Low level hum in background of recordings

[bbrady32]

Hi there,

I am running Audacity 2.4.2 on an MacOs Catalina version 10.15.7. I have a Samson G-Track Pro microphone and am getting a low level hum that is noticeable in my recordings. At first, I thought it may be a problem with my headphones because when I unplugged them, it seemed to go away at first. But, in reality, it didn't go away. There is no fan running, or any obvious ambient noise in my recording area. I have attached a quick recording that you can hear it in. The recording is unedited/treated, other than the recommended Audacity mastering steps for RMS.

Thanks for your help,
B

[Trebor]

Muffin.wav (1.57 MiB)

Notch90Hz,Q=5.png (28.98 KiB) Viewed 403 times

https://manual.audacityteam.org/man/notch_filter.html

[kozikowski]

That's not wall power hum. That's 90Hz. I would have called computer fan.

You know those pictures you see of people swinging metal detectors back and forth at the beach?

Screen Shot 2021-01-05 at 07.37.42.png (438.37 KiB) Viewed 403 times

You can do that with your microphone. Aim it back and forth over your studio. You can set Audacity to listen in real time while you do this. Wear your headphones. The side grill above the company name is the "front" of the microphone.

This is how I found a bad speaker in my music room. Constant super-low volume hum coming from somewhere.


I think you missed a step. Post another test and this time don't do anything to it.

Koz

[Trebor]

That's not wall power hum. That's 90Hz. I would have called computer fan ...

As it's exactly 1.5x mains frequency, my money is on Intermodulation between 60Hz mains and 30Hz subharmonic, (or maybe 30Hz artefact from the A/D converter).

[bbrady32]

Ok, here is a naked recording. I had my headphones on and plugged into the mic.

[bbrady32]

And, actually I don't hear it in this one. This is the problem. It's intermittent.

[bbrady32]

And another, where I can hear it. No editing.

[kozikowski]

my money is on Intermodulation between 60Hz mains and 30Hz subharmonic

In which case the metal-detector, sweeping the microphone across the room won't do anything. Tests with negative results still tell you stuff.

It could be lights, too. Nobody actually dims LED lights. They finagle time-sharing and pulse width management (because it's cheap and inexpensive) That could certainly give you intermodulation products. I have an air cleaner which "appears" to have a yellow light on the front. If I flip my fingers in front of it, it's actually switching very rapidly between red and green lights.

The easy way to test that is turn all the room lights off.

I don't actually believe this one, but it's easy to test. Make a test recording and leave your phone in another room.

Are you reading from a tablet like normal people? Those can have a 90Hz display. Temporarily put that in another room.

Koz

[kozikowski]

You got it! There is no 90Hz hum in that sound file.

I applied Audiobook Mastering and the file passes ACX Audiobook testing (but not by much).

Screen Shot 2021-01-05 at 09.11.48.png (24.58 KiB) Viewed 391 times

You still need to find that noise. There's nothing more crazy-making than sound damage that comes and goes.

I drag-selected some clean background noise and Analyse > Plot Spectrum at those settings. I pulled the display sideways to make it easier to see. This is the first example, with the noise.

Screen Shot 2021-01-05 at 09.22.02.png (148.18 KiB) Viewed 391 times

See that big purple spike?

Koz

[kozikowski]

I'm using the ACX Audiobook testing and mastering because, in general, if you can pass that, you can submit anywhere else. Their standards pass broadcast proof of performance.

You are warned to use all three Audiobook Mastering tools. Don't add any and don't leave any out. They clean up after each other.

Koz