Continuous noise with changing frequency

Hi all, I’m seeing continuous noise at a frequency at all times of day. However, the frequency seems to wander a bit, ranging somewhere between 20 - 30 Hz over the course of the day.

The shake is underneath a makeshift Faraday aluminum-foil-covered plastic box covering all sides except the ground which it is in direct contact with, which I think would filter out some of the external RF noise.

Any ideas what this mystery signal could be and how to eliminate it?

Realized shortly after I posted this that the highest amplitude signal here was my hot tub pump, which cycles on and off throughout the day. Here’s what it looks like when it switches off:

A very curious signal indeed guillochon, spectrogram plots are always fascinating to look at, because they reveal a hidden world of noises and vibrations (in the case of what our Shake records).

The stronger signal at 11:34 could be an helicopter passing over (or close to) your house, while it’s the first time I see that ‘wavy’ signal. Have you tried to momentarily shut down other possible vibrating apparatuses in your house, and see if that has any effect on what you are seeing?

What about removing the Faraday box? Does that change anything?

The wavy signal is also the helicopter, helicopters tend to circle around here so you’re seeing it doppler shift towards/away from me over and over.

The faraday cage is padded with foam and I do notice a slightly lower level of overall noise with it on top of the shake, maybe like 30% less “white” noise. I saw in another thread trying a “pillow” test to isolate RF from ground shaking and I’ll prolly give that a go.

My first impression of the bright part of the trace (not the big loud noise) was a helicopter, but looking closer I had some doubts. First because it was just a more intense part of that longer periodic signal, and secondly because the slope was quite different - all of my helicopter traces with multiple signals like that tend to have very similar slope to the ‘S’ pattern (or so I thought … see below).

If helicopters to circle your place for a long time, and many circles, then yes, I can believe that it is a helicopter caused that.

I just went back and looked at a helicopter pass over my house:

The higher frequency trace does indeed have a noticeably steeper slope. So yes, helicopter would seem like a good guess.

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I’m pretty confident it’s a helicopter because I was staring at the spectrogram while the helicopter was doing donuts over my neighborhood :laughing:.

The signals I’m trying to eliminate are the constant “hums” at various frequencies that don’t evolve quickly. In addition to the hot tub signal you can see some fainter constant frequencies around 20Hz that I see even in the dead of night. I like the detective adventure of figuring out what those are :slight_smile:

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Some more data: I put the shake on a pillow, without the faraday box on top. It’s a lot noisier overall (a pillow is not particularly stable), and that hum at 20Hz is still there.

So now that the seismic noise seems to be eliminated, I’m wondering if it’s either the USB charging brick or the unshielded cat-5 ethernet I’m using to connect to the shake. I’m gonna try swapping those out next to see if that makes a difference.

Well, here is one to add to the “library”.
Pressure washing the road before applying a new seal layer.
The shake is in the barn at the top of the image, back edge, away from the road.
Distance to road ~86m.

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