I have done a lot of work with barometric pressure with respect to caves.
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Caves are often a large volume which equilibrates with surface pressure
with huge volumes of air flow.
When a wind is blowing, there are often enormous eddies, mixing air from
the surface with the convection ceiling often a kilometre or two from the
ground. These eddies cause fluctuations or gusting, with a period about
2 - 10 minutes in the velocity of the wind at ground level.
The variation of velocity is accompanied by variations of barometric pressure.
These are so obvious that they can be seen as continuous movement of the
needle of a low friction barometer or altimeter. Aircraft altimeters have
too much friction, but the instruments made by Wallace & Tiernan, if examined
with a magnifying glass, can be seen to be in continuous motion.
Digital barometers based on fairly insensitive IC chips do not show the
effect very well.
So the frequency band that you should look at has periods between 30
seconds and 5 minutes. The variations are also related to micro-seisms,
which are found in the ground, oceans and the air, due to coupling
of energy from the atmosphere to the ground and water.
On 5 Apr 2020, at 00:28, Chris <email@example.com> wrote:
Never underestimate the power of the wind [and water waves] to produce infrasonics. i.e. Sub-20Hz frequencies.
I have experimented for decades with large, audio subwoofers with responses well below 10Hz at levels well above 110dB[C].
So I can attest to the fact that most building structures will readily vibrate in sympathy with low frequency sound.
By its very nature infrasonics are often completely inaudible to most humans. Except when something rattles in sympathy. Anywhere in the building!
Audio enthusiasts, with a flair for the dramatic, often relate their solid concrete floors having shown visible surface waves when LF frequencies are reproduced at very high levels.
Measuring [or even detecting] infrasonics requires a suitable microphone with an extended LF audio response and audio frequency response, graphing software.
It is quite possible that the clock case is being rapidly and cyclically pressurized and evacuated much like an aneroid barometer capsule or drum.
Or even that the apparently solid, clock wall is vibrating. No doubt water waves are similarly responsible for exciting building structures.
Both water and air are likely simultaneous VLF energy sources in Bepi's particular environment.
Windows and doors provide almost no protection from VLF audio waves.
They will usually vibrate in sympathy and instantly re-transmit the airborne "noise" on their exteriors directly into the interiors of a building.
YouTube is well furnished with videos of doors and windows vibrating over an inch out of true when subjected to infrasonics.
Rooms, themselves, can often amplify [or exaggerate] certain VLF if the dimensions of standing waves between opposite walls match a particular audio frequency.
On 04/04/2020 14:33, Andrew Nahum wrote:
The association with wind speed is fascinating. I wonder if the wind could in some way be pressurising your house? Do you have a barograph actually next to your clock or is your pressure data that for the Venice area taken from the public weather service?
The empty city must be quite wonderful in a way! Andrew Nahum
I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends. But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:
<Pasted Graphic 1.jpg>
The clock period shown here is averaged over the 5 period impulsing interval.
The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.
The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.
I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.
During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.
The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.
<Fluct vs wind.jpg>
The relationship with atmospheric pressure is shown below:
<fluct vs press.jpg>
I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.