How does wind generate sound? What happens on a chemical or cellular level that allows that to happen? Also (semi-related) when driving, why is there a noise that feels like a vacuum every other second when one window is open at high speeds?

284 views

How does wind generate sound? What happens on a chemical or cellular level that allows that to happen? Also (semi-related) when driving, why is there a noise that feels like a vacuum every other second when one window is open at high speeds?

In: Physics

2 Answers

Anonymous 0 Comments

(Tl;DR at the end)

I’ll try to answer this to the best of my knowledge while keeping it simple.

First of all, what is sound? Sound is our interpretation of pressure waves hitting our ears.

What is a pressure wave? Basically moving differences in pressure. Just like a water wave has crests and troughs, so does a pressure wave. While in a wave you can see the “peaks” and “dents” in the water, in a pressure wave those are basically differences in density.

So much material in some space = high pressure, little material in the same space = low pressure. A wave is thereby some form of alternating “pockets” of density that move.

Ok so some waves hit our ear and we interpret these waves as sound.

What is wind? Wind is moving air right? But why does air move? Because of pressure differences! As warm air expands it occupies more space with less molecules and thereby warm air has lower density, lower pressure, than cold air. The result is a pressure gradient, a difference in pressure, between regions of air with different temperatures.

As the universe likes to keep things stable this pressure wants to equalise. It does so (or tries to) by moving matter from high pressure regions to low pressure regions. If air does this movement we call it – wind.

So wind is caused by pressure and we can hear pressure waves (of certain frequencies (the amount of crests and troughs moving through a given area per second)).

When wind hits objects, just like when you hit them, it causes the objects to move. This goes from a small vibration to taking the object with it. In the case of vibrations the fast “back and forth” movement basically creates moving packets of air of different densities, so a pressure wave – which we can hear.

Now in the car, if you have your window slightly opened you’ll year a “whum whum whum” sound. This is caused by air basically blowing “over” the opening. This creates air movement in one direction, either out of or into the car. This movement however creates an imbalance and as we made clear before the universe likes things to be balanced (as all things should be). Thus the movement in one direction ultimately results in an opposite movement to counter it. However due to inertia this movement “overshoots” resulting, again, in a counter-movement. This then basically happens at some frequency related to the speed of the car, which you can hear.

This effect is called “Helmholtz Resonance”, which is basically the same effect that happens when you blow over the opening of a bottle.

TL;DR: 1. Wind creates sound by hitting objects, resulting in pressure waves that we can hear.
2. Helmholtz Resonance caused by air pushing in and out of the car.

Anonymous 0 Comments

A: wind makes sound only because of the physical items around. Rustling leaves, blowing against the side of a house or through a tunnel, etc.

B: It’s actually is creating a vacuum. The air is being sucked out of the vehicle.