Solar Panel 15kHz noise issue

[Stuart79]

Morning all. I need a bit of advice and guidance with a minor noise issue that's perplexing me. I have a sharp 15khz peak visible on the analyser well above the self-noise of anything I go to record and I'm struggling to work out where it might be coming from. Offline analysis shows the peak to be around 14969hz. The peak is around 12-15db above the rest of the self-noise in this range.

Have methodically tried various combinations of the following:
- Plugging everything into the same distribution board
- A different distribution board
- Three different audio interfaces (one bus-powered)
- Four different microphones
- Different mic cables
- Two different laptops (plugged in and unplugged) and a desktop
- Different rooms in the house from top floor to ground floor
- An unbalanced line signal into a line input
- The same unbalanced line signal into a Radial JDI into a line input. (This brought the peak down more in line with the rest of the self-noise but it was still poking out a little.)

The frequency of the peak is always the same whichever kit I'm using. I have a simple setup anyway, but stripped it back to just input source>interface>recorder for testing purposes. I'm guessing there must be a source of interference somewhere nearby, but emphasise the 'guess' aspect of it.

My questions are:
1) Can anyone suggest what the interference might be coming from?
2) Can anyone suggest how I might address it? (Trying some double screened mic cable was the only thing I could think of.)

Thanks in anticipation.

[SafeandSound Mastering]

A few Sat morning thoughts.

Do you have any cat/rodent repellents situated domestically ?

At high SR do you ahve further harmonics ? (That might be even higher in level) and you see a sub harmonic ?

I think we can discount cathode ray tube VDU's ?

My TFT office machine is so old I can hear it whining in standby and when on different freq/tones though, it could be acoustic bourne.

Some switch mode PSU's (in PC's and wall warts) whine quite a lot.

Does the level change with mic gain ? Or even better cupping your hands very gently around the mic capsule if so could be acoustic 15kHz will be effective and quickly attenuating by hand cupping.

If you have very high harmonics up at 29kHz or beyond then it could be components gone bad in signal path.

After re-reading your extensive testing some of those may be ruled out.

Do you have the same smoke detectors/lighting/ some common system in mains outlets/alarm system/ integrated in house tech - cameras etc. in the rooms you tested ? Design fault ? Anything common in the rooms ?

"The same unbalanced line signal into a Radial JDI into a line input. (This brought the peak down more in line with the rest of the self-noise but it was still poking out a little.)"

That does somewhat suggest balanced/phase inversion is killing it off some. So it maybe electomagnetic bourne, possibly from your own or neghbouring property. Consider your own domestic tech common to both rooms first.

For sure it is tricky one.. good luck.

[Wonks]

Do you have solar panels with an associated frequency converter/inverter? They can put out that sort of frequency noise.

[Stuart79]

Thanks for the quick responses.

- No animal repellent
- No CRT monitors anywhere
- The noise increases proportionally with an increase in mic gain
- The noise is present whether the laptops were plugged in or not. I ran a test with a bus-powered interface into two different laptops, laptops plugged in first and then unplugged. The noise was present either way. I've assumed that this would rule out the mains/power supplies but please correct me if I'm wrong on this
- I made one recording at a higher sample rate which showed a harmonic an octave above at a lower level
- There are solar panels on the property that were installed last June. I had considered that these might have something to do with it but my knowledge of solar panels is zero. I've been trying to dig through older recordings made in the same conditions before last June to rule these out if possible. More digging required at time of writing.
- There are some basic tech items around the house but nothing outrageous. Things I've considered are: wi-fi signal booster, Alexa, and a Phonak TV connector for hearing aids.

I have the opportunity to record elsewhere this afternoon. Hopefully this will allow me to rule out equipment issues.

I'll give the cupping technique a go later as well. Who doesn't love a bit of cupping?

Thanks again!

[RichardT]

I would start by turning off the electricity supply for a couple of minutes and seeing if the problem still happens.

[The Red Bladder]

Offline analysis shows the peak to be around 14969hz. The peak is around 12-15db above the rest of the self-noise in this range.

That is a typical symptom of a poor converter that is suffering from a timing or clocking problem. This also leads to harsh or even distorted sounds at harmonies of that frequency. I used to see that in early ProTools converters before they went HD.

