How to track the source of high-pitched audio interference?
April 11, 2012 7:30 AM   Subscribe

I have high-pitched ringing in my the recording set-up of my new home voice recording studio (it seems to have a pulse, not steady, and occaionally disappears). Where should I start trying to get rid of it?

I'm using a brand new Mac Powerbook (OSX Lion), Mbox 3 mini (runs to computer via USB A/B), balanced XLR cable. Have checked mic and cables at a nearby studio and they seemed fine. Ringing resistant to the following:
1) unplugging computer from power
2) turning off ProTools, etc.
3) removing firewire drive
4) replacing the XLR cable with different/shorter one
5) turning off all lights in house (includes a dozen dimmers)
6) unplugging house WiFi, cordless phones, small stereo in same room
7) turning off furnace, ceiling fan downstairs, etc.
Strangely, if I hold my hand/arm a few inches above the mic, the ringing goes away, which makes me think it's more "in the air" than "in the wires," but what do I know! (With a prior cable, touching the end where it joined the mic stopped the ringing, but this newer cable isn't plain metal right there.)

So, should I start by trying to upgrade all my cables to more shielding? Add power conditioning of some sort to the circuit? Get an electrician to come and look for poor wiring? (or could I walk around my house with a multimeter and somehow check for current leakage in outlets and wall switches? how??) Turn off banks of fuses and see if anything drives it out? (then what??) The n-space for investigation seems huge, so I'd love to get some advice on where to start and/or how to block RFI.

The only other information I have is that we get poor radio reception in this room as well, but don't know why. (The furnace chimney may run behind the wall with our stereo against it.) Live in an urban area, room is on 4th floor of our house, attached to another unit on one side... Help!
posted by acm to Technology (21 answers total) 1 user marked this as a favorite
What does your physical space look like? What is between your mic and the walls? What are the walls made of?
posted by Jairus at 7:39 AM on April 11, 2012

I wouldn't worry about upgrading cables if you've already tried several. Unless you are using the absolute cheapest XLR cables you could find, pretty much all cables are shielded fairly well.

Does this happen in other rooms in the house, or just that one room? Have you tried it on the bottom floor?

Do you know your neighbors well enough to ask to plug in your stuff in their place and try it? I'm just trying to figure out ways to eliminate possible problems.

Have you tried a different mic in your house? If putting your hand several inches above the mic makes a difference, it may be an issue with the mic.

The fact that you were able to remove the noise by touching the old cable leads me to believe it is a grounding issue, but I'm not sure what your best approach would be to fix that, since you've already said that you tried it without the power plugged into your computer, which is always my first suggestion when troubleshooting a laptop.
posted by markblasco at 7:42 AM on April 11, 2012

Can you upload a sample of the sound?
posted by tomierna at 7:47 AM on April 11, 2012

Is there any chance that what you are hearing is not electrical but good old feedback? Like, what are you hearing the ringing in? The reason I ask is the high pitch, plus the fact that if you very, very slightly change the audio environment (holding your hand, damping the mic's resonance) it goes away.

It is possible to get feedback from headphone leakage into a microphone if the gain structure favors it. This doesn't sound like a shielding problem. It COULD be a leakage problem but the things that fix it make it sound like it's not.

Without more information, the first thing I would do would be eliminating feedback as a source. What is the ringing coming from? Assuming it's headphones when you are in front of your mic, try muting the headphones, or muting the mic, or turning them both on but acoustically isolating them, and see if the ringing is controllable that way.

If the problem *is* feedback, you are going to have to try different headphones, use less gain on one or both ends, or fiddle with your positioning relative to the mic's pickup pattern.
posted by jeb at 7:47 AM on April 11, 2012

Response by poster: Physical space: bedroom, in a corner, regular drywall walls, house <5>
Haven't tried other rooms, because this is the only space that is a long-term solution, but I could definitely move everything for a test. Have considered borrowing a mic to test that -- a good first thing to do?

Feedback: I'm using professional cup-style headphones; don't really know how to take them out of the loop and still detect ringing, although I guess I could record a stretch of audio at maximum gain with headphones unplugged and then listen to it. Didn't try that. However, it's not input-dependent, as I get it when sitting perfectly still with nothing else being recorded.

