Question Electronic Noise from Microphone

Oct 3, 2019
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I have solved this issue (somewhat) but would like someone to educate me on this matter if possible.

I have a Shure Sm7B setup to very close to my tabletop (suspended 1cm with boom arm) to record some ASMR for my Youtube channel. HOWEVER, I realized it picks up terrible buzzing noises because of my surge protector taped directly under my cheap 1.5 inch thick ikea table. As soon as I move my mic to the left, the noise deafens. ( need it to be in the middle cuz aesthetics)

(regardless of what is plugged in, when I turn off the power strip, the noise is completely gone so its 100% confirmed its coming from there)

Can someone explain what is going on? Is it possible I could do something cool like get some copper sheets in there and make the noise dissapear? Otherwise I am fine with destroying my perfect cable management for the sake of clean audio. But I would still like to know exactly what is going on, or if potentially my power strip is bad.
 
Feb 16, 2019
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Like any dynamic mic, you have a diaphragm, voice coil and magnet. Like a loudspeaker, this is also a transducer which converts mechanical pressure (sound waves) into electrical signals and vice versa. Plug your mic into the headphone out jack and it'll sound pretty good! Plug a full-range speaker into a mic input jack and it won't, but you can speak into it. The fact that it has a coil is why it can pick up signals we can't hear. Electromagnetic energy at certain frequencies can easily be picked up by the coil winding in your microphone, which has high sensitivity to small signals. The lines of the magnetic field pulse at the sine wave frequency of 60Hz (AC power), which comes across as a hum or buzz, 60 times per second. The proximity of the source of the field to the mic coil has everything to do with what you're experiencing, by moving your mic around. You're changing the phase (can't hear) and amplitude (energy level).

You're on the right track, by wanting to shield. But first, you must look at what you have plugged into the power strip. A simple power distribution strip will not emit a magnetic field. Electromagnetic interference (EMI) occurs basically for this reason: Any electrical current, flowing through a conductor, will produce a magnetic field. Wind a coil of wire, attach a battery and you have an electromagnet. If you have any power supply connected to anything, such as a PC peripheral that draws any electrical current, it will emit EMI. The more the current, the stronger the interference. Power supplies are transformers, which are notorious for EMI.

Try this:
Relocate the power strip, if possible. OR- one by one, relocate the power supplies (transformers) to see which one solves the problem....try the biggest one (highest current rating on the label) FIRST. If you have several, that's a great source for EMI and they must be relocated.

I have this app on my smart phone (Galaxy S8+) and it's pretty amazing: "Physics Toolbox Suite". It has a magnetometer! If you can use it, you can "see" where the EMI is coming from.
 
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