I am not saying that is what is happening here and the above-mentioned causes may be the right answer, but my 30 Cents worth is to try a different and better AD box.

[Lostgallifreyan]

I don't know what the source of that noise is but there is a likely way to eliminate solar panels as a risk. To be efficient, they are used with MPPT charge control (Maximum Peak Power Tracking), which means that even in relatively steady illumination, they periodically scan for the best product of amps and volts to be efficient in transferring energy.

These scans can make easily detected interferences on all low RF bandwidths and go well into HF too, and if you try using a detuned radio and comparing a few test tunings at night, and again at day, you'll recognise their variable signals. If your 15 KHz noise is very steady, the odds are that nothing as variable as solar panel charge control is causing it.

Something you might try: record some of it, pitch bend it down an octave or two (complete with that harmonic and more if you can get them). Once you can hear it over a period of time, you may be able to deduce things your gear can't tell you.

I'll back the hand-cupping thing, but for a different reason than attenuation. To do it, you have to get close to the mic, so you'll disturb the nodes of cancellation and reinforcement in acoustics. It's how ultrasonic motion detectors work (but at around 38 KHz). Moving the mic would also reveal this. If nether method reveals it, then the source is elsewhere, possibly carried into wiring from other sources. Electromagnetic..

If it only affects the mic, and not a single-coil guitar, for example, then consider oscillation in a sensitive preamp. I have heard such things with a Marantz PMD-660 recorder powering Oktava MK-219 mics, when the thing was battery powered. The problems went away when using its mains adapter because the power source was decoupled enough to prevent oscillation feeding back through the supply.

[Stuart79]

Just to update on this: I've tried the same basic setup of mic>interface>laptop in a different building and there's no interference. That suggests that maybe there is some sort of interference In the place where I normally record.

Regarding converters: I tried three different audio interfaces in my initial tests. An Audient id44 (my usual interface), an Audient id4 and an old Focusrite 18i8. They all exhibited exactly the same interference. I think it's unlikely all three would exhibit the same converter problem. As much as I'd like to be able to justify the purchase of a fancier interface...

[Lostgallifreyan]

Depending on timing and duration, consider RF sources on mains, because you may be hearing a demodulated result of a much higher frequency source. Rectification in a dirty contact might cause this demodulation.

A lot of people use LED and CFL lighting, plasmascreen TV's, things with patterns of field strength you might look for with a radio, to detect if that might be relevant. Things that put digital signals on mains wiring are nasty, and might cause a 15 KHz modulation to end up in your audio wiring. Switchmode PSU's too..

The one thing that might work best is to look at your grounding. It's the only certain thing I can think of that might fix this, if you have ruled out acoustic sources.

[Stuart79]

I would start by turning off the electricity supply for a couple of minutes and seeing if the problem still happens.

Do you mean to turn off the power and record with the power off? Or just turn it off for a couple of minutes before switching it back on to see if whatever the problem is resets itself?

[Wonks]

Run the system with as much power off as you can, ideally all. If no interference, then it’s something electrical in the house. Though I’m not sure how your solar panels connect in.

It’s then a question of turning off items until the interference disappears.

[RichardT]

I would start by turning off the electricity supply for a couple of minutes and seeing if the problem still happens.

Do you mean to turn off the power and record with the power off? Or just turn it off for a couple of minutes before switching it back on to see if whatever the problem is resets itself?

The first one - run your laptops off battery and use the bus-powered interface.

[Hugh Robjohns]

My inclination is RF or powering interference, rather than anything acoustic. I doubt its a converter issue.

The jdi transformer bring the level down suggest RF getting in the cables, to me.

Turning off everything electrical might help to track down the culprit...

[Stuart79]

Pretty sure it's the solar panels causing my issue. The noise is present in all audio recordings made in this location after their installation. Recordings made prior to this date are blissfully clean.

Is there an easy way to overcome this? Would double shielded or star quad cable be of any use in reducing the interference?

[Wonks]

Unlikely, it’s probably on the mains.

[Stuart79]

Unlikely, it’s probably on the mains.

Even though I'm getting the interference when no gear is plugged into anything?

[Folderol]

What mic are you using, and how is it connected. Is it balanced or unbalanced?
If it's a passive one, ideally the cable should be a twisted pair inside a screen.

