Glass table = hum? Why?

January 24, 2011 10:14 AM

When I set my microphone stand on a table I get a hum. How can I fix that without spending a lot of money or holding the microphone the whole time?

This is a follow up to this question:
http://ask.metafilter.com/171268/Help-me-rid-myself-of-this-annoying-hum

After hours and hours of troubleshooting I have discovered the source of the hum--my table. I don't know WHY, and as mentioned in that previous question, this is somewhat new behavior as I recorded like this for years without any hum.

But with my microphone stands sitting on the table, I get a hum. If I lift the stands into the air, there is no hum.

My setup is that I have two microphone stands, metal with rubber feet, standing about 8" high. At the top is your standard plastic mic clip. In the mic clip is my mic. Also attached to the Mic stand is a pop filter.

I set these on my desk, an office desk with a glass top.

I am guessing the noise to be vibration related, but I have turned off all other electronic devices on the table and still get the hum. Further, I have tried "buffering" the mic and the table with bubble wrap, carpet, and paper, and all three still produce the hum.

I have looked into tripod stands for the floor, but due to the set-up, to keep those out of the way would require a 4' or longer boom arm, and the one I tried had balance issues with the boom extended all the way out.

I'd prefer to not have to spend over $200 on mic stands to fix this problem, but not sure how else to fix it. Any ideas would be greatly, greatly appreciated.

posted by bodgy to technology (24 answers total)

Try standing the mic on a bath towel, folded over a few times so it's thick? That should isolate it from most of the vibration.
posted by DarkForest at 10:18 AM on January 24, 2011

Put a vibration-resistant pad under the feet of the table. It's the building that's vibrating. You need to isolate the table from the vibration.
posted by The World Famous at 10:20 AM on January 24, 2011

Oh, now i see you have already tried something like that...
posted by DarkForest at 10:20 AM on January 24, 2011

Use a noise-blocking stand and a long enough cable to distance the mic from the computer. Often times the vibrations from a computer's fans and drives will vibrate the computer desk and the surrounding area. If the microphone and its stand are on the computer's desk, the microphone often will pick up the vibrations and produce a noise on the audio track (often referred to as a "warble" sound: a soft, repeating hum). To help prevent this, use a ceiling-suspended microphone stand or a full-size floor stand that can have its height adjusted. If these (pricey) options are not available, an alternative is to support a desk stand using a sound-insulating lift, such as a flimsy cardboard shoe box or a number of newspapers. These things insulate the noise rather well, making it difficult for any vibration noises to flow through to the microphone. Almost any lift made of non-rigid, flexible material will do.

from http://wiki.audacityteam.org/wiki/Reducing_noise
posted by fozzie33 at 10:23 AM on January 24, 2011

What is the table made of?

Since I am an electrical engineering student and I run into problems with tables in labs all the time I would not rule out interference, does the hum sound like 60 Hz? Just thinking out loud here but even if a device is off on the table, the power line to it is still live. You could either try grounding the table (if its metal) or setting the microphone stand on something grounded.
posted by token-ring at 10:24 AM on January 24, 2011

Hang it from the ceiling.
posted by at the crossroads at 10:25 AM on January 24, 2011

Fozzie - I have googled but can't find anything about a "noise blocking" microphone stand... any ideas on that?

Token Ring - Not sure what 60 hz sounds like, but if you do you can hear the hum here: http://cilff.com/wtf.mp3 If you go about 2/3 into the track I really turn up the amp so the hum is more humm-y than hiss-y

Also, the table is made of glass with metal frame.

The microphone stand has rubber pads, so wouldn't that handle the ground?

And it's not the computer. I have removed the computer from the equation by turning off and removing all computer equipment from the table, and the hum comes right out of the mixing board to my ears. And if I raise the mic off the table just so it's not touching the sound goes away completely.

For the ceiling mount ideas, I hit Amazon but don't see ceiling mount microphones. Anyone have a link for that? (and again I'd prefer to NOT spend $200 or more on mic stands...)
posted by bodgy at 10:33 AM on January 24, 2011

Try a suspension mount for the mic.
posted by rhizome at 10:42 AM on January 24, 2011

Hmm, sounds higher-pitched than the usual 60Hz hum typically associated with ground issues.

