High frequency spikes during location recording - what are they?

[bigster]

I just got back from a 4 day location recording at a small modern concert hall in Birmingham UK where I experienced some strange electrical glitches, and I'm trying to understand what they were and why they occurred. I'd be extremely grateful for any insights anyone may have.

My setup is one I have used for probably a decade now, and which has always been rock solid and trouble-free. It's a simple two-device MOTU AVB combo: a Stage B-16 in the live room, 624 in the control room (just for handling monitoring control, talkback etc), and a 50m Cat-6 ethernet cable between them. For this session a full 16 mics were plugged into the Stage B-16 - Schoeps CMC62, Sennheiser MKH 8040 pair + an MKH 40/30 combo, 3x Austrian Audio OC818s, various Line Audio CM3s and CM4s. Also, there were line outputs to two Genelec 8010s for talkback. All known good, as it were, as were my XLRs, that in fact I'd tested only about a week previously. No new elements in the setup, which has performed flawlessly on about 5 classical location sessions already this year. Recording to an M1 MacBook Pro and the Reaper-based ReaClassical DAW, 96kHz 24 bit. The whole lot was powered from a single mains outlet in a wall box in the live room that also had some (unused) audio and other tie-lines, though in trying to troubleshoot I also later switched to a conventional mains outlet in the same room, and that did not change anything.

Essentially, here's what I saw on a spectrum analyser on any of the mic channels, when phantom power was enabled:

A narrow spike at exactly 20kHz, and a louder one at 40kHz. Identical on every channel. The 20kHz spike was static in level, while the 40kHz one pulsed with about 10dB variance over about 5 seconds. Only there when phantom was on - if I disabled it on a channel I got a typical line-level noise floor about 110dB down, and no spikes - so it seems the unwanted signal was coming in before the AD conversion stage. There was no change if had only one channel of phantom on, or all 16, btw.

Something else weird: one of my Sennheiser 8040s was generating another tone, high in the audible spectrum. I'm afraid there wasn't time to spot what that frequency was - the session was underway and I had no option but to swap out to another pair. I did have time to change cable, and to unscrew and reseat the capsule/preamp, but this didn't make any difference.

Today, back home, I've rigged everything again, and of course it is all as clean as quiet as could be, here. Not even a hint of the spikes in the spectrum, and both 8040s sounding identical and completely quiet, as they've always been.

Luckily, in Birmingham the spikes were above the audible range (and in any case are easy to notch out) and didn't affect the capture at all - the recording has come out sounding very nice indeed. That's a huge relief... But I'd be fascinated to know what they were and how I might deal with something similar happening in future. Other electric gear in the same concert hall was a large complement of ceiling-mounted LED lighting (a prime candidate maybe?), and a digital lighting desk. There was a powered-up SSL digital mixer in a gallery too, but no powered PA speakers. I couldn't see any powered-up radio mic stuff, or anything else similar. I briefly tested both mains outlets I used with a simple plug-in tester - that reported all OK, earth present, and a mains voltage of 247V. I did all the other obvious troubleshoots I could, in the very limited time that was available - rebooting computer and interfaces, trying both the Stage B-16 and 624 as the AVB master clock, unplugging groups of mics - nothing changed.

I'm still stumped as to where in the signal chain the interference was originating. It surely wasn't acoustic, as none of the mics except for the 8040s have a response that goes that high. Was it every single mic in the room picking up RF interference identically? Seems unlikely. Was it a mains/earth issue with the MOTU Stage B-16 and its phantom power? In which case, why only in this venue, and not the many dozens of others in which I've used it? Maybe the mains was carrying some sort of data signal (for lighting?), but again, how did this end up bleeding into my preamps?

Any suggestions or explanations very gratefully received!

[SafeandSound Mastering]

Maybe a rodent repellent device, possibly. Lights, solar power are other possibilities.

It used the be CRT VDU's that could cause issues like these in home studios but out and about and in 2025 probably not.

[Hugh Robjohns]

Stage lighting and/or motion motors would be a strong possibility.

Since you mention it going away with phantom then DC-DC converter problem in the preamp generating the +48V is another possibility.

And lastly, tuning issue in the MKH mics — it's a known problem with the MKH design. If the modulator/demodulator circuitry drifts you can get whistles.

And finally, something in the location intermodulating with something in your system to produce the 20/40k aliases...

Also, check for SMPS power supplies near the mics or xlr cables.

But at least those two unwanted signals are (a) inaudible to everyone (-66dB at 20k) and (b) easy to notch out.

