RFI/EMI mystery

[rubinomusic]

Hi there. I'm in Los Angeles and I have some noise in my recording chain and I'm wondering if anyone can help!

PROBLEM: Steady whining noise in recording chain, eq analyzer shows peaks at exactly 4k, 6k, 8k, and 10k. Also occasional static registering around 300-500 hz.

LOCATION: On 2nd floor of 2 story building with parking garage in dense section of LA. Unit has dedicated circuit panel.

TESTING:
1) I turned off all electricity at the breakers including lighting and HVAC except one circuit with two receptacles with nothing plugged in except preamp. Plugged in condenser mic with Mogami cable to Presonus TubePre. Plugged headphones into output of preamp and noise was still there.

2) Tried two different condenser mics (Blue Mouse and Neumann TLM-103), three cables, and two preamps (though both preamps were of cheap quality). Changing only one variable at a time of course. Mogami cable reduced noise compared to Hosa but still there.

3) Touching the mic or moving it a few inches in different directions reduces the noise.

4) Different receptacles outside my unit but still on 2nd floor show same problem. Repeated experiment on 1st floor and noise reduced quite a bit. Tried again in parking garage: completely clean! Tried again in different building: completely clean!

5) Tried dynamic mic - mostly gone. Tried preamp with phantom power on and cable plugged in but no mic attached - greatly reduced. Nothing surprising here probably.

Is it possible there's something on my floor or on the roof causing a ton of interference? Any suggestions? (other than moving...)

Thanks as always for everyone's amazing help on this board,

[Soundman2020]

It sounds like you have a problem in your power feed somewhere. Is there any chance you can get hold of a professional power line analyzer, which should be able to help you trace where this is coming from. You might need the help of an electrician. You'll have to start at your own distribution panel, and work backwards through the rest of the building until you find the source. You say you have a dedicated panel, but that's not strictly true: at some point in the building, your power comes from the same cables as someone else's: that's where I'd be looking.

Also, you might try to find out if something in the building has changed: I'm assuming that you didn't have this noise before, and now you suddenly have it, so the question to ask is: what changed? Did a new tenant move in somewhere recently? Did someone install new equipment, or "repair" old equipment? Did you change something yourself, in your own studio?

I've had to troubleshoot similar problems for some of my clients, and believe me, it often isn't easy to trace this kind of thing, so I wish you luck!


- Stuart -

[Speedskater]

Any chance that you could do a recording test with a battery powered system?

[BriHar]

Do you have a hospital in the immediate vicinity? Some of the equipment they use can cause such whining.
The fact that it disappears in the garage infers that it is most likely airborn i.e. RFI. The other building you checked is probably further away from the source, or sheilded.

[Speedskater]

Jim Brown is the AES Committee Chair on EMI/RFI interference and a HAM (amateur radio operator). Several of his 50 some papers touch on interference tracing, some from a HAM point of view and some from an audio engineer viewpoint.

http://www.audiosystemsgroup.com/publish.htm

[rubinomusic]

Brilliant idea on trying with a battery system. I think I can use my laptop for that and determine weather it's power or interference (my guess). Then I'll look into those research papers to see if I can track it down. There's no hospital nearby so maybe it's something else. I'll take a look on the roof. Could it be something like a direct tv dish?

You guys are so helpful. So much appreciated!

[Speedskater]

A small UPS could power a simple mic pre-amp or mixer that has phantom voltage and can run headphones.

What are the nearby building like?

[rubinomusic]

So I tracked down the source of the noise! I powered up my laptop, threw it in a bag with my mbox 2 which runs on usb and has phantom power, hooked in a condenser mic and a pair of phones. Instant mobile RFI tracking system! I walked around the building with it like a ghostbuster (especially since I was carrying a weird looking Blue mic I got lots of strange looks). I came to a utility closet on the first floor just below me and my ghost busting equipment went berserk (the noise got reallllly loud). I opened the door and got slimed. Well, no, but I discovered a bunch of networking equipment. Wireless modem, cat5 routers, and hard drives. This closet is also where the phone panel is. There were no other visible electronics in here (though who knows what's in the walls) but I'd bet a lot that it's that networking equipment, right? I will knock on my neighbors door tomorrow and see if they'll let me turn it off for a few and see if the noise goes away. My big question: What do I do about it? Is there some way to isolate the RF emissions from the equipment? Would it help if my vocalist wore a pointy aluminum foil hat?

[BriHar]

You may find that legally, the onus falls on your neighbours to suppress such emissions, but you'll have to get a qualified technician to do the tests properly and verify whether the emissions exceed allowed tolerances.

[Soundman2020]

It's great that you tracked down the source, but you still don't know for sure if the interference is only airborne RFI, or if you are also getting it through your power lines. That's why I suggested getting a professional in here, with a power line analyzer. It might only be airborne, but you won't know for sure until you test.

Like Brian said, it might be the legal responsibility of whoever installed that new stuff to fix the emissions issue, but that might not be so easy (or cheap) to prove.

If it is also on your power feed, then power line filters might be effective for that (note the choice of words: "might"!). But that won't solve the airborne problem.

Anyway, assuming that it is only airborne (not on the power feed as well), and that it won't be turned off any time soon, how do you treat it? There's not many options: one is to put ferrite beads on all your audio cables. Not a small task, not cheap, and not necessarily 100% effective. The "major overkill" option is to build a Faraday cage around your studio, and ground it. There might be other options too, but those are the two that come to mind.

- Stuart -