[time-nuts] Re: in-ground clock room

[Poul-Henning Kamp]

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Hendrik writes:

> go to ebay and get a cheap geophone. [...]

I did that prior to building our house, because we built about 60m
from the main rail-road through Denmark, and I needed vibration
data to design of the house.

There are a number of footnotes about geophones, the primary one being
that they have a resonance frequency in the 4-7 Hz range and therefore
provide no usable information below that.

The second is that it is anyones guess what their sensitivity is,
in particular if you buy it on eBay from somebody who found it
lying in a field after percussive oil-exploration, so buy more
than one.

The good news is that it is easy to calibrated a geophone for
absolute response using common T&M kit.

I feed the geophone directly into a TI ADS1282 evaluation board, that ADC
is built specifically for geophones and it works great.

It is now bolted to the foundation of our house, and here is a recent plot:

	http://phk.freebsd.dk/misc/_.18856.pdf

The 50 & 100 Hz lines are electical noise.

The vertical red impulses at night are typical of passing trains, around
5Z construction work on the rails started.

See if you can guess when they used the huge vibration compactor :-/

I have no data from any other building to compare with, and we did
succeed in building a very quiet house: You really have to pay
attention to hear the train-traffic.

Amongst the many faint domestic signals I can identify in these
plots are our ground-circuit heat-pump, the "GenVex" ventilation,
the cooling in my lab, the fans in my Dell-server, the freezer, the
washer, the dryer, and the occational good "rocking-through" on the
Rauna-Njord speakers.

I've tried playing the recordings back through head-phones, but
none of those signals were audible.

The vibration compactor on the other hand...

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.