First up is the worst-case scenario, with the mic on its hard mount - essentially making it mechanically coupled to the disk drive’s motor. I ran three tests at a fixed input gain level, using the USB mic most sensitive to hum. The mics without shockmounts, whether external or internal, captured more of the 100Hz hum. The vibrations from the spinning disk traveled across the desk, up the boom arm, and into the microphone. (See my shootout and review of podcast/broadcast microphone boom arms.) Sitting on the same surface, three feet away, was a Seagate external USB disk drive, which runs 24×7 as a dedicated backup drive (running the OSX “Time Machine” utility).
For every test, I had mounted the mic to a boom arm attached to my desktop. The problem was obvious, once I found it. Clearly the hum was real - not a defective mic, not a bad cable, not some sort of weird USB crosstalk within the computer. Then I tried an analog mic through an external ADC. The hum was still there! It was less audible, but my spectrum analyzer showed a pronounced spike at 100Hz. None of these made any difference at all. turned off the HVAC and every other appliance that might be causing line noise.
unplugged and reset every connection in the audio chain.
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