Can't get rid of this humming noise in recordings

Question

I have a brand new Blue Yeti USB microphone on a shock mount attached to a boom - no vibration noise possibilities in this setup. I have tested this in a dead silent enclosed room using a Macbook Pro with the fans on the lowest RPM (basically off). I have also tested using a desktop PC, and tested using a wall-powered USB hub. In all of these scenarios, there exists a subtle high-pitched hum that sounds very electronic and unlike ambient noise. What is causing it and how do I get rid of it? I've tried two different USB cords: one short and one long. Here is an unedited audio clip of the noise: https://dl.dropboxusercontent.com/u/7345145/noise.wav

The deep humming is probably my desktop computer fans - I'm referring to the higher pitched buzzing noise over it.

The gain is at the lowest setting on the microphone dials, and it is set to cardioid mode. The microphone level setting in Windows 7 is set to 22, though as I said I tried this on a Macbook Pro and heard the same electronic noise. I would really appreciate any advise with this, I'm at the point of pulling my hair out.

Edit: I ran a male to male 3.5mm headphone cord from the mic output to the line in on my Creative Titanium HD card input. At first test the results looked much better (and quieter) in the waveform, but after amplifying the levels to -0.1db for my voice, I could also hear the same electronic hum again. Here is an audio clip of the hum: https://dl.dropboxusercontent.com/u/7345145/noise_2.wav

Answer 1

I don't think it's coming from your microphone because there is a small stereo width to the noise at all frequencies. The microphone can only produce perfectly mono sounds. Here's an enhanced spectrum of the noise: -

Very noticeable are the sequence of peaks at precisely 1kHz, 2kHz, 3, 4, 5, 6, 7 (not 8), 9, 10, 11, 12kHz etc....

These are very unlikely to be made by the microphone because they are pretty much spot on the 1kHz mark plus harmonics.

Try accessing the microphone wires and shorting them out if possible - does the noise go away? Alternatively, does the mic have an on-off switch to kill audio - what does it sound like then?

Are there any other controls on the microphone? Do you have it at maximum gain? Does it have an AGC circuit that, in the absense of sound ramps up the amplification?

EDIT - following a discussion in comments it became clear that the owner of the mic had its gain set at minimum. THIS IS NOT IDEAL AT ALL. Analogue gain needs to be set near maximum to ensure that sufficient signal feeds the analogue to digital converter or you will get noise and noticeable fragmentation of the signal due to trying to digitize a weak signal and not getting enough "bits". Nearly all ADCs have a small amount of "digital" processing signal imparted to the analogue signal they are trying to capture.

Answer 2

Well, it turns out that our worthless electricition did not properly ground the outlets in this room - I used a tester to verify this. This would explain the humming noise in my recordings and also some other unmentioned audio problems I've had in the past. I'm going to fix the grounding for the outlet and assume this problem will be solved. I was able to verify the line noise went away when the mic was powered by a grounded power source.

Update: I re-checked the grounding of my electrical outlet and it is properly grounded. I misread the output of the tester. This means that something about my desktop pc was causing the noise. Rather than spend more hours trying to narrow down what the source of the electronic interference or grounding issue was, I went out to Radio Shack and bought a "Ground Loop Isolator" RCA cord and used that on the aux output of the microphone. This brought the line noise from -40db down to -85db and I can no longer hear it - I can only see it on the spectral graph. I consider this "good enough".