[supabee]

Another possible culprit : was any hearing impaired device working in the room ? I got such strange audio artifacts once in a theater that got a system for audio prothesis systems enabled (I don't remember the name of the device)

[bigster]

Many thanks everyone. I think any or all of the above are distinct possibilities in that room. It was disconcerting and unexpected, but thank goodness survivable. Right, better crack on with the editing now…

[Stuart79]

Any idea if there were solar panels at the venue? I had trouble domestically with a an issue that looked much the same as your screenshot and it turned out to be solar power related.

[shufflebeat]

I’m not equipped or qualified to offer any wisdom but would just like to comment on the framing of the question, which looks like a great example of troubleshooting process.

Nice work.

[forumuser840717]

Just to check, when you say a spike are you referring to a very short duration click-like event or a very narrow bandwidth but relatively long term tone-like event?

If it's the latter, these have become increasingly common over the last 15 years or so as venues install systems which use continuous or sometimes intermittent ultrasonic tones and/or tone bursts to check the signal integrity/continuity on their PA and, particularly, emergency anouncement systems. They used to be in just a few of the larger halls with newer PA/safety systems but as places re-furb or install new hardware, they're cropping up with increasing regularity in public buildings all over the place! Systems using ultrasonic monitoring have been in use for decades but I've only really noticed them becoming visible on recording systems in this century.

There are also fire detection and intruder/space monitoring systems which emit ultrasonic tones - normally around 20 / 22 /24 kHz but they do vary.

There used to be a hearing-impaired relay system that used something like 18.9kHz as a carrier tone which was installed in at least two UK halls but that seems to have gone away at some point in the last 10 years as I didn't notice it the last times I worked in them. It's quite possible that there are similar systems using other high frequency carriers but the ones I've noticed more recently seem to use some kind of IR emitters.

There's one venue I work in occasionally where the tones have been badly thought out and they fight with each other and produce beat notes and weird harmonics. Thankfully, they're normally inaudible and easily removed but they make for interesting displays on spectrum analysers and spectrograms.

(There was a paper on it about 10-ish years ago from the IoA. I probably still have a hard copy of it somewhere but it's likely to be online if one searches.
EDIT: Can't find that one online but here's a related one from JASA

EDIT EDIT: The IoA paper is apparently here but the link doesn't work for me.)

If it's the former, I've had them quite often, in location recordings, usually resulting from badly suppressed electric motors of some sort, ranging from lift (elevator) motors (which could be well away from the recording area yet still spraying very short, high energy clicks all over everything) through to fridges and the like hiding in dressing rooms or bars, and even bell ringing motors chiming the time in little country churches!

[Hugh Robjohns]

It was constant tones at around 20kHz and (stronger) 40kHz — as shown in his RSA display:



Your IoA paper link works for me.

[forumuser840717]

Ah, ok. Then I'd suspect that the 20kHz is some kind of active monitoring/surveillance safety system as they're pretty common nowadays.

40kHz seems rather high for those systems but maybe it's one of the fire/intruder detection systems. Impressive if the mics are all resolving 40kHz at though, so I'd guess at maybe some other cause. Given that it's a pulsed tone rather than continuous I guess it's probably not just a harmonic/resultant of the continuous 20kHz tone unless it's part of the same detection system and/or is some kind of interference product that's slowly phasing in and out but that seems a tad unliklely. I do know one venue that has a 21.something kHz tone that runs for seven seconds followed by three seconds off and also a much higher one (didn't bother looking exactly how high as I just rolled them both off on the mix!) that started about 2 seconds after the 21kHz tone and ran for six seconds. From past work there, I know that the 21kHz tone was part of their fire alarm/anouncement system. Never asked about the higher one that appeared more recently.

The IoA link works for me now too - I was connecting via a mobile hotspot that kept returning a connection error but it's fine from home.

[Hugh Robjohns]

I didn't know much about the ultrasonic continuity detection in PAVA systems, so I did some hoogling.

It seems there is a format which sends 20kHz out and an end-of-line unit connected at the end of a chain of 100V line speakers doubles that tone to 40kHz and sends it back along the line to the amp where it is detected if all is well.

Apparently its a very reliable system and easier to implement than most other monitoring systems.

So it might be down to that, as that could also explain why the 40k is stronger than the 20k...

[bigster]

It seems there is a format which sends 20kHz out and an end-of-line unit connected at the end of a chain of 100V line speakers doubles that tone to 40kHz and sends it back along the line to the amp where it is detected if all is well.

So it might be down to that, as that could also explain why the 40k is stronger than the 20k...

Well I never. Fascinating. Good googling! I did try searching but nothing came up for me. The building has a system along those lines, for sure.