawawa waaawaaawaaa wwoooooowww wwwooooooowwwoooowwwww waahwaahwaah wawawawa... After reading the thread you pointed out, Qiaozhi indicates that for very low delays a shield would more likely be required. Perhaps I should wind a second identical coil and shield one of them for the sake of a split test. My detectors are only really going to be used out in the bush, far far away from any kind of EMI from powerlines/transmission towers etc so I'm hoping that won't be a factor either. I guess the truth will come out when I place it on the ground outside.

I'm not adding a static drain to the SurfPI, as I built these primarily to act as a known working "control" test surface for my coils. Though I'm currently working on an MPC based PI with DSP, which will indeed have a static drain. I'm far, far more skilled with data science and analysis than I am with electronics and so I'll likely have plenty of questions regarding the drain when I'm up to that point

I have heard this effect many times, particularly with high pulse currents. I put it down to the interaction of the pulsed magnetic field from the coil and the earth's magnetic field. This makes the coil vibrate and depending on the type of shell it is in and the type of encapsulation, you get a sounding board effect. Changing the electrical damping of the coil will have little or no effect: neither will shielding. Try orienting the coil to a different plane. The sound should be at a minimum when the earth's field is in the horizontal plane of the coil.

Eric.

Thanks Eric, rotating the coil on all axis in free air seems to have nil effect, provided of course I have my ear in the same location relative to the coil.

I grabbed a small stack of tiny neodymium magnets to test the effects from magnetic fields - Placing the magnets <20mm from the innermost turn makes it buzz LOUD!!!

Also, when approaching the coil with the magnet, the detector hardly reacts. But when removing the magnet, the detector reacts strongly. Not sure what this means if anything.

[fo0bar]

Are you adding any type of shield or static drain?

Sorry, misinterpreted 'static drain' - I'll try wrapping some plain enamelled wire around it once I remove it from the paper backing and report back on how that goes.

[Qiaozhi]

I grabbed a small stack of tiny neodymium magnets to test the effects from magnetic fields - Placing the magnets <20mm from the innermost turn makes it buzz LOUD!!!

Also, when approaching the coil with the magnet, the detector hardly reacts. But when removing the magnet, the detector reacts strongly. Not sure what this means if anything.

In order to test the effectiveness of the Earth Field elimination, the magnet needs to be in motion. Just placing it near the coil will not have any effect, apart from possible detection of the magnet material. The test needs to be performed by waving the magnet a few inches away from the coil, and perpendicular to the surface.

[waltr]

Will certainly keep posted, with pictures I wasn't planning on adding a shield as it seems to be performing quite reliably without it, provided I'm away from interference. I haven't taken this coil outside for testing as yet, but indoors it doesn't react to my hand or a brick. I don't really want to add extra weight and/or points of failure that don't need to be there, and right now the performance is quite good - I've now pushed this little coil to ~7cm on a 0.05 gram lump of sterling in free-air without any shield so I don't feel it's really necessary.

That said, I do get a strong warbling when the delay is right down at minimum. The warbling shifts frequencies and amplitude, kind of like two fans resonating harmonically, going wawawawawa waaawaaawaaa wwoooooowww wwwooooooowwwoooowwwww waahwaahwaah wawawawa... After reading the thread you pointed out, Qiaozhi indicates that for very low delays a shield would more likely be required. Perhaps I should wind a second identical coil and shield one of them for the sake of a split test. My detectors are only really going to be used out in the bush, far far away from any kind of EMI from powerlines/transmission towers etc so I'm hoping that won't be a factor either. I guess the truth will come out when I place it on the ground outside.

I'm not adding a static drain to the SurfPI, as I built these primarily to act as a known working "control" test surface for my coils. Though I'm currently working on an MPC based PI with DSP, which will indeed have a static drain. I'm far, far more skilled with data science and analysis than I am with electronics and so I'll likely have plenty of questions regarding the drain when I'm up to that point



Thanks Eric, rotating the coil on all axis in free air seems to have nil effect, provided of course I have my ear in the same location relative to the coil.

I grabbed a small stack of tiny neodymium magnets to test the effects from magnetic fields - Placing the magnets <20mm from the innermost turn makes it buzz LOUD!!!

Also, when approaching the coil with the magnet, the detector hardly reacts. But when removing the magnet, the detector reacts strongly. Not sure what this means if anything.

I had that wobbling tone and be adjusting the PPS rate was about to eliminate it. Since changing the PPS rate fixed this issue I am sure it is caused by a 'beat' of the PPS and the AC mains frequencies.
In fact when I did the adjustment to wobble was at aabout 1Hz and adjuting the PPS rate change the frequency of the wobble. Adjuting in one direction increased the wobble freq, then adjust the other direction until the wobble went away.
Yea, 'shield' verse 'drain' terminology - for me and the shielded verse unshielded discussion it seems that a full shield is not needed and a simple drain wire is all that is needed. I still need to run tests on this.
A simple drain should keep electrostatic potentials off the coil and this seems to be the cause of falsing everyone talks about when the coil touches grass, etc.
In theory a Shield would be to block high frequency but this is not what seems to be the falsing cause.

In order to test the effectiveness of the Earth Field elimination, the magnet needs to be in motion. Just placing it near the coil will not have any effect, apart from possible detection of the magnet material. The test needs to be performed by waving the magnet a few inches away from the coil, and perpendicular to the surface.

Thanks for clarifying this.
My detector, HH2, does react strongly to a swinging magnet so guessing the Main and EFE sampling is not correct.

[Qiaozhi]

My detector, HH2, does react strongly to a swinging magnet so guessing the Main and EFE sampling is not correct.

Monitor both the main and secondary pulse widths. They should be the same width for effective Earth Field elimination.

[waltr]

Monitor both the main and secondary pulse widths. They should be the same width for effective Earth Field elimination.

They are the same at 15usec and 20usec apart (delay 2).
Possibly I have the integrator inputs not balanced or gain matched. I will be looking at this.

[green]

Neodymium magnets are detectable. I use ceramic disk or fridge magnets to test for EF.