Humming noise from electrical appliances

TYU wrote on 10/14/2004, 1:25 AM
I am doing narration to an instruction video.
I do all voice recording using simple microphone. Recently I moved to another office. Everything is the same: the computer, chair desk, furnishing etc.

Now the microphone picks up a strong humming sound that I can't hear myself. I narrowed the problem some arcane electricity problem. The humming sound comes when any power cord is plugged in the wall outlet. I tried using a laptop computer running on batteries and recording with my mike. All is fine if nothing is plugged in. If I plug in a, say, printer which is off and not connected to any computer the humming sound is picked up by the microphone.

Mind you the humming sound is not heard - only the microphone picks it up.

The humming problem was not in my old office room. It had all the same computer peripherals plugged on with no problems with recording.

Any ideas how to fix it?


farss wrote on 10/14/2004, 2:31 AM
Sounds like an earthing problem. Have you got a multimeter?

If so check the AC voltage between some of the metal parts of your gear and say a water pipe that's buried in the ground. If it's over a few volts I'd suggest calling an electrician.

The only other possibility is that the mic is picking up a strong CA magnetic field. Usually due to nearby transformer. Anything like that nearby, maybe under / over the room? If thats the cause the effect should change over time if it's a utility transformer, late at night when the load is less you should get less hum.

TYU wrote on 10/14/2004, 4:36 AM
Thanks for a quick reply. None of the wall connectors are earthed.

There is a room with a lot of transformer below. It makes a humming noise which is not heard upstairs, however. Anyway when using the laptop with nothing else plugged in in the room the humming sound won't get picked up with the mike. Only when something is plugged in (albeit on turned off).

I have a multimeter. I use it to check battery charge. I connected the red connector to heater pipe and the black connector to a metal part of the printer and got nothing. A regular battery (1.5V) shows 1.690 when the ends are checked similarly.

Should a better microphone work better and not pick the humming sound? The mike is just a run-of-the-mill computer mike which I've found to be sufficient.
farss wrote on 10/14/2004, 6:10 AM
Firstly you need to switch the multimeter to AC Volts!
However if there's lots of big transformers underneath they could be the culprits, I've had them make CRTs go very wobbly.
If it's a dynamic mic they could easily pickup external magnetic fields although most cheap 'computer' mics are electret condensor mics.
Either way a better mic would be a good step but it could possibly make your problem worse, really hard to know. A good sound card with balanced mic input and a matching mic is certainly a step in the irght direction and not just to fix your hum problem either!
If you're working with a lot of gear I'd be getting pretty nervous about not having things earthed, just from a safety point of view, all that stands between you and death is the insulation in the numerous power supplies. Every piece of gear is connected by conductors so a failure in any piece of gear makes everything live.

Bill Ravens wrote on 10/14/2004, 6:39 AM
This is a common problem experienced by most sound engineers. It's called 60 Hz hum. The 60 cycle AC, particularly if you're near a transformer, is picked up by the microphone acting as a receiver antenna. Many times, this is caused by an incomplete ground, as was mentioned above. Ironically, the same thing can happen if you have too many grounds, resulting in a ground loop effect. Grounding of electrical equipment is truly a black art. You might want to try an electrical system isolator to isolate your mike from the wall socket. These isolators are available at most sound shops and run about $100. Check here...
TYU wrote on 10/14/2004, 7:12 AM
Thanks for all involved. I brought an extension cord from a nearby room (there it's earthed) and the problem went the way of the Dodo (ie. extinction in my case).

johnmeyer wrote on 10/14/2004, 8:32 AM
If any of the equipment has two prong connectors, you can sometimes fix the problem by reverseing the plug. Having said this, do NOT put a "cheater" plug on a three prong plug to turn it into a two-prong plug and then do the reverse. While this "trick" can actually eliminate the hum, it also has a tendency to eliminate the talent, via electrocution. Most people do not consider this a wise tradeoff. However, if the equipment started out with a two prong plug, and the plug is not "polarized" (with one prong bigger than the other) then you can safely try reversing the plug.