Hmm, you just beat me with new info.
Are you saying you're getting it with no mic plugged in? If so, does plugging in a shorting link kill the interference?

[Stuart79]

What mic are you using, and how is it connected. Is it balanced or unbalanced?
If it's a passive one, ideally the cable should be a twisted pair inside a screen.

Hmm, you just beat me with new info.
Are you saying you're getting it with no mic plugged in? If so, does plugging in a shorting link kill the interference?

I meant when no power supplies are plugged in to the mains.

The interference gets into any recording: mic or line. I have tried all of the microphones I have at my disposal. A Sennheiser MD421, an AEA KU5A (phantom powered), a Telefunken TF51 (valve with its own power supply). As mentioned in my original post, I've also taken an unbalanced line out of one of my synths.

[Wonks]

The interference may well be airborne straight from the inverter, or it could be sticking into the mains and the house wiring is acting as an aerial for it.

[Lostgallifreyan]

Did you try what I suggested?

Record it, slow it down, listen to it. Solar panel systems aren't likely to generate that frequency directly in a strength that would get through any normal shield, so there's almost certainly RF involvement, along with demodulation, probably from rectification in a dirty contact. When I mean 'dirty', I mean as little as an atom-thick oxide layer.

Tracking this down could be HARD, so start with simple means. If the recorded sound has some pattern to it, try walking around with a small portable radio tuned off station, in as many bands as you can use. Small 'Tecsun' portables are great for this, especially those with little plugin ferrite antennas. They have many bands, and are amazing for tracking localised emission sources. RF engineers can get more mileage out of these, than far more exotic gear. If you hear anything with the same pattern of timing, pulses, warbles, etc, as in the 15 KHz signal, you stand a chance of tracking to source.

You might get an answer in less than half an hour if you try this.

If the source turns out to be external to your space, then cleaning contacts and fixing grounding problems is what you need to do. If you ever did anything to 'lift' a ground, this is definitely true. Finding new sources may just be revealing problems you already had, and didn't know. That sounds bad, but the new source is actually good, it's a clue, so you need to follow it or you'll be guessing forever.

[Lostgallifreyan]

Regarding converters: I tried three different audio interfaces in my initial tests. An Audient id44 (my usual interface), an Audient id4 and an old Focusrite 18i8. They all exhibited exactly the same interference. I think it's unlikely all three would exhibit the same converter problem.

Agreed, but did you try different sample rates? If you do, and the noise pitch changes, it might be the product of aliasing. The outcome may not be diagnostically useful, but it's easy, and worth trying because if it does change you can calculate an original frequency.

[Stuart79]

Regarding converters: I tried three different audio interfaces in my initial tests. An Audient id44 (my usual interface), an Audient id4 and an old Focusrite 18i8. They all exhibited exactly the same interference. I think it's unlikely all three would exhibit the same converter problem.

Agreed, but did you try different sample rates? If you do, and the noise pitch changes, it might be the product of aliasing. The outcome may not be diagnostically useful, but it's easy to try because if it does change you can calculate an original frequency.

Yes, and the frequency of interference remained the same.

[Lostgallifreyan]

Yes, and the frequency of interference remained the same.

Ok, good, that narrows it down. Try the listening test, try to see if an RF source matches its character.. That test could narrow it down a LOT.

[Stuart79]

Yes, and the frequency of interference remained the same.

Ok, good, that narrows it down. Try the listening test, try to see if an RF source matches its character.. That test could narrow it down a LOT.

I don't have immediate access to the type of radio you describe. I'm going to try switching power off at the mains one thing at a time to try and isolate the problem.

[Lostgallifreyan]

I don't have immediate access to.the type of radio you describe. I'm going to try switching power off at the mains one thing at a time to try and isolate the problem.

Unpowering specific stuff is well worth a go, though might take a while. Studios might have loads of things.. I do it like that anyway, but any small AM/LW radio might give you a shot at saving time if you wave it around any likely suspects.

One other thing: the solar install.. whatever its method, it almost certainly varies in emissions between night and day. The difference should be literally as clear as... Is it?

When I have tough interference problems, figuring out times of day when it changes can give huge clues.