I don't know what to make of the fact that this is intermittant -- that is, I have made at least two recordings that had no ringing (out of a dozen or so sessions there), which somehow makes me doubt that it's fundamental to my own set-up. But if something like my positioning relative to the mic might affect things, then I wouldn't have necessarily caught the pattern.

Could upload a sample later this evening -- where would I put it? (sorry for continued ignorance...) It sounds a little like a dot-matrix printer busily at work down the hall...
posted by acm at 8:11 AM on April 11, 2012

Response by poster: (sorry, that was "house is less than 50 years old")
posted by acm at 8:11 AM on April 11, 2012

Try recording a short clip in protools. When you play it back, is the sound still there or does it disappear? This will test to see of the noise is coming from the mic (or cable, preamp, mbox input) or coming from the headphones (or headphone amp, mbox output). Also try recording and playback with different software, QuickTime would work.

Is your mbox sitting on top of the laptop? Near it or near other electronics? Try putting it inside a thick metal container (maybe a heavy pot or frying pan) to block any RF that may be entering it. The internals should be well shielded, but maybe there is a break or cold solder.

Can you describe the ringing further? Ideally, can you determine it's frequency? The two most common sources of RFI are 60hz hum from wall power and GSM buzz from cell phones, but I've never heard those described as "ringing."

Higher frequency noises are often caused by switching power supplies that share a common ground with the audio source. You could try a cheap "ground lift adapter" which is just an XLR barrel connector that drops the ground connection.

I would also suggest trying a third XLR. It wouldn't be crazy to have two cables with a bad shield. You can also use a multimeter to test for continuity between all three pins and the metal on the connector.

Since everything in the setup is portable, what happens if you take the whole rig and sit in your car or outside somewhere?

I agree that it might be plain ol' feedback. Is your mic cranked? Headphones turned up to 11?

Finally: what is the make and model of that microphone? Condenser? Dynamic? Does is require phantom power? If not, are you pumping it full of phantom anyway?
posted by stephennelson at 8:11 AM on April 11, 2012

Bingo. "dot-matrix printer busily at work down the hall..." That's a cell phone. Turn it off or airplane mode it, or at least take it out of your pocket near the mic.
posted by stephennelson at 8:15 AM on April 11, 2012 [1 favorite]

This may be a dumb question, but where are your headphones plugged in-- your computer or the Mbox? This sounds an awful lot like an artifact I get from my Mac Pro's sound card, which sucks (gotta get a headphone amp). The "dot matrix printer" comparison in particular made me think of it.
posted by supercres at 8:15 AM on April 11, 2012

However, it's not input-dependent, as I get it when sitting perfectly still with nothing else being recorded.

When you say nothing *else*, do you mean something is being recorded? Because if you hear it when nothing is recording from any input, then it's definitely not feedback.

The dot matrix printer noise...makes me think of noise leaking out of a laptop into a cheap D/A converter, like an onboard headphone jack, but that's unlikely to be the problem if you are using the mbox.

Are you using the Mbox for both input and output, or are you using the Mbox only for input and the computer's headphone jack for output by any chance?

You can throw a sample on soundcloud if you manage to record one.
posted by jeb at 8:17 AM on April 11, 2012

For this sample, could be better. Free, no account, and it will disappear in 7 days. I want to hear this too.
posted by caclwmr4 at 8:22 AM on April 11, 2012

Jinx, jeb. My money is on D/A issues or the cell phone. If it's one of these, the easy test is... how loud is it? If it's just barely audible (and you have headphones plugged into your laptop), and always the same volume, it's the sound card. If it's louder -- sometimes painfully loud, in my experience -- and not always the exact same volume, it could be a cell phone.
posted by supercres at 8:32 AM on April 11, 2012

Response by poster: Sound definitely on the recording -- spent quite some time with a local studio trying to subtract it, but it has some harmonics that make it take several notches to remove even partially (and too close to voice to remove completely). You do have to crank up the system to hear it, but it's real. Will happily excise several chewy samples and post them this afternoon.

Mbox is on the same table as laptop, but a couple of feet away. Have tried moving equipment around, repositioning cables, blah blah... Am on 3rd XLR cable, still ringing. Will try a big pan! Sweetwater will look at Mbox, but hate to be without it if other tests might diagnose...

Definitely main frequency is knowable, I just can't recall (have emailed my studio guy) -- I want to say B or D (above middle C), so let's say 400-600 kHz.

