Still dealing with The Hum (Update from July 2024)

[–] • 0 pt

As for the hum itself...70/77 Hz is noteworthy, no pun intended. It's not AC power frequency, it's not any standard harmonic of AC power frequency and it's not at all common in the world. That rules out a huge number of manmade sources. Not sure if you've measured any higher order harmonics at 140, 280, etc. but there presence or absence would help rule out some natural phenomenon as well. If there is only the fundamental harmonic, well that makes this a lot spookier since that would indicate a manmade source of very deliberate purpose.

If you're so inclined, look for some surface conduction transducers to use as microphones. These transducers are often used as speaker drivers that attach to a wall or a hard surface to turn the whole thing into a speaker. You can use them as microphones just the same. These could be attached to a sheet of plywood or even insulation panel and placed on bare ground to see if the hum is emanating from the earth or the atmosphere. You could even just use a large woofer driver as a mic and get good results too. I used to listen to my whole house as a teenager with such a setup. Never could make conversations out clearly but it was fun nonetheless.

BTW, I checked Spectroid on my phone to be sure the app doesn't have a natural "whistle" at 70/77 Hz. Some FFT apps tend to have false indications at specific frequencies, but my phone doesn't indicate that. But...phone hardware varies and the phone itself could have various "whistles" in it but it's rare to see one at such a low frequency. Hmm, I do see a whistle at 50 Hz. I'm not in Europe and have 60 Hz AC. Weird.

Definitely condenser over dynamic microphones for this application. I can vouch for that Behringer ECM 8000 metrics microphone. I use those with my Behringer DSP 8024 active EQ/spectrum analyzer room correction units. They are good for a wide range of frequencies and should do well at the ~70 Hz you have with the hum. I'm more a fan of Tascam over Zoom, but I think Zoom has really stepped up their game over the last few years. As for the hum itself...70/77 Hz is noteworthy, no pun intended. It's not AC power frequency, it's not any standard harmonic of AC power frequency and it's not at all common in the world. That rules out a huge number of manmade sources. Not sure if you've measured any higher order harmonics at 140, 280, etc. but there presence or absence would help rule out some natural phenomenon as well. If there is only the fundamental harmonic, well that makes this a lot spookier since that would indicate a manmade source of very deliberate purpose. If you're so inclined, look for some surface conduction transducers to use as microphones. These transducers are often used as speaker drivers that attach to a wall or a hard surface to turn the whole thing into a speaker. You can use them as microphones just the same. These could be attached to a sheet of plywood or even insulation panel and placed on bare ground to see if the hum is emanating from the earth or the atmosphere. You could even just use a large woofer driver as a mic and get good results too. I used to listen to my whole house as a teenager with such a setup. Never could make conversations out clearly but it was fun nonetheless. BTW, I checked Spectroid on my phone to be sure the app doesn't have a natural "whistle" at 70/77 Hz. Some FFT apps tend to have false indications at specific frequencies, but my phone doesn't indicate that. But...phone hardware varies and the phone itself could have various "whistles" in it but it's rare to see one at such a low frequency. Hmm, I do see a whistle at 50 Hz. I'm not in Europe and have 60 Hz AC. Weird.

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[–] • 1 pt

(edited )

Ordered the Behringer plus a wind screen fur thingie and a good short XLR cable.

For the recorder, I'm currently looking at the Tascam Portacapture X6, might be overkill but it has the 32 bit resolution, which I would like to have.

Thank you so much for your input, it helps and is very much appreciated.

I'll keep you in the loop.

ETA: Also looking into surface transducers. Any recommendations? Only found this in a cursory search: https://www.sparkfun.com/surface-transducer-large.html

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