Can anyone tell me why your ears “hurt” (for lack of a better word) if you’re driving down the interstate and only have one window cracked open? It doesn’t seem to happen until I’m going over 55-60 MPH and opening a parallel window fixes the issue, but I can’t find an exact reason why it happens. I assume it has something to do with air pressure, but why does it only happen when you’ve reached a certain speed? I’ve searched online for an answer, but have yet to find a satisfactory one. Please fellow posters, help me fight my ignorance! Hopefully, this question makes sense and I’m not the only one who has experienced this issue. Thanks!
With one window open, your car becomes a Helmholtz resonator, similar to an empty wine/beer bottle when you blow across the open end.
The frequency is dependent on the volume of the enclosure (larger means a softer “spring”, so lower frequency). It’s also dependent on the volume of the air that’s actually moving back and forth (more volume in motion = more mass/inertia = lower frequency). For bottles, this is the neck volume. For a car it’s the dimensions of the window opening; this opening doesn’t have any real length like a bottle neck, but for calculating the “neck volume” there’s an applicable length anyway (see “end correction” at above link), since the air isn’t suddenly at zero speed on the inside or outside of the car’s window opening.
The discomfort is because the pressure fluctuations are relatively large. You’ll feel the same discomfort if you put your ear next to a subwoofer playing a comparably low test frequency at high volume.
When you open another window on the other side of the car, you no longer have an enclosed cavity with a single port; you destroy the resonator system.
As for why some minimum speed is required to induce resonance, I don’t know.
This is why I love this place.
Some previous threads on the subject:
https://boards.straightdope.com/sdmb/showthread.php?t=813332
https://boards.straightdope.com/sdmb/showthread.php?t=793772
You’re amazing! Thank you so much!
I’ve only been a seedy lurker for a few months and I feel like I’m courting someone entirely out of my league. Hopefully, SDMB loves me as much as I love it.
yes, this very low frequency phenomenon (as explained by Machine Elf) is sometimes called “thrum” or “thrumming” in the industry. If your vehicle has 4 doors, cracking the rear windows open a little bit can mitigate it.
this used to be a fairly significant problem with sunroofs/moonroofs for the same reason. on modern cars, you’ll notice that a little “spoiler” or wind deflector pops up along the front edge of the moonroof opening when it’s retracted.
Thank you! The embarrassing thing is I actually tried to search the board to see if this had been previously discussed before starting this thread. I obviously need to sharpen my searching skills! Much appreciated.
Don’t let us get a big head. The fart jokes are always RIiiiiight under the surface.
Thank you for this answer - I had wanted to know, and then forgot to ask.
About the speed… Is there zero resonance when the air isn’t moving, like I’m guessing? And, beyond that, I wonder whether resonance begins at the first hint of air motion and gradually increases with the speed of air flow, or whether there’s a threshold speed for resonance to begin.
Or maybe I’m looking at it all wrong.
I do know that when playing a (modern-style European) flute (maybe other flutes too, I don’t know), if you blow the air too slow or too fast, you don’t get the resonance you want; the air stream has to be within a certain speed range to make the flute work.
Obviously, with a musical instrument of refined design, a lot of factors have been manipulated in order to get the desired musical effect - so maybe a flute isn’t a worthwhile example. Oh well. 
The information I read (can’t remember cite) is that the pressure disturbance is initially caused by vortex formation. The fast-moving air outside the window “rubbing” against the still air inside the window makes up a small whirl of clear air – a vortex – that makes its way from front edge of opening to back edge. At the back edge, it runs into the end of the opening and dumps its energy into the cabin of the vehicle as a pressure pulse.
When the car’s speed is too low, the vortices are pretty lazy and pressure pulses are trivial (and probably discharge into the slipstream more than into the cabin). At high enough speed, they pop appreciably, and when their generation rate matches the Helmholtz frequency of the resonator system (vehicle cabin and opening size), they pulses reinforce each other in an ear-popping, vertigo-inducing standing wave.
The only depressing part is that I thought I was pretty smart until I took up with these knotheads.
Did you actually use the board’s search function? If so, that’s why you came up empty. Go to Google and type your search terms along with “site:straightdope.com” in the search field, and you will get much, much better results.