AC hum field and a dynamic mic

FAQ #3895 Updated June 28, 2011

Question:

I'm encountering what appears to be an induced AC hum in a dynamic microphone.  We've tried several makes and models with similar results.  The source of the hum seems to be an AC distribution panel about 6 feet from our Voice Over booth, through the wall and across the hall.  I've gotten the result with not only our main mixer, but with a battery operated field recorder.  I've tried replacing the mic with a 150 ohm load and the hum disappears.  I've not tried a low pass filter as the hum seems centered around 60 hz. The hum does not manifest in condenser mics, which seems to confirm the source.

Is there a way to shield the panel from the booth, or is some other solution in order?

Answer:

You are correct.   A dynamic mic has a coil of wire as the voice coil, and this will indeed create an audible hum.    Shielding the panel would be difficult, but could be done using Mu-Metal - a type of metal used to shield transformers.

http://en.wikipedia.org/wiki/Mu-metal

Also consider a dynamic mic with an internal hum bucking coil:   Shure SM7B; Shure SM63
 

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