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dmazin

This is SO cool.

I am actually currently working on a project to record the sound of the London Underground passing under me.

We can very clearly hear the Northern Line under us. It's < 30 meters below us.

I have become obsessed with getting high-quality, low frequency recordings of it passing under us.

Why? I don't know. I just can't take my mind off it.

For example, there are two tunnels (north and south bound). By correlating it with actual TfL data, can I figure out the sound signature of each?

More intriguingly, I know that there are maintenance vehicles that operate under us in off hours. Can I "catch" them?

I'm not sure what else I might do with this project, but the idea of capturing the sound of this semi-ephemeral creature that operates below me has captivated me.

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That's awesome. What sensors have you considered?

I'm interested in extremely weak, high-frequency vibrations of everyday things while in resting state (which is sort of the opposite of what you're after, as I understand), but have not gotten far in acquiring the sensors. I'd love to get a laser doppler vibrometer, but they're pricey.

Well, I was just going to put a contact mic in my cellar. My understanding is that this is the best way to pick up low-frequency vibrations.

I know very little about audio engineering. I wonder what else I might be able to use to pick up the vibration signature?

I completely understand your drive to pick up those high-frequency vibrations! There's a whole secret world of vibrations out there, and we can analyze it!

You might struggle using microphones if it's very low frequency because most human audio microphones intentionally filter frequencies that are below (and above) human hearing range. Perhaps fixing to he contact mic to the right resonant object might serve to translate the low frequency to a higher frequency?

Could you use the accelerometer on a smart phone to 'hear' the vibrations?

I don't know if a contact mic really has any filtering though; it's just a piezo element that I plug into a pre-amp.

However, what you're saying is completely legit. I read that I might want to lay a large, sturdy thing on the floor and put the mic on that, for the same reason that you gave. I used a large old pane of glass that I found in the cellar.

All that came to my mind is making sure to physically bond it to the floor. But then you still need to have something that moves relative to the magnet and then I got confused again.

What do seismographs do? People also use acoustic pings to measure soil and rock density underground. What are those devices using for their “microphone”?

And yeah, would an accelerometer work? I don’t know what the temporal resolution is on your standard Amazon / AliExpress accelerometer is but it’s probably pretty decent?

"Geophone" is the device you seek.

Amplifying and digitizing the signal is straightforward for an experienced EE, but you might look into acquiring some Raspberry Shake hardware which has it all already done.

Oh, snap. Thank you. Guess I’m about to be $350 poorer!

Stupid question, but how far can you get with a laser, a mirror, and a light sensor?

That's actually a really cool idea. I am not sure what variance there would be on a light sensor though.

I remember building a toy interferometer in high school - a laser, splitter and a mirror. You could definitely see the interference patterns when you'd press on a wall with a finger.

Maybe something like that could be turned into a reasonable microphone?

I have pondered adding 4 microphones to the corners of my house. I suspect with some processing and filtering you could likely tell quite a bit about the house. When the front door or garage opens, what rooms are occupied, when it's raining or windy, what appliances are on, when someone parks in the driveway, when the mail comes, duty cycle of the fridge and furnace, when it's sunny out (expansion noises on the sunny side), when someone is showering (and which shower), when toilets are used, etc. Certainly should be able to detect when a window is broken or a door forced.

I was at an Open House at the Lamont-Doherty Earth Observatory [0] a few weeks ago, and one of the researchers was doing something adjacent to this -- recording the sound of earthquakes for analysis. Will try to find who it was.

[0] https://lamont.columbia.edu/

I am sure that your work in discriminating what's happening deep underground with nothing more than passively recording surreptitiously available signals at the surface level on non-specialist hardware is of interest to a few governmental offices.

My kind of people. Good luck!

I used to live about 300 yards from where the subway went and I felt like I could hear it. I’m not sure if it was residual sounds from the sidewalk emergency exits or from the earth ringing like a bell.

Ha! If you happen to live in the battle area of South Wimbledon then the noise from the northern line in the early hours of the morning can wake you up! It’s roughly 30 meters under too!

What I learned is that the Northern Line has the most noise complaints (from people who live near it) of any TfL line!

These are electric trains. I'd be interested to know if a big coil of wire picks up a signature as well.

Woah. A bit of reading shows that the signal would be incredibly weak, especially because of shielding.

I wonder what other ways there might be to detect it.

Someone else mentioned literal vibration, but I figure a contact mic would already pick that up – I mean, sound is already vibration...

The oldest deep London Underground lines have iron tunnel segments, which might make this method particularly difficult.

https://www.ltmuseum.co.uk/collections/collections-online/in...

Have you looked into whether a Raspberry Shake might be adequate?

Good shout — I may have to look into this.

Could you use a ground mount seismic sensor?

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