Thread: Infrasound?

I have two questions about infrasound.
1)As I understand it, Something can generate audible sound and infrasound simultaneously. Is that correct?
2)What could cause infrasound in the outdoors?

This question would be better posed in Physics, much more likely to get a good and thorough answer there.

Infrasound, afaik, is simply a low frequency pressure wave. Any oscillating object in nature that creates a pressure difference with less than 20 hertz is making inaudible sound. and an object that is oscillating at multiple frequencies can indeed make both inaudible and audible sounds.

Originally Posted by WVBIG

2)What could cause infrasound in the outdoors?

Same kinds of things as produce audible sounds, as long as they "operate" at lower frequencies (either exclusively or beside making the audible sounds).
Wind blowing between trees or against terrain features - I can imagine wind "whistling" in infrasound in the opening of a water well. Flowing water. Ocean waves. Animals, especially large ones (I think elephants communicate over long distances by stomping their feet and sending infrasound vibes through the ground).

And of course many man-made noises also have an infrasound component.

Thanks to both of you. That was pretty much what I thought. I was asking because now there is a theory that Bigfoot may use infrasound. This is based on witnesses reporting some of the same symptoms known to be caused by infrasound just before a sighting or in an area with a history of sightings or possible evidence discoveries. Personally, it sounds to me like something that won't be known until Bigfoot is proven to exist. Too many other sources of infrasound to waste time on the hypothesis at this time

I recently built an infrasound monitor that records sound
frequencies between 0.01 Hz and 20 Hz. My sensor range is
+/- 25 Pascals (0.1 " WC) and the resolution is 1/1000 Pascal.

Here is an example of the typical infrasound signals
that I see from my porch in a residential neighborhood.
I am near Washington, DC and the major source of infrasound
other than wind noise seems to be helicopters from police/militiary
that fly nearby several times a day

1. My typical 24 hour display on a calm day:

http://www.infiltec.com/Infrasound@home/091029a.png

The red outline is around typical helicopter infrasound signal
with two other helicopter signals next to it.
The green outline is around typical wind noise data.
The blue outline is around typical signal spike from a door closing
in my building that causes a damped oscillation at about 10 Hz.

2. The red outline helicopter infrasound signal expanded:

http://www.infiltec.com/Infrasound@home/091029B.png

3. Power spectra of red outlined helicopter infrasound signal:

http://www.infiltec.com/Infrasound@home/091029C.png

The power peak is around 17 Hz but it is spread out because of
+/- doppler shift as the helicopter goes by me.
Note the steep roll off of the 8 pole elliptic
20 Hz low pass filter that I use for antialiasing.

The free seismic Amaseis datalogging software allows me to block
any segment of time data and blow it up, and then
plot the power spectra of the selected time data.

Hope this helps,

Dave
http://www.infiltec.com/Infrasound@home