Do you have monitors (speakers) near the setup? Is it possible that the glass table is acting as a sounding board and you're getting a sort of faint feedback loop? Does it get worse if the mic is closer to the table? Do you still get the hum if you have no loudspeakers but just listen with headphones?
posted by xedrik at 10:54 AM on January 24, 2011

http://www.amazon.com/Samson-SP01-Shockmount-Spider-Condenser/dp/B000LQLDM2/ref=sr_1_1?ie=UTF8&qid=1295895229&sr=8-1
posted by fozzie33 at 10:55 AM on January 24, 2011

Floor mic stands are like 30 bucks.
posted by Potomac Avenue at 10:56 AM on January 24, 2011

Where does the cable run? I'm imagining a glass table with a metal frame, and it's possible the metal frame is carrying some tiny amount of voltage, maybe induced by a power cable that's nearby. If the mic cable is laying across this metal frame when it's sitting on the desk, but not laying across it when you pick it up, that could cause the behavior you describe.

To my knowledge, steady hum like that can only come from either vibration or electrical interference. If a folded-up bath towel doesn't get rid of it, it's not vibration, so a shockmount won't help. Check the signal path for anything metal that could possibly have any current in it.
posted by echo target at 11:32 AM on January 24, 2011

I was all ready to call this 60 Hz but I ran the mp3 through a spectrum analyzer and the hum is at 89.6 Hz, so it's not line frequency and is most likely from hard drives or fans or something else mechanical.
posted by Rhomboid at 11:32 AM on January 24, 2011

Well, depending on what sort of audio processing you're doing, you could use a 89.6 Hz notch filter....
posted by schmod at 12:04 PM on January 24, 2011

The cable is fairly short and sits on the glass, not on any metal.

THe problem with the floor mic stands are that we are sitting and reading from computers when recording, so we need clear desk access. The optimal situation would be hanging the mics from a table mount, a boom, or the ceiling, but I'm still looking for some sub-$200 solution as the $25 boom mount I got tips over when extended to a 3' length with the mic and pop filter on it...

The Spider-Condenser I'm not sure how it would help. I've tried 3 brands of mics and they all have this problem...
posted by bodgy at 12:15 PM on January 24, 2011

does the hum sound like 60 Hz?
Depending on which country you live in, the hum could also be a 50Hz one, or a multiple of that.
posted by ddaavviidd at 12:28 PM on January 24, 2011

the $25 boom mount I got tips over when extended to a 3' length with the mic and pop filter on it

It probably wouldn't take much weight on the legs of the stand to counterbalance that, maybe a heavy bedspread or something like that.
posted by InfidelZombie at 12:33 PM on January 24, 2011

the $25 boom mount I got tips over when extended to a 3' length with the mic and pop filter on it

Throw a bag of sand on the base of the mic stand, should counterbalance it just fine.
posted by platinum at 12:54 PM on January 24, 2011

You could also try adding mass to the mic itself. They sell pairs of sand-filled strap-on exercise weights for like $10. If putting one of those on the mic causes any noticeable change then that would be a pretty good indicator that it's not EMI but mechanical. (You could even try just gripping the body of the mic firmly with your hand and see if the vibration changes.)
posted by Rhomboid at 1:12 PM on January 24, 2011

What happens to the hum when you grab the mic desk stand with your bare hand without picking it up off the table?
posted by exphysicist345 at 3:03 PM on January 24, 2011 [2 favorites]

Well, as a last resort, my [well known] engineer friend in Nashville recommends surrounding your entire system (literally, everything) with as many coat hangers and other metal objects in parallel to your main input lines as you can until the hum goes away. At a certain point, all that complex grounding interference will cancel itself out.
posted by digitalprimate at 4:59 PM on January 24, 2011

Dumb question: Could you be the impedance of the mic when you pick it up?

Try picking it up wearing gloves and see if the hum stops then.

If not, then you need to play around with the grounding of your mic and the board.
posted by Orb2069 at 5:57 PM on January 24, 2011

Coat hangers. Right.

The shockmount is to keep most vibrations away from the mic, and definitely dampen sympathetic hums and crap like that. Have you tried turning the channels up with the XLRs plugged in without microphones, testing only the cables?

Sounds like you really want lavaliers, though.
posted by rhizome at 12:34 AM on January 25, 2011

... changing the impedance, that is. Sorry.
posted by Orb2069 at 2:33 AM on January 25, 2011