Why Does Wind Change Pitch as It Blows Harder?

[JoAuSc]

I know the basic answer, that the movement of air sets things vibrating which makes noise, but I don't know the specifics. For example, if the wind is blowing hard, you can often hear the wind howling at a certain pitch. This seems to be dependent on velocity because the harder the wind blows, the higher the pitch is. How does this happen? Are sounds made by the wind primarily due to the air itself vibrating or to solid objects vibrating when the air blows against them? Also, is there an easy way to predict what the frequency of the howl will be based on how fast the wind is blowing?

 

[marlon]

I know the basic answer, that the movement of air sets things vibrating which makes noise, but I don't know the specifics. For example, if the wind is blowing hard, you can often hear the wind howling at a certain pitch. This seems to be dependent on velocity because the harder the wind blows, the higher the pitch is. How does this happen? Are sounds made by the wind primarily due to the air itself vibrating or to solid objects vibrating when the air blows against them? Also, is there an easy way to predict what the frequency of the howl will be based on how fast the wind is blowing?

What we call sound are in fact just pressurewaves propagating through the air. Our ears and brain detect and convert these pressure waves to electric signals which we interprete as being sound. The frequency of these waves determines the altitude of the tone, while the amplitude defines the volume of the sound.

When waves (eg pressure waves) pass through openings that have about the same magnitude (or less) than the wavelength of the incident wave, diffraction will occur. Once passed through the opening, diffraction will "generate" a phase difference between waves starting from the top and bottom of the opening. This phase difference leads to interference. Phenomena like diffraction and interference of waves will alter the caracteristics of the incident wave. For example, some frequencies can be canceled out while others may become bigger. This is what happens in constructive and destructive interference. Different frequencies lead to different sounds because of the connection between frequency and sound.

Here's more

marlon

 

[Astronuc]

I know the basic answer, that the movement of air sets things vibrating which makes noise, but I don't know the specifics. For example, if the wind is blowing hard, you can often hear the wind howling at a certain pitch. This seems to be dependent on velocity because the harder the wind blows, the higher the pitch is. How does this happen? Are sounds made by the wind primarily due to the air itself vibrating or to solid objects vibrating when the air blows against them? Also, is there an easy way to predict what the frequency of the howl will be based on how fast the wind is blowing?

Adding to what marlon mentioned, noise is the result of vibrations in the air which excite the ear drums at those frequencies to which the ear is sensitive.

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/ear.html#c1
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/hearcon.html#c1
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/earsens.html#c2

Anyway, when wind blows it may interact with structures or cavities, and the structures or the air in the cavities may vibrate at certain natural frequencies, which are based on mass (of structure) and geometric properties, and gas properties.

This is a particular type of fluid-structure interaction and is very complicated to model in precise detail.

 

[miket]

I know the basic answer, that the movement of air sets things vibrating which makes noise, but I don't know the specifics. For example, if the wind is blowing hard, you can often hear the wind howling at a certain pitch. This seems to be dependent on velocity because the harder the wind blows, the higher the pitch is. How does this happen? Are sounds made by the wind primarily due to the air itself vibrating or to solid objects vibrating when the air blows against them? Also, is there an easy way to predict what the frequency of the howl will be based on how fast the wind is blowing?

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.

Mike