I went for a hike today. It was quite windy and also quite noisy. It seemed to me that most of the noise came for the windy hitting the trees. The sum total of all that noise is the sound of wind.
I imagined that I was in the middle of a huge parking lot with a lot of wind all around me. Would I hear anything? It is the sound of wind or the sound of air flapping the skin on my ears?
Nope, no sound in a parking lot. This gets into some pretty complex areas of the physiology and physics of sound as well as fluid flow, but the main fact (generalisation) to realise is that fluid that is moving in straight lines doesn’t make any noise. Don’t believe me? Get a garden hose and turn it on. Listen to it, no sound. Now kink the hose a little and you will hear the water running through it quite easily.
Sound as we hear it is caused by pressure waves in the air. In simple terms that means that something has pushed the air in one location, and that push has been shunted form air molecule to air molecule all the way to our ears. Or to put it even more simply, something got the air moving faster and that movement has made it to our ears and been trnaslated into sound.
Now if sound is just caused by air moving then it might seem like wind would have a sound, but it doesn’t. That’s because sound is movement within the air. IOW we only hear a sound if part of the air moving faster relative to all the rest. It is the change in the speed of the air we respond to, not the speed itself. In contrast wind is a movement of air. With a steady wind there is no change for us to detect. The pressure from a constant wind is, well, constant. With no change in speed we can’t hear wind itself.
As you’ve noticed, we can hear wind if it slams into something like a leaf, that’s because striking the leaf causes the air in that location to slow down. Now we have an area of air that is moving at a different speeed to the rest, and that we can detect as sound.
We also hear wind whistling in our ears. It’s not the wind flapping your ears, your ears themsleves don’t move. It’s because the air is slamming into the little knobs on the external part of the ear and slowing down. Once again, a change in speed of the air, and we detect that as a sound.
In a parking lot wind won’t make any sound because there is nothing for it to strike, hence nothing to slow it down. The air will flow in nice staright lines and wind speed will remain constant. With no change in speed there is no sound.
Good question. In every instance I can think of, the sound of wind has been generated by resistance against something; against your ear, against trees, etc… The whistling sound seems to come from hitting an object a certain way to create a “tunnel” that makes the sound. If you could theoretically produce wind in a wall-less environment away from the source, does it make sound? If so, maybe it’s ultrasound (out of range in frequency of human hearing).
Question: What about sonic booms? Still a matter of air hitting something else, or is it air on air?
If a strong wind knocks over a tree in the forest …
Blake, that is an an awesome answer.
The actual sonic boom is caused by air hitting air. There’s nothing unique about sonic booms though. There are lots of noises caused by air hitting air, anything from the hiss when you open a soda bottle to the roar of an approaching tornado. The thing is that in all these cases the sound is caused because the air isn’t flowing in straight lines. Some parts of the air are flowing faster than others and that is what makes the noise. As soon as you get non-laminar flow in air or any other fluid it’s going to generate sound.
The noise of a sonic boom is no exception. The clap occurs as the temporary vaccum caused by the passing missile fills with gas again. The passing missile leaves two separated walls of air, much like the Red Sea as Moses strolled across, and it closes in much the same way. When the two walls of air collide they rebound off each other producing a wave of moving air that is travelling much faster than the air around it. That boom is then transmitted like any other sound: from air molecule to air molecule.
>Nope, no sound in a parking lot. This gets into some pretty complex areas of the physiology and physics of sound as well as fluid flow, but the main fact (generalisation) to realise is that fluid that is moving in straight lines doesn’t make any noise.
But I don’t think this is quite true. Even if the surfaces that bound the flow of air are straight, such as a parking lot, whether the air moves in straight lines still depends on whether turbulence develops. “Wind” generally describes air moving at a few or even a few dozen meters per second, and through dimensions that are generally many meters. Therefore the Reynolds number is generally high enough that wind is turbulent, and great vortices appear (these are the things that spin rain droplets into the sheets you see moving across the ground in thunderstorms, which was discovered at the University of Delaware [see recent Delaware discussion] only a few years ago). Energy cascades downward through vortices of smaller and smaller sizes, creating sound as it goes.
Moreover, there is a characteristic turbulent sound generated by your outer ears as the wind goes around them. You will notice that this sound depends greatly on what direction you face. There’s no flapping skin here, just air turbulence. Maybe it’s fair to debate whether this is the sound of wind or a more complicated combination of wind and human ears, and therefore some kind of artifact. But it is purely air noise with no objects flapping.
No. A sonic boom is the shock wave coming off the aircraft/missile. It’s the sudden increase in pressure that makes the sound, not the air clapping in behind the object after it passes (the air doesn’t do this.)