Room Noise

We are still wrapping panels, building a booth and treating the room.
I'm checking the room noise with a neat little application, Jack Alsa Audio Analyzer, jaaa for short, from the folks at Kokkini Zita. Sunday night, which should be very quiet, I see a spike at 58 hertz. I put a red marker on it; it is the one on the left side of the line. I can hear the hum, barely. I check again this morning and it's still there. I'm not sure what this is. We have radon fans running, and maybe some of the fan sound is bleeding into the studio. I don't want to turn them off. But maybe I can run the recordings through a high pass filter.

The LKSM-12, which is my primary recording guitar, is tuned down a minor third. The lowest string is about 68 hertz. I try the glame high pass filter set at 68 hertz, 4 pole. This is a very sharp cutoff. And it seems to work, but filters tend to introduce phase shifts, particularly when they are set very steep. I would be better off if the annoying sound was further from the good sound. This prompts me to crank up the LKSM-12 another semitone, putting the lowest string at 73 hertz, and a full third of an octave above the sound I want to cut. The string tension is still well below spec for the this guitar - 231 pounds, up from 206 pounds. The LKSM comes from the factory setup over 300 pounds, so as long as I can handle the tension I am sure the guitar can.

I'm still not sure what to do about the mysterious 58.3 hertz sound. When I calculated room modes based on the dimensions, there was one at about 63 hertz. Calculators are only approximate; rooms aren't geometric diagrams. So this could be a very low level sound that is resonating. Maybe as we add more traps it will lessen. For now I'm trying a couple of filter plugins. The most promising is Steve Harris's Single Band Parametric, set to minus 16 db at 58.3 hertz, .33 octaves. It drops the 58 hertz bump down to the level of the ambient sound.

Update March 17th - St.Paddy's Day:
The 58.3 hertz sound is coming from the main radon pipe fan. Luckily, it has an on/off switch. My hair may fall out, but my sound will be clean.