Wavy Windows
Why does rolling down a single window in a car hurt your ears?
I'm sure that most of you have experienced it before. It's a warm, sunny day and you roll down a window to feel the gentle wind. However, as soon as you open it, you hear a piercing, wavering noise and your ears begin hurting. Reluctantly, you close the windows (or open one on the other side, but we'll come back to that). Before I explain exactly why this happens, let's talk about something similar that happens on smaller scale. This will be easier to recreate and will prove why the car window phenomena happens. Redirect your attention to Figure 1. These are some classic glass bottleneck containers, usually used for soda. Suppose I remove the cap of the bottle and gently blow air across the top of the opening. What do you think will happen?
Figure 1
First, let's go back a couple seconds. Before I blow over the bottle, the air inside the bottle (for this article, when I refer to "air inside the bottle", I am referring to the air above the liquid) and the air in the surrounding room have the same pressure. They are in equilibrium and there is negligible movement of air between the inside and outside the container. However, after I blow, there is extra air pushed into the bottle. Now the air in the bottle is at a higher pressure than the air outside the bottle. As a result, the air in the bottle gets pushed out into the surrounding air, trying to create a pressure equilibrium.
However, when the air is pushed back out of the bottle, inertia carries too much of it out. Now, to create balance again, the surrounding air of high preesure pushes air back into the bottle pressure. But, once again, inertia has carried too much air into the bottle. You may have realized that we're in the same situation that we started with. Though there may be less of a difference now, the air pressure inside the bottle is still greater than that of the surrounding air. This cycle will continue as air oscillates in and out of the top of the bottle (Figure 2). This oscillation of air pressure is exactly what creates sound.
Figure 2
Figure 3
Now this is all very theoretical. I do have a way to prove it though. Suppose the bottle was emptied of its contents. If we repeat the blowing experiment, there is more space for the air to enter and compress in the bottle. The oscillations will continue to occur using the same principles we just discussed, but it will take more time for each cycle of air pressure fluctuations. Namely, the frequency of the oscillations will decrease. It's also helpful to know that a lower pitched oscillation is correlated with a lower pitched sound. With this in mind, our theory is easy to test. If the bottle creates a higher pitched noise when filled as compared to when empty, then our explanation is corroborated. Take a look at Figure 3.
Just as we expected, the bottle creates a higher pitched noise when empty. Now that we understand how this smaller scale phenomena works, we can extend it to our car windown scenario. As soon as we open the window, the same oscillations begin occuring. Air pushes in and out of the window, but at a much lower frequency than it did with the bottle. This is a frequency so low that our human ears can't perceive much of the noise it creates, but it does hurts our ears. However, there is a simple solution. If you open another window, air can enter and leave through each window, preventing the oscillation and protecting your ears.