Mic is Rode NT-1A (cardioid condenser), requires phantom power. I've done a couple of tests powering off the phantom (letting mic go silent); once I got a big increase in the ringing, but yesterday the ringing went away, sigh. (If I unplug the XLR from the Mbox, everything goes silent.)

Headphones plugged into MBox, which is set up to be my combined I/O for ProTools.

I don't carry a cell phone, but a simple luddite one has been charging in my spouse's nightstand, and I didn't think to unplug or move that, so that move to top of my test list. Appreciate all the good ideas!!!
posted by acm at 8:39 AM on April 11, 2012

Response by poster: crap, spouse confirms that cell phone was out of room during testing, and probably not on even downstairs. so that's out...
posted by acm at 8:46 AM on April 11, 2012

Cell phone issues will be intermittent chirping more than anything else, so if you are getting a relatively consistent sound, that's not likely to be it.

You should definitely try moving your setup to different rooms, as well as trying a different mic. If it works in different rooms than there is something about the room you are in which is causing it (assuming you share a wall with a neighbor, they may have something sending out a signal there). The only way to protect about that would be shielding. Maybe a tin foil hat for your gear (which will also keep away the alien mind control rays) ;)

If you still get the noise with the phantom power turned off to your mic (and having it off for several minutes to make absolutely sure the mic isn't doing anything), than I'm a bit stumped, since that would rule out the electronics in the mic, which would be my best guess at this point other than some strange interference.

So, maybe try these things (other than what I already suggested):

Plug the mic into both inputs on the mbox, is there a difference between the two?
Try moving the mic around while listening, including turning it around and upside down.
Take a piece of tin foil and drape it over the mic.
Connect all of your XLR cables together to get a really long cable, and plug the mic into that. Does it make a difference?
If you can, turn off all of the breakers in the house except the room you are in. If it is something in your house (such as the fridge), that will help to find out.
Try a ground lift on the mic, although I'd hate for you to go out and buy this only to find out that it doesn't solve the problem, since this is something you likely won't ever need.
posted by markblasco at 9:03 AM on April 11, 2012

Response by poster: Just found a sound sample that I made for a Sweetwater technician when I started this quest. You will have to crank your sound to hear the ringing, but it's there -- be forewarned that voice comes in about halfway through (at not necessarily painful levels). I uploaded it to wikisend here. Let me know if hearing it changes your diagnosis or suggestions at all!
posted by acm at 9:47 AM on April 11, 2012

Okay, so if you turn off the phantom power, you still get the noise? But you unplug the XLR from the Mbox and it goes quiet?

Turn off the phantom power, see if you're still getting the noise, and then unplug the xlr from the mic. if you're still getting the noise, then unplug it from the Mbox.
posted by dubold at 9:51 AM on April 11, 2012

That sounds like digital interference, but not GPRS cellphone dits.

I'd say better, shorter XLR cable may help, but you might try some sort of outbound phantom power as opposed to using the MBox builtin.
posted by tomierna at 11:22 AM on April 11, 2012

Best answer: Agreed, not a cell phone. Doesn't sound like something that is being picked up along the XLR either. If you are in a rush, it's worth trying what tonierna suggested and getting an external phantom power supply (even an inexpensive Mackie mixer will do the trick). Feed the mbox input with a line level signal from the mixer, letting all the amplification happen outside. You will not be getting rid of the source of the buzz, but you will essentially lower it by about 40db, which will likely bury it in the noise floor.

If time isn't an issue, I'd send in the mbox.
posted by stephennelson at 11:37 AM on April 11, 2012

Is everything plugged into the same electrical circuit? Try running from a power board. In the pre digital days, we used to occasionally have issues with ground loops that were fixed by putting everything on the same circuit.
posted by dantodd at 3:11 PM on April 11, 2012

Response by poster: Aren't ground loops usually low frequency? I seem to see a lot of discussion of how to prevent them, but it doesn't seem like that's what I have. (Did you listen to the sample, dantodd?)

I have a heap of ideas from you guys to try out, and I think I'll send the Mbox for diagnosis. In all honesty, I don't really understand where I'd put a Mackie mixer into the setup, but if all other tests fail (including the tinfoil hat! hah), that may be the way to go -- that's a lot of subtraction! Will post a new thread if/when I make any progress on this one.
posted by acm at 7:08 PM on April 11, 2012

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