There are cheap FFT spectrometers on eBay and Aliexpress. They’ll do the job.
FFT becomes very inaccurate at very low frequencies due to the nature of FFT itself. You can only rely on FFT as a presence detector for low frequencies and not really do any measurement or analysis with it. The cheaper the FFT spectrometer, the worse it will be at low frequency analysis. 70 Hz is already in the range of high error for non-scientific instrument grade FFT.
I would probably go with a standard waveform measurement (oscilloscope) using a good low frequency microphone made for recording kick/bass drums. Use an active parametric equalizer with a 12 or 24 db roll off and cut all frequencies above 2X the frequency you're looking to measure. This will get rid of the junk higher frequencies if you just want the raw base-frequency hum. Ditch the parametric EQ if you want to determine if the hum has any higher harmonics.
The microphone you use will need a means to collect sound better if the hum is very low in volume. A parabolic dish with the microphone located at the focal point will help greatly, but it will make the microphone directional which will make it difficult to locate the source direction since low frequencies are hard to determine spatially due to their long wavelengths. The reflector dish can also color the sound but less of an issue at low frequencies. Experimentation is key here.
lol You're gonna scare the shit outta him. A free audio analyzer app on a phone with a (usb/lightning) microphone to walk around and spot the source/origin of that noise will do. :)
lol You're gonna scare the shit outta him. A free audio analyzer app on a phone with a (usb/lightning) microphone to walk around and spot the source/origin of that noise will do. :)
Yeah this setup can get quite pricey, but it would be necessary for proper analysis of the sound. His phone might do alright, but it seems like he already gave that a go and didn't have good results. Might be able to buy a cheap used Tascam or Zoom portable digital recorder on eBay and get better pickup. My Tascam DR-70D picks up a lot of things I wouldn't have expected it to with its built in mics. I got mine cheap on eBay ($25) because it had a broken headphone jack which I fixed in under an hour.
Thanks for the writeup. Might save me a lot of time if this persists. The hum has subsided, became intermittent and seems to be gone for now. I want to be prepared for when it returns.
Good idea re the scope, bought a Rigol 16 bit one some weeks ago, should do the trick. I have a Shure Beta 58A microphone left over from a project that didn't materialize. Not ideal but I'll start there, before investing into a different microphone better suited for low frequencies.
Edit: Typos
Thanks for the writeup. Might save me a lot of time if this perists. The hum has subsided, became intermittent and seems to be gone for now. I want to be prepared for when it returns.
Those pesky aliens keep moving around. Hard to find them that way. :)
Good idea re the scope, bought a Rigol 16 bit one some weeks ago, should do the trick. I have a Shure Beta 58A microphone left over from a project that didn't materialise. Not ideal but I'll start there, before investing into a different microphone better suited for low frequencies.
The Shure 58A should do much better than most common microphones. With the scope's sensitivity you should be able to see something show up. It will also help you get a fix on the frequency/periodicity of the hum and the scope may have math functions to help isolate or enhance it depending on the model. This setup should do well if the hum returns.
Just to correct my idiocy re scope. It's not 16 bit, of course. It's the 12 bit DHO924S. Cool bit of kit, nonetheless.
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