Best way to record/measure structure bourn noise?

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mrclunk

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Over the last year we've been having an issue with very low level train/underground noise coming into the studio.
In 30 years at this location we've never had even a hint of this before.

It sounds like noisy points or a gap in the track has opened up. After we made some enquires to the Transport for London noise department it went away, but now after Christmas its back..

I need to make some recording as evidence. What would be the best way?
The noise floor of the kit tend to swamp the actually sound and i'm not sure the mic i tried has the low frequency response.. I tried a DPA lav stuck to a wall but didn't really capture it.

Contact mics on pillars maybe?
Any one have experience in this?
thanks
 

abbey road d enfer

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Contact mics won't do; what they will pick up does not relate to the sound level in the room.
You must do two recordings with a good LDC, one without the train noise and one with it. It is necessary to make noise measurements with a calibrated sound meter.
 
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Tubetec

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Why not try a mems tri axial accelerometer into REW .
Actually producing a calibrated noise level measurement might not be needed.
Catagorising the sound with a video of an FFT/Spec measurement might be more useful .
So if you set up the accel board bonding it to the conctrete , you'll easily be able to make a visual comparison of before and after work on the tracks and send it Network Rail for evaluation , it should be relatively easy to compare the energies involved in the sounds being created .
Theres a few mems accels on ebay for less than a tenner should do the trick . Again Im not sure what if any good a sound recording of this noise is worth to anyone in an office , they simply wont have the equipment to reproduce it unless they wheel in a few PA subs .
 

Tubetec

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The Cosmos ADC might be worth investing in for the job because of its good low end performance , where inevitably a regular audio interface will tail off the frequency range of interest >20hz , you could always try and get the rail Co to agree to cover any associated costs , and bill them at the end like any other customer , you never know , if you make your presentation in a professional and easy to understand way they might call on your services again .
 

mrclunk

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thanks, I don't really need a calibrated level. I think they will send someone down to do that. The level we're getting is well below the problem threshold for action anyway i believe, it's more working out why its started now after 30 years and where..

I'd just like a clearer recording. I suppose our lowest SNR mic/pre in omni?
It would be useful to clearly distinguish the amount of bogies/carriages on the train, as that will help us narrow it down. It's not actually regular enough for a tube train, possibly a heavy freight train on the overground which is further away... But I have sat by that line listening out for the same rhythm to no avail..

After speaking informally to a couple of tunnelling people, the head engineer of a firm that digs tube and power tunnels, he said it could be another branch line they've decided to use that's not on the maps. Even when they're designing multi billion pound networks underground they don't know whats down there, just get told to reroute sections by x amount if there's an issue.

The mems idea is interesting (and sounds fun!)
 
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Bo Deadly

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Could it be "dig-down extension" where people are digging out there basements, sometimes multiple levels down? It's a thing in London right now. Search "london basement conversion".

I would just look at vibration sensors on Mouser and look at datasheets to see which ones have good low frequency performance. Just picking something almost at random, look at part 1006015-1 to get a bearing. Then you would need to design a circuit to interface with the part and be able to reproduce low frequencies. Mount it firmly inside a die-cast box such that the sensor base (not the mass) is hard coupled to the box and then place the box on the floor with something heavy on top of it and with heavy blankets or sound proofing material on top of that. Note that you cannot just plug a sensor like that into your USB audio interface. It will of course give you results but the results will be garbage. You'll need a special circuit that is designed to work specifically with that sensor and has the right filtering and such. Then you'll need to find an audio interface that can actually record sub-frequencies. I think MOTU devices can record DC. Then feed that into GNU Octave and run FFT on 1-100Hz and plot.

At least that's what I would do. The exact procedure depends on available skills.
 
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mrclunk

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Could it be "dig-down extension" where people are digging out there basements, sometimes multiple levels down? It's a thing in London right now. Search "london basement conversion".
That was a possibility but we ruled it out. We original thought it was a new tunnel being dug nearby for London power networks. 32.5 km tunnelling project under London begins | National Grid Group

But after a meeting, we found out they hadn't even started digging yet. They were actually very helpful, geeked out with them.

Will investigate the Vibrations sensors, i have a Quant Asylum interface that will work with some sort of high impedance amp.
 

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