Does your podcasting or voiceover microphone hum? Do you hear a constant, low-pitched rumble, whine, or buzz in your audio tracks?

The problem might not be the microphone. The hum might exist in your room. The mic is just making it obvious.

I recently had a bad hum problem with a particular USB mic. I tried everything I could think of to make the noise go away:

changed USB cables

added a ferrite choke to the USB cable

changed USB ports on the computer

changed from desktop to laptop

turned off all the fluorescent lights in the house

unplugged and reset every connection in the audio chain

turned off the HVAC and every other appliance that might be causing line noise

None of these made any difference at all.

I tried a different USB mic. The hum was still there! It was less audible, but my spectrum analyzer showed a pronounced spike at 100Hz. Then I tried an analog mic through an external ADC. The 100Hz spike was still there. Clearly the hum was real — not a defective mic, not a bad cable, not some sort of weird USB crosstalk within the computer.

The problem was obvious, once I found it. For every test, I had mounted the mic to a boom arm attached to my desktop. (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). The vibrations from the spinning disk traveled across the desk, up the boom arm, and into the microphone. The mics without shockmounts, whether external or internal, captured more of the 100Hz hum.

I ran three tests at a fixed input gain level, using the USB mic most sensitive to hum. 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. The 100Hz hum peaks at -44dB. (Click any of these images to zoom in.)



I replaced the mic’s hard mount with an elastic shockmount. The shockmount bought me 20dB of isolation; the hum dropped to -64dB.



Then I un-mounted the external Seagate drive from the desktop, and let its motor spin down. Predictably, the hum disappeared into the noise floor of the mic’s USB circuitry, around -100dB.

How to fix podcasting microphone hum

The lesson is that if your mic is picking up unwanted noise, the problem might not be some sort of esoteric EMI or RFI, or a bad mic or cable, but actual mechanical noise in your environment. Try this: take the mic off its desktop stand (USB mics nearly always come with desktop stands) and hold it it your hand. If that kills the hum, then there’s something on your desk that’s making the noise your mic is hearing. The culprit is probably the computer itself, or a disk drive, or a stereo component with a fan.

If you can’t get the noise source off the desk, you could try isolating it with Sorbothane feet or some other decoupling solution. I tested this idea by laying the Seagate external disk drive sideways on three Sorbothane feet; the 100Hz noise heard by the mic dropped by 30dB, well below the threshold of audibility. In other words, decoupling the disk drive from the desktop worked better at suppressing hum than putting the mic into a shockmount.

Another option is to mount the mic on a heavy floor stand. And I always recommend using a shockmount for the mic as well.

Bonus tip – freeware RTA

By the way, the RTA screenshots above are from BlueCat Software’s freeware FreqAnalyst spectrum analyzer plug-in. It is a great tool, and the price is right.

Posted in Acoustics, Microphones, Podcasting | 17 Comments »