The Sounds of Wind and Water

Rome, New York

[We are stopping this week to visit family.]

Years ago I wrote about the sounds on a boat; especially sounds at night when you are trying to sleep.  Now we have fresh experience.  Sleeping in a tent, few if any sounds are blocked.  Also, because of our diverse camping locations, we were exposed to many environments.  I found the nighttime sounds to be a lot of fun.  Several times, I laid there trying to figure out what I was hearing.

Not all sounds are mysteries.  In several places, the sounds of stiff wind were very loud.  In several other places, the sounds of swift running water were very loud.  One night it struck me,  I couldn't tell the difference.  Wind and water sound alike.  So much alike, that I'm sure that I could not tell the difference merely by listening.  It is a white noise.  To my ears (which can not hear high frequencies) it sounds rich in the low frequencies. That is surprising.

To repeat, the sounds of wind in a forest, wind in the open desert, white water rapids in a mountain stream, and swiftly flowing water in the much larger but calmer Green River in Dinosaur National Monument, all sound alike.  I exclude the moaning sound of very high winds; it never blew that hard.

So what exactly is the mechanism of wind noise?   I always thought that it was the sound produced by the wind in the tree tops.  Now, I don't believe that any more because it sounds the same when there are few trees around.  When I research this question online, I find answers such as this:

In many cases the sound is not due to direct vibration of wires, branches or whatever. Let us assume we have a horizontal wire with the wind blowing past it horizontally. Vortices are created in the air downstream of the wire, alternately above and below the wire at a frequency which depends on the wind speed. The frequency f is given by the relation f = Sr u/d

where u is the wind speed and d is the wire diameter. Sr is a number which is about 0.2 for a circular cylinder and is called the Strouhal number.

As an example, a wire 5mm in diameter in a wind of 10ms-1 gives a frequency of 400Hz.
The pattern of vortices is known as a von Karman street.

That sounds good until I note that the wind sounds the same in open desert or in a forest.   You might ask if it sounds different at sea?   The answer is that we don't know.  At sea wind noise mingles with the noises of the water surface so we never hear the wind alone.

So what exactly is the mechanism of running water noise?  No matter what the answer, it is very surprising that the sound is indistinguishable from the sound of wind.

Perhaps readers can enlighten me.