In preparation for attempts to record the Hum using classic audio techniques using highly sensitive electret microphones, I made some night recordings using a Zoom H4n recorder, out of the box, while I learn to use the unit. I noticed something unusual. The two screenshots I’ve attached are from the well known and widely used Audacity software. The first recording was taken from the basement of the house, with the mains power on but no fans or motors running in the house. The bright (loud) spikes are me setting the recorder in place or moving it. The Hum, perceived by me at roughly 56.5 Hz, was very loud as I made the recording.
The second recording was made from inside my car in the driveway (obviously with the ignition off and windows up). Notice the steady modulation in the lowest end of the audible range.
Perhaps the most important result from this is that when I listen to the recordings, I cannot hear the Hum in them. Obviously I can’t conclude anything yet, but as I learn more and begin to master the technology, I will report on what I find.
If you are an audio expert – particularly one who can comment professionally on recording low frequency environmental sounds – feel free to post here or contact me at firstname.lastname@example.org