Here are some results of an experiment into the diurnal variation in background infrasound levels at station R21C0.
I expected to see an increase in frequencies above 10Hz as a result of “cultural noise” or man made activity during the day. I didn’t expect to see an increase right across the infrasound spectrum.
I selected time periods during the day and the night where there are no obvious very close range noise events from inside the house. Passing trucks during the day are fair game, they are included, but I avoided doors closures, etc.
Apart from cultural noise differences, there are minor wind and temperature differences. Cultural noise possibly not as big an influence as expected. Perhaps wind has more effect than expected.
Interesting observations! In my experience, wind is a big factor. I am not sure if it directly creates ultrasound noise, or if the noise is an effect produced by wind interaction with structures and trees. However, windy days with tree movement are distinctive on my infrasound detector.
I think you are right - wind speed will be a large factor. At some stage I’ll do some more experiments looking purely at wind (as far as practical). While I tried to keep the difference in wind speed to minimum, it’s still there, and given the fluid friction (that produces noise and heat) is proportional to the square of the velocity, it’s very likely to be significant. And it’s a valid part of the difference between day and night.
Turbulence is what produces the infrasound from wind. Turbulence is produced from fluid shear (wind shear in air) which results from interaction with a surface or different fluid (air) speeds. As long as the Reynolds number is less than 2000, the flow will be laminar. Above 2000 the flow is turbulent.
Laminar flow is essentially quiet. That’s the reason engineers go to great pains to make sure the air flow in ventilation and air conditioning systems is laminar. That way the noise is minimised (to only those small areas of turbulent flow around sharp edges).
I don’t think I’ve seen laminar flow in wind apart from dead calm. While ever the wind speed and gust speed measured on a weather station are different the flow is turbulent. My gut feel tells me that the infrasound from interaction with a surface will be louder than that from the turbulent air flow itself, and that both will have different charactistic frequencies, but I have no idea how to set up an experiment to separate the two.
It would be interesting to see if the results of others doing this experiment to maybe see what is common and what is unique to